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The diet of harbour porpoise (Phocoena phocoena) in the northeast Atlantic

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The harbour porpoise (Phocoena phocoena) is probably the most abundant small cetacean in the northeast Atlantic and as such is an important top predator. It is also one of the most threatened species, particularly as a consequence of fishery by-catch. Porpoises feed mainly on small shoaling fishes from both demersal and pelagic habitats. Many prey items are probably taken on, or very close to, the sea bed. Even though a wide range of species has been recorded in the diet, porpoises in any one area tend to feed primarily on two to four main species (e.g. whiting (Merlangius merlangus) and sandeels (Ammodytidae) in Scottish waters). Evidence for selective predation is equivocal. Many studies provide evidence of geographic, seasonal, interannual, ontogenetic or sexual differences in prey types or prey sizes, and such dif-ferences are often (speculatively) interpreted in terms of prey availability. A few studies demon-strate trends in diet selection that are consistent with changes in prey abundance. However, lack of availability of prey abundance data at an appropriate spatial and temporal scale is often a problem. Porpoise diets overlap extensively with diets of other piscivorous marine predators (notably seals). Many of the main prey species are also taken by commercial fisheries, although por-poises tend to take smaller fishes than those targeted by fisheries. Given their high abundance, porpoises clearly remove substantial quantities of fish. The literature on porpoise diets in the northeast Atlantic suggests that there has been a long-term shift from predation on clupeid fish (mainly herring Clupea harengus) to predation on sandeels and gadoid fish, possibly related to the decline in herring stocks since the mid-1960s. Evidence from studies on seals suggests that such a shift could have adverse health con-sequences. Food consumption brings porpoises into contact with two important threats – persistent organic contaminants and fishing nets, both of which have potentially serious impacts.
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Oceanography and Marine Biology: an Annual Review 2003, 41, 355–390
© R.N. Gibson and R.J.A. Atkinson, Editors
Taylor & Francis
THE DIET OF HARBOUR PORPOISE
(PHOCOENA PHOCOENA) IN THE
NORTHEAST ATLANTIC
M. B. SANTOS & G. J. PIERCE
Department of Zoology, University of Aberdeen, Tillydrone Avenue,
Aberdeen, AB24 2TZ, UK
e-mail: m.b.santos@abdn.ac.uk
Abstract The harbour porpoise (Phocoena phocoena) is probably the most abundant small
cetacean in the northeast Atlantic and as such is an important top predator. It is also one of the
most threatened species, particularly as a consequence of fishery by-catch.
Porpoises feed mainly on small shoaling fishes from both demersal and pelagic habitats.
Many prey items are probably taken on, or very close to, the sea bed. Even though a wide range
of species has been recorded in the diet, porpoises in any one area tend to feed primarily on two
to four main species (e.g. whiting (Merlangius merlangus) and sandeels (Ammodytidae) in
Scottish waters).
Evidence for selective predation is equivocal. Many studies provide evidence of geographic,
seasonal, interannual, ontogenetic or sexual differences in prey types or prey sizes, and such dif-
ferences are often (speculatively) interpreted in terms of prey availability. A few studies demon-
strate trends in diet selection that are consistent with changes in prey abundance. However, lack
of availability of prey abundance data at an appropriate spatial and temporal scale is often a
problem.
Porpoise diets overlap extensively with diets of other piscivorous marine predators (notably
seals). Many of the main prey species are also taken by commercial fisheries, although por-
poises tend to take smaller fishes than those targeted by fisheries. Given their high abundance,
porpoises clearly remove substantial quantities of fish.
The literature on porpoise diets in the northeast Atlantic suggests that there has been a long-
term shift from predation on clupeid fish (mainly herring Clupea harengus) to predation on
sandeels and gadoid fish, possibly related to the decline in herring stocks since the mid-1960s.
Evidence from studies on seals suggests that such a shift could have adverse health con-
sequences.
Food consumption brings porpoises into contact with two important threats – persistent
organic contaminants and fishing nets, both of which have potentially serious impacts.
Introduction
The harbour or common porpoise, Phocoena phocoena (Linnaeus 1758), is one of the six
species recognised in the family Phocoenidae (Read 1999). Its common name, porpoise,
derives from the Latin porcus piscus (pigfish) and was used in Ancient Rome. Linnaeus
(1758) distinguished it from the common dolphin by calling it Delphinus phocoena, from
the Greek word phokia (seal), due to its lack of beak and its seal-like appearance.
M.B. SANTOS & G.J. PIERCE
356
Harbour porpoises, one of the most common cetaceans in European waters (Watson
1985), are small with an average adult length of 150cm to 160cm and an average weight of
45kg to 60kg (Gaskin et al. 1974). Maximum sizes of animals stranded in the UK have been
reported as 163cm and 54kg in males and 189cm and 81kg in females (Lockyer 1995)
although more recent work by Santos et al. (2001a) reported maximum sizes for males and
females as 170cm (55kg) and 171cm (55.5kg), respectively, for porpoises stranded in Scot-
land. Harbour porpoises stranded in northwest Spain and Portugal seem to be larger, and
several specimens have measured more than 200cm (Donovan & Bjørge 1995, Sequeira
1996).
There is some variation in the maximum ages reported for different harbour porpoise
populations: no porpoises over 17yr of age have been found in the Bay of Fundy (eastern
Canada) (Read & Gaskin 1990, Read & Hohn 1995) while animals up to 24yr old have been
reported in UK waters (Lockyer 1995) and off California (Hohn & Brownell 1990).
Harbour porpoises are widespread in coastal waters of the Northern Hemisphere; their
range extends northwards from 14–15°N in the North Atlantic and from 30°N in the North
Pacific (Evans 1980). They are also found in the Black Sea and some records suggest the
existence of a separate population off northwestern Africa (Smeenk et al. 1992). Historically,
the species was also found in the western Mediterranean but there are no confirmed records
since the nineteenth century (Marchessaux 1980, Donovan & Bjørge 1995). The species is
also found over offshore shallows (e.g. the Georges and Grand Banks, stretching from New-
foundland to southern New England on the edge of the North American continental shelf) and
around islands such as the Faeroes and Iceland. Individuals have also been recorded consider-
able distances up rivers (e.g. 320km up the river Mass in Holland, Gaskin 1984).
Intraspecific differences in morphometric and meristic skull characters have been
demonstrated between the North Pacific, North Atlantic and Black Sea (Tomilin 1957,
Kinze 1985, Miyazaki et al. 1987, Yurick & Gaskin 1987, Amano & Miyazaki 1992). The
differences between these three areas, suggesting reproductive isolation, have been con-
firmed by studies on mitochondrial DNA (Rosel et al. 1995, Wang et al. 1996). Rosel et al.
(1995) suggested that there are at least three subspecies: P. phocoena phocoena (Atlantic),
P. phocoena vomerina (Pacific) and P. phocoena relicta (Black Sea). The existence of
further population subdivisions within these three major areas has been proposed by several
authors using a variety of methods (e.g. Yurick 1977, Gaskin 1984, Yurick & Gaskin 1987,
Andersen 1993, Rosel et al. 1995, 1999, Tiedemann et al. 1996, Wang et al. 1996, Börjesson
& Berggren 1997, Walton 1997, Wang & Berggren 1997, Berrow et al. 1998, Lockyer 1999,
Westgate & Tolley 1999, Tolley et al. 1999, 2001, Tolley & Heldal 2002) and putative stock
boundaries have been suggested by the International Whaling Commission’s (IWC) Scient-
ific Committee (Donovan & Bjørge 1995).
Historically, harbour porpoises have been taken in European waters for human consump-
tion and for fish bait (Watson 1985). It is believed that, in some areas, the porpoise hunt
goes back to the Stone Age (e.g. inner Danish waters, Möhl 1970 quoted in Kinze 1995).
Porpoise meat was greatly appreciated during the Middle Ages. It was served in the English
court at coronation banquets and on other important occasions (Whymper 1883). In
Denmark, a harbour porpoise fishery was first mentioned in 1357 and continued until the
Second World War (Kinze 1995). Kinze estimated an average catch of at least 1000 animals
took place every year in Danish waters for three centuries. Along the coast of Norway, large
numbers were taken as early as the eleventh century (Watson 1985). At present hunting con-
tinues, on a small scale, only in Greenland and the Faeroes (IWC 1996).
THE DIET OF HARBOUR PORPOISE IN THE NE ATLANTIC
357
Accidental (by-catch) mortality in fishing gear is recognised as the main threat at present
for harbour porpoise populations in the northeast Atlantic (ASCOBANS 1994, IWC 1994,
1996, Donovan & Bjørge 1995, Tregenza et al. 1997, Vinther 1999). The harbour porpoise
suffers a high degree of by-catch mortality across its range (Read & Gaskin 1988). In north-
ern Europe, it is the most frequently caught cetacean species in fishing nets and it is caught
in a wide variety of fishing gear, mainly gill nets but also salmon drift nets, pound nets for
herring and salmon and mid-water trawls (Lindroth 1962, van Utrech 1978, Andersen &
Clausen 1983, Kinze 1990a, Northridge & Lankester 1992, Clausen & Kinze 1993 quoted in
Lowry & Teilmann 1994, Tregenza et al. 1997, Vinther 1999). There are historical records
of porpoises caught in gill nets as long ago as the sixteenth century in Europe (Belon 1551,
quoted in Harmer 1927), but it is clear that increased fishing effort together with innovations
in net design and use of thinner synthetic fibres (which reduces detectability) have resulted
in steeply increased catch rates.
The impact of by-catches on harbour porpoise populations is, at the moment, difficult to
assess but there is concern about the sustainability of these catches. In 1990 and 1991, the
Scientific Committee of the IWC recommended, with the “highest priority”, the reduction of
by-catch for harbour porpoise (IWC 1991, 1992). It also noted the need to improve know-
ledge of stock identity and migration, and obtain reliable figures on by-catches and on abun-
dance.
An additional cause of harbour porpoise mortality in Scotland is the bottlenose dolphin
(Tursiops truncatus), particularly in the Moray Firth area. Post-mortem examinations of a
number of porpoise carcasses (all with multiple skeletal fractures and damage to internal
organs), provided the first evidence of dolphin attacks, when measurements of tooth marks
present in the skin corresponded with the tooth spacing in Tursiops mandibles (Ross &
Wilson 1996). Subsequently, bottlenose dolphins were observed attacking porpoises and
repeatedly throwing them out of the water. The reasons for these interactions are unknown,
but could include competition for food, play, practice-fishing or sexual behaviour (Ross &
Wilson 1996).
In July 1994, a survey of Small Cetacean Abundance in the North Sea (SCANS) was
carried out to estimate numbers of harbour porpoises and other small cetaceans in the North
Sea and adjacent waters. To date this survey has provided the most accurate and complete
information on population figures for harbour porpoises in the area. Previous ship and aerial
surveys were on a smaller scale (e.g. Heide-Jørgensen et al. 1992, 1993, Bjørge & Øien
1995). The population estimates for harbour porpoise obtained from SCANS are given in
Table 1. As pointed out by Hammond et al. (1995), these figures were calculated from
observations that took place in only one month.
Porpoises clearly exist in large numbers in the northeast Atlantic (Hammond et al. 1995)
and, as such, are likely to have a quantitatively important place in the marine food web. In
addition to their ecological importance, as fish eaters they may compete with commercial
fisheries. Diet has consequences for individual fitness and, ultimately, population status and
it is of interest to determine whether the large-scale fluctuations in fish stocks seen in the
second half of the twentieth century have influenced porpoise diet and population trends. In
so far as porpoises prey on species also eaten by humans, the health consequences of diet
choice (e.g. in terms of nutrition or bioaccumulation of contaminants) are of direct relevance
to the human population.
The general biology of harbour porpoises is reviewed in Bjørge & Donovan (1995) and
Read (1999). However, to date there has been no major review of diets of harbour porpoises
M.B. SANTOS & G.J. PIERCE
358
and little attention has been given to their ecological role as marine top predators. The
present paper reviews information on the diet of harbour porpoises in the northeast Atlantic
and will try to answer the following questions:
(1) What is their feeding niche: what do harbour porpoises eat and where do they
obtain their food?
(2) Are they specialist or generalist predators and is their diet related to prey abun-
dance (opportunistic or selective)?
(3) Do porpoises compete with other top predators such as seals and other
cetaceans, and do they compete with fisheries?
(4) Is there evidence of long-term trends or changes in diet composition?
(5) What are the consequences of diet composition for individual health and popu-
lation status (e.g. is the diet nutritionally adequate, does diet choice play a role
in fishery by-catch mortality, and are there adverse consequences of contaminant
bioaccumulation)?
(6) What important questions require further research?
Study methods
Almost all published accounts of porpoise diet are based on stomach contents analysis. The
largest scale studies are associated with hunting of porpoises in the Black Sea (Tsalkin 1940,
quoted in Tomilin 1957) but most studies are on by-caught and stranded animals. The
methodology of stomach contents analysis was reviewed by Pierce & Boyle (1991).
The present review focuses mainly on results from stomach contents analysis. However,
Table 1 Abundance estimates of harbour porpoise for the different blocks surveyed by the Small
Cetacean Abundance in the North Sea (SCANS) survey (data from Hammond et al. 1995), CVcoef-
ficient of variation.
Block Area covered in the survey CV Porpoise abundance
A Celtic Shelf 0.57 36280
B Channel and South tip North Sea 0 0
C East coast of Britain 0.18 16939
D West northern North Sea (excluding J) 0.25 37144
E North central North Sea 0.49 31419
F Central North Sea (56°N–58°N) 0.25 92340
G South central North Sea 0.34 38616
H South eastern North Sea 0.29 4211
I Kattegat 0.34 36046
J Waters off Shetland and Orkney 0.34 24335
L Coastal area S and W of Jutland 0.47 11870
M Off SW coast of Norway 0.27 5666
X Bay of Kiel 0.48 588
Y Northern Wadden Sea 0.27 5912
Total 0.14 341366
(95% confidence intervals) (260000–449000)
THE DIET OF HARBOUR PORPOISE IN THE NE ATLANTIC
359
some information on diet can also be obtained indirectly from studies on distribution and
behaviour. Relatively new techniques for obtaining information on marine mammal diets such
as fatty acid signature analysis of blubber and stable isotope analysis have not yet been applied
to porpoises on a large scale. Fatty acids are mainly of dietary origin and stable isotope values
vary geographically and, in a systematic way, with trophic level. In contrast with stomach con-
tents analysis, these new techniques can provide information on the prey ingested over a longer
period (days to months; e.g. Tiezen 1978, Hobson 1990, Iverson et al. 1995, Kirsch et al. 1998,
2000, Hooker et al. 2001). However, stable isotope analysis provides relatively coarse level
data on diet, providing data mainly on the trophic level at which an animal feeds (see Pauly et
al. 1998). On the other hand, fatty acid analysis has the potential to provide quantitative data
on diet, although realisation of this potential requires information on the fatty acid composition
of all putative prey, including data on variability, and the computing power to solve the
equivalent of an extensive series of simultaneous equations. Fatty acid signature analysis is
well-established as a technique for studies on seals (e.g. Iverson et al. 1997). Doubts were
raised as to its applicability to cetaceans due to blubber stratification: the outer blubber layers
being relatively metabolically inert while the inner layers, closer to the muscle, are metaboli-
cally active. Also, blubber from different parts of the body may show differences in fatty acid
composition, as found for some cetaceans (Ackman & Lamothe 1989). Recent studies in
harbour porpoise have shown that dietary fatty acids are concentrated in the inner blubber
(Koopman et al. 1996, Koopman 1998) and the fatty acid composition seems to be uniform in
the body with the exception of the caudal peduncle.
What do harbour porpoises eat?
The earliest published information available on the diet of harbour porpoises derives from
the examination of single specimens, which were captured accidentally in fishing gear or
stranded, during the nineteenth century. Dewhurst (1834) records that “porpoises live upon
small fish though they will eat any offal and garbage that is thrown into the sea”. Southwell
(1881) observed that “the food consists of fish and it follows the shoals of herring, etc.,
amongst which it commits great depredation; it has a taste for salmon (Salmo salar) and is
sometimes taken in the salmon-nets”. Van Beneden (1889) stated that “the porpoise preys on
fish, including herring, but may also eat crustaceans, cephalopods and even marine plants”.
Descriptions of porpoise diets from the late nineteenth and early twentieth centuries
include records of a wide variety of fishes and, in some cases, cephalopods and crustaceans
in stomachs. Scott (1903), examining the stomach contents of a porpoise caught in a salmon
net in Scotland, found remains of fish (flesh and otoliths), of which the most frequent was
whiting (Merlangius merlangus). The high numbers of otoliths found (240), in the opinion
of the author, showed “how destructive these cetaceans can be when they get among a shoal
of fishes”. Various small-scale studies in the northeast Atlantic from the early part of the
twentieth century through to the 1970s document predation by porpoises on whiting, herring
(Clupea harengus) and sometimes other fish species such as capelin (Mallotus villosus),
mackerel (Scomber scombrus), sole (Solea solea), cod (Gadus morhua), eel (Anguilla
anguilla), as well as shrimps and cuttlefish (Millais 1906, Stephen 1926, Harmer 1927,
Freund 1932 quoted in Tomilin 1957, Fraser 1946, Darling 1947, Matthews 1952, Hardy
1959, Slijper 1962, Andersen 1965, Källquist 1975 quoted in Otterlind 1976).
M.B. SANTOS & G.J. PIERCE
360
Similar types of prey were recorded in other areas: for example, Pacific herring (Clupea
pallasii), capelin, Pacific sardine (Sardinops coerulea) and smelt (Osmeridae) in the North
Pacific (Sleptsov 1952 quoted in Tomilin 1957, Wilke & Kenyon 1952, Scheffer 1953, Fink
1959); herring, cod, mackerel (Scomber scombrus), hake (Urophycis tenuis), pollack (Pol-
lachius virens), squid (Loligo pealii), smelt (Osmerus mordax), silver hake (Merluccius
bilinearis) and redfish (Sebastes marinus) off eastern Canada (Sergeant & Fisher 1957,
Smith & Gaskin 1974).
The most extensive published study on harbour porpoise diet was carried out in the Black
Sea, where 4000 stomachs were examined by Tsalkin (1940 quoted in Tomilin 1957). The
diet consisted mainly of benthic fish (several goby species, Gobius rotan, G. melanostomus,
G. syrman, Mesogobius batrachocephalus; Black Sea flounder, Pleuronectes flesus flesus;
Black Sea sole, Solea nasuta; bream, Abramis brama and Black Sea whiting, Gadus
euxinus). Pelagic species (Black Sea silverside, Atherina pontica; Black Sea anchovy,
Engraulis encrasicholus; pikeperch, Lucioperca lucioperca; mullet, Mugil sp. and Black Sea
shad, Caspialosa sp.) were also found but were thought to be taken only when they occurred
in large and dense schools.
In the northeast Atlantic, the first detailed study on harbour porpoise diet was carried out
by Rae (1965). This author, concerned with the possible role of porpoises as predators of
salmon, examined the stomach contents of 52 animals by-caught in different fishing gears in
Scotland from 1959 to 1965. Herring and whiting were the main prey found. In a later study,
from 1965 to 1971, a further 30 by-caught and 11 stranded porpoises were examined (Rae
1973). In these 41 stomachs, the main prey were clupeids (herring and sprat Sprattus sprat-
tus) and small gadoids (mainly whiting). No evidence of predation on salmon was found in
either study. Lindroth (1962) examined the stomach contents of 50 harbour porpoises from
the Baltic Sea captured from 1960–1, again with the aim of finding out whether porpoises
prey on salmon, and also found no evidence of salmon in the diet. Remains of sprat, herring,
Baltic cod (G. m. callarias), gobies (Aphya minuta) and sandeels (Ammodytes sp.) were
found.
As schemes to record and examine stranded cetaceans were set up in different countries
and research programmes started collecting by-caught specimens during the 1980s and
1990s, more studies on harbour porpoise diets were carried out (see Table 2). The results of
these studies demonstrated that harbour porpoises feed on both pelagic schooling fish
species, for example, herring, capelin, whiting, blue whiting Micromesistius potassou,
sardine, northern anchovy, and demersal or benthic fish, for example, hake, Trisopterus spp.,
sandeels (Ammodytidae), gobies, sole, dab Limanda limanda, Greenland halibut Rein-
hardtius hippoglossoides (Desportes 1985, Sekiguchi 1987, Kinze 1989, Lick 1991a,b,
Aarefjord et al. 1995, Börjesson & Berggren 1996, Malinga & Kuklil 1996, Rogan &
Berrow 1996, Santos 1998). Other prey species such as cephalopods (ommastrephids, sepi-
olids, loliginids, gonatids), other molluscs, crustaceans (e.g. euphausiids) and polychaetes
(Nereis) were also recorded (Smith & Gaskin 1974, Desportes 1985, Sekiguchi 1987, Kinze
1989, Smith & Read 1992, Gaskin et al. 1993, Gearin et al. 1994, Rogan & Berrow 1996,
Santos 1998). Plastic and other foreign objects such as nylon fishing line and a banana peel
have also been recorded in the stomachs (Kastelein & Lavaleije 1992, Baird & Hooker
2000).
THE DIET OF HARBOUR PORPOISE IN THE NE ATLANTIC
361
Table 2 Main studies on harbour porpoise diets since the 1980s (sstranded animals, b by-caught, mstranded and by-caught, d direct hunt). *No
data on number of stomachs with food provided, aadult animals, ccalves (from Smith & Read 1992).
Area Stomachs analysed Stomachs with Main Prey Reference
food remains
NE Atlantic
France 17 (s) 8 Blue whiting, scad, hake Desportes 1985
Denmark, Sweden, Norway 247 (m) 179 Herring, gadids Aarefjord et al. 1995
Germany 54 (m) 36 Sole, cod Lick 1991a,b
34 (m) * Sandeels, sole Benke & Siebert 1996
Ireland 26 (m) 19 Trisopterus spp., whiting, herring Rogan & Berrow 1996
United Kingdom 250 (b) 100 Gadids, sandeels, gobies Martin 1996
Scotland 72 (m) 72 Whiting, sandeels Santos 1998
The Netherlands 62 (m) 62 Whiting Santos 1998
Denmark 58 (m) 58 Cod, viviparous blenny, whiting Santos 1998
NW Spain 6 (m) 6 Scad, sandeels, Trisopterus spp. Santos 1998
Skagerrak and Kattegat Seas
Sweden 171 (m) 145 Herring, sprat, whiting Berggren 1996, Börjesson & Berggren
1996
Baltic Sea
Germany 48 (m) 42 Cod, gobies, herring Lick 1991a,b
27 (m) * Gobies, herring, cod Benke & Siebert 1996
Poland 16 (b) 14 Cod, gobies, herring Malinga & Kuklik 1996
NW Atlantic
Eastern Canada 160 (b) 127 Herring, silver hake, cod Recchia & Read 1989
a149 (b) a136 Clupeids, gadids Smith & Read 1992
c31 (b) c24 Euphausiids
138 (b) 111 Capelin, herring Fontaine et al. 1994
95 (b) 95 Herring, silver hake, red and Gannon et al. 1998
white hakes
Greenland 18 (d) 18 Greenland halibut, haddock Kinze 1989
20 (d) 20 Capelin, halibut Kinze 1990b
Pacific Ocean
Northern Japan 20 (b) 5 Ommastrephid squids, herring, Gaskin et al. 1993
anchovy, hake
Washington 100 (b) 94 Pacific herring, squid, smelts Gearin et al. 1994
California 15 (m) 9 Northern anchovy Sekiguchi 1987
M.B. SANTOS & G.J. PIERCE
362
Are porpoises opportunistic or selective predators?
As Dunnet (1996) pointed out, opportunism, selection and availability are “in fact shorthand
for very complex biological interactions about which we know only a little in quantitative
terms”. Strictly applied, the term “opportunistic” predation can be taken to imply that prey
are taken as encountered, with the implication that prey availability is the only criterion
affecting diet choice. Following the theory of optimal diet selection (e.g. Pulliam 1974),
observation of opportunistic predation (sensu stricto) might be taken to imply that high
quality prey are relatively rarely encountered. However, the term is probably used at least as
often as a catch-all phrase which indicates no more than that evidence for active selection
was not found (and often was not sought).
The diversity of prey eaten and the geographical variation found in the diet have led some
authors to consider the harbour porpoise to be an opportunistic feeder (e.g. Martin 1996, Teil-
mann & Dietz 1996), not limited to shallow waters, but able also to feed pelagically on mid-
water species from deeper habitats (IWC 1996). In fact, few studies in the northeast Atlantic
have been able to address this question because information on prey abundance, for all the
species at an appropriate spatial and temporal scale, is rarely available. Donovan & Bjørge
(1995) note that, to answer the question of whether harbour porpoise feeding patterns follow
prey availability, it would be necessary to study “the distribution of prey and target species on
a very small spatial scale, much smaller than presently documented in fishery literature”.
Santos (1998) compared the rank order of importance (by weight) of different fish species
in harbour porpoise diets and fishery landings in Scotland, excluding species eaten by por-
poises but not commercially fished, and found a positive correlation in 3yr out of the 5yr
studied. Species that are not fished made up between 0.3% (1996) and 2.6% (1994) of the
diet by weight but their absence in landings data simply reflected low (or zero) market value.
Using catch statistic data as a measure of species abundance has potential risks since fishery
landings are also affected by changes in market demand, fishing effort, establishment of
management measures such as minimum landing size, total annual catches, area closures,
etc. (Hislop 1996). Nevertheless, fishery-catch data can sometimes provide a reasonable
estimate of the abundance, distribution and availability of the prey species (Evans 1975),
and the correlation between the importance rankings of certain species in commercial
catches and in porpoise diets (Santos 1998) remains of interest. To the extent that fisheries
take species in proportion to their abundance, this result provides weak support for the
notion of porpoises as an opportunistic species.
Porpoise distribution around Shetland was spatially correlated with sandeel distribution in
1992 and 1993 but there was no correlation between the seasonal patterns of porpoise
numbers and prey abundance (Evans & Borges 1995).
The nature of feeding strategies can be revealed by studies of dietary variation. Against a
background of varying fish abundance, piscivorous predators, especially those foraging
opportunistically, might be expected to show regional, seasonal or interannual variation in
diet. On the other hand, evidence of sex- or age-related variation in diet may be consistent
with opportunistic predation (if there is sex- or age-related habitat segregation) or provide
evidence for the role of other factors (e.g. if both sexes occupy the same habitat). Despite the
existence of numerous studies on harbour porpoise diet there has been little quantitative
analysis of patterns in diet in the northeast Atlantic. This is mainly due to the lack of appro-
priate sample sizes that would allow disentanglement of the effects of all the factors poten-
tially affecting diet variability.
THE DIET OF HARBOUR PORPOISE IN THE NE ATLANTIC
363
Geographical variation
Several studies on harbour porpoise diets in the northeast Atlantic have revealed geographi-
cal variation in the main prey consumed (in the UK, Martin 1996, Santos 1998; in Norway,
Denmark and Sweden, Aarefjord et al. 1995, Berggren 1996; in Germany, Lick 1991a,b,
Benke & Siebert 1996; in Ireland, Rogan & Berrow 1996). One caveat is that not all of these
studies were contemporaneous and the confounding effect of temporal shifts in diet cannot
always be ruled out.
Based on examination of prey remains from the stomachs of 100 porpoises stranded and
by-caught on the British coast from 1989 to 1994, the most important prey in terms of
biomass were gadoids (whiting, haddock, Norway pout Trisopterus esmarkii and pollack),
while sandeels and gobies were by far the most frequently eaten. Differences were found
between diets in different areas of the British coast, with sandeels taken in bigger numbers
on the east coast, while Norway pout was taken by more than half of the porpoises from
Shetland, but was not eaten elsewhere (Martin 1996). The greater dietary importance of
Trisopterus in Shetland than elsewhere in Scotland is also supported by results in Santos
(1998). Harbour porpoises from Irish waters fed mainly on Trisopterus spp. (Rogan &
Berrow 1996).
Significant between-area differences in the diet were reported by Santos (1998), who
analysed the stomach contents of 198 stranded and by-caught porpoises from Denmark, The
Netherlands, Scotland and Galicia (NW Spain) mainly from 1989 to 1996. The author found
various significant between-area differences in the diet. Porpoises from Scotland had eaten
significantly more sandeels than had those from Denmark and Holland, also more sepiolids
and fewer cod than in Denmark. Danish porpoises took significantly more gobies than did
porpoises in Scotland, while viviparous blennies (Zoarces viviparus) were present only in
the Danish diet. In Holland, porpoises had taken significantly more gobies, dragonets and
squid Loligo forbesi than had porpoises from Scotland, and more sepiolids than porpoises
from Denmark. In Galicia, despite a small sample size, a wider range of prey was recorded
than in other areas, including some species found only in the Galician diet (e.g. silvery pout
Gadiculus argenteus thori, argentine Argentina sp.).
Regional differences in the diet were also found by Aarefjord et al. (1995), who exam-
ined stranded and by-caught specimens from Norwegian, Danish and Swedish waters
(1985–90). Overall, herring was the most important single prey species, while gadoids made
up more than half of the total prey weight. However, harbour porpoises from the Danish
North Sea and the Baltic took mainly cod, whiting, sandeels and gobies, while saithe, blue
whiting and capelin were more frequent in porpoises from the Norwegian waters. Herring
and sprat were found to be the main food of harbour porpoises stranded and by-caught
during 1988–93 in the Swedish Skagerrak and Kattegat Seas (Berggren 1996).
In German waters, results from Lick (1991a,b) and Benke & Siebert (1996), from strand-
ings and by-catches in 1985–90 and 1991–3, respectively, indicate geographic differences in
the diet of harbour porpoises. In porpoises from the North Sea from 1991–3, sandeels made
up almost 40% of the prey weight, with a further 30% being common sole (Solea solea). In
contrast, in the Baltic Sea over 50% of the total prey weight was made up of gobies, while
23% was herring and 15% was cod (Benke & Siebert 1996).
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Seasonal variation
Seasonal variation in harbour porpoise distribution has been described as a general inshore
movement in summer and offshore movement in winter, although east–west and north–south
migrations have also been proposed in different locations (e.g. Tomilin 1957, Gaskin et al.
1974, Gaskin 1977, 1984, Taylor & Dawson 1984, Gaskin & Watson 1985, Northridge et al.
1995, Read & Westgate 1997). Seasonal movements are believed to be related to prey avail-
ability or to breeding habitat (Gaskin 1977, Northridge et al. 1995, Read & Westgate 1997).
Camphuysen & Leopold (1993) interpreted the higher numbers of porpoise sightings and
strandings on the Dutch coast in winter than in summer as indicating a seasonal east-west
movement.
Aspects of the ecology of some of the prey species may assist in interpreting seasonal dif-
ferences in diet (Santos 1998). Sandeels spend most of the autumn and winter (September to
March) buried in the sand, with the exception of the spawning period between December
and January (Macer 1966, Reay 1970, Langham 1971, Winslade 1974, Wright 1996). This is
consistent with the higher prevalence of sandeels in spring and summer diets than in autumn
and winter diets of Scottish harbour porpoises (Santos 1998). Although, arguably, echolocat-
ing porpoises should be able to detect sandeels in the sand it is perhaps energetically more
costly to catch them. The higher importance of whiting in the winter diet could relate to the
lower availability of sandeels but is also consistent with trends in whiting abundance. Trawl
survey data for the east coast of Scotland indicate that whiting, poor cod and Norway pout
are more abundant in inshore waters in winter than in summer (Santos et al., unpubl. data).
Seasonal variation was also documented in the diet of harbour porpoises by-caught in
Swedish waters: while herring was the main prey all year round, the contribution of sprat
and whiting varied seasonally (Börjesson & Berggren 1996).
Seasonal differences have also been reported in the size of prey eaten by porpoises
(Santos 1998). In Scotland, smaller whiting were taken in autumn and bigger whiting were
eaten in spring and summer, while bigger sandeels were taken in winter and spring than
summer. In Denmark, smaller viviparous blennies and whiting were taken in spring than
summer. In Holland, smaller gobies were taken in autumn than in winter and spring. Such
trends may also be interpreted with reference to fish life cycles, in that small fishes are most
likely to be taken when 0-group fishes move into the area used by porpoises. Thus, large
numbers of 0-group sandeels become available in the summer months, when they are preyed
upon by harbour porpoises and a wide variety of birds, fishes and other marine mammals
(e.g. Furness & Hislop 1981, Jonsgåard 1982, Perkins et al. 1982, Furness 1987, Daan 1989,
Harris & Riddiford 1989, Monaghan et al. 1989, Pierce et al. 1989, 1991a,b, Harris &
Wanless 1991, Storey 1993, Thompson et al. 1991).
Seasonal differences in the diet of harbour porpoises have been reported also in studies
outside the northeast Atlantic. In Atlantic Canadian and United States waters, prey diversity
was higher in winter than in summer, with porpoises eating mainly herring, silver hake and
pearlsides (Maurolicus weitzmani) in the autumn (Palka et al. 1996). Gannon et al. (1998)
analysed the stomach contents of 95 harbour porpoises by-caught in autumn in the Gulf of
Maine and compared the results with previous studies of harbour porpoises by-caught in
summer in the Bay of Fundy (Smith & Gaskin 1974, Recchia & Read 1989, Smith & Read
1992). They noted that herring was the main prey for porpoises in both autumn and summer
but was less dominant in the autumn diet than in summer. They also found that porpoises ate
a wider variety of prey and of prey sizes in autumn than in summer.
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365
Interannual variation
Interannual variation in the diet might be expected to follow variation in the availability of
preferred prey. Declines in the availability of common prey could lead harbour porpoises to
switch to other prey species and/or prey sizes. For some of the commercial fish species eaten
by porpoises, estimates of abundance and size distributions are available. However, this is
not the case for many other prey species (e.g. the smaller inshore fish such as blennies,
gobies and eels).
Significant interannual differences in the average size of fishes eaten (e.g. for sandeels
and whiting in Scotland, viviparous blenny and whiting in Denmark, and gobies in Holland)
were described by Santos (1998). In terms of amounts eaten, significant interannual vari-
ation was found only for herring, and the significance of this variation was strongly influ-
enced by a single porpoise killed by bottlenose dolphins in the Moray Firth in November
1994, which had eaten many small herring (150mm). In any case, the interannual changes
were apparently unrelated to changes in herring abundance. However, it is worth noting that
of the three main studies on porpoise diets in UK waters, only the earliest (Rae 1965, 1973)
records herring as forming a major part of the diet and this change could reflect the decline
in herring abundance in the North Sea since the 1960s. This topic is revisited in more detail
below.
Outside the northeast Atlantic, Recchia & Read (1989) found some differences in the diet
of harbour porpoises by-caught in groundfish gill nets in the Bay of Fundy (Canada) during
two time periods (1969–72 and 1985–7). Porpoises from 1969–72 had taken mackerel and
silver hake less often than porpoises from 1985 to 1987 although herring, silver hake and
cod remained the main prey in the diet for both samples.
The main prey (by percentage occurrence) of harbour porpoises off northern Washington
differed between the years 1988–90. In 1988 it was Pacific herring, followed by squid and
smelt, while in 1989 smelt was the main prey followed by squid (Loligo opalescens) and
gadoids. In 1990, Pacific herring was again the main prey followed by smelt and gadoids
(Gearin et al. 1994).
Ontogenetic variation
Differences in diet between adult and juvenile porpoises in the northeast Atlantic have been
found in several studies (Lick 1991a,b, Benke & Siebert 1996, Börjesson & Berggren 1996,
Santos 1998). Juveniles cannot dive as deep as adults and could be prevented by their small
size from catching and eating big prey.
Differences in the diet of young (120cm total length) and adult porpoises were found in
a sample of 78 stomachs from porpoises stranded and by-caught in Germany. Young por-
poises took more gobies, while adult porpoise took more flatfishes and gadoids and had a
bigger variety of prey species in the stomach (Lick 1991a,b). Similar results were found in a
sample of 61 porpoises by-caught and stranded in Germany during 1991–3 (Benke & Siebert
1996). Börjesson & Berggren (1996) also noted that gobies were important in the diet of
calves (1-yr old) from porpoises by-caught off Swedish waters. The authors concluded
that the small size of gobies could make them a suitable prey for calves. Santos (1998) found
that, in Scotland, adult harbour porpoises ate bigger whiting than did juveniles, while in
Denmark juveniles ate bigger viviparous blenny and whiting than adults, and in Holland
M.B. SANTOS & G.J. PIERCE
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adults took bigger gobies and sandeels than juveniles. The author considered that it was pos-
sible that most of these differences related to adult porpoises feeding further offshore than
juveniles. In addition, the analysis showed that, in Holland, smaller porpoises took fewer
whiting but more gobies than did bigger porpoises.
In other studies in the northeast Atlantic, no significant differences were found between
the diets of calves (113cm total length) and adult porpoises in Scandinavian waters,
although gobies were the most frequent prey in the stomach of 0 and 1-yr old porpoises
(Aarefjord et al. 1995), or between diets of juvenile and adult porpoises in a sample of
animals stranded and by-caught in the UK (Martin 1996).
Outside the northeast Atlantic, although no differences were found between the diets of
juvenile and adult porpoises in Canadian waters (Smith & Gaskin 1974), comparison of a
sample of 31 calves (1-yr old) and 149 adult porpoises by-caught in gill nets in the Bay of
Fundy from 1985 to 1991 revealed that calves were taking euphausiids during their first
summer while adult porpoises ate herring (Smith & Read 1992). The authors suggested that
calves “learn” to forage on euphausiids before starting to take larger prey such as fishes. A
comparison of the diets of 13 calves and 74 juvenile and adult porpoises by-caught in gill
nets in the Gulf of Maine in 1989 and 1991–4 showed that calves had eaten a greater propor-
tion of pearlsides and euphausiids than adults and had also taken smaller herring and silver
hake (Gannon et al. 1998).
Diet of male and female harbour porpoises
Differences in diets of males and females might be expected if the sexes tend to inhabit dif-
ferent areas, as a consequence of the larger average body size in females, and if the foraging
behaviour of females is affected by the presence of nursing calves.
Segregation of harbour porpoises in groups of different sex and/or age has been proposed
by several authors to explain differences in by-catch figures. The predominance of mature
males in the catch in the Baltic Sea was explained by Tomilin (1957) as a consequence of
adult males forming separate groups that are more “mobile” than groups comprising juve-
niles or females with calves. Such segregation by age and sex has also been suggested as an
explanation of high catches of sub-adult males in nets in offshore Canadian waters (Kinze
1994). In contrast, females accompanied by calves would tend to be associated with shal-
lower waters (Kinze 1994). If this segregation takes place, females with calves would not
only have a different distribution from males but could also be restricted in their search for
food (e.g. by not being able to dive very deep or search long distances). Seasonal differences
in the prey composition of adult female harbour porpoises, in a sample of 119 porpoises by-
caught in the Swedish Kattegat and Skagerrak fisheries, were interpreted by Börjesson &
Berggren (1996) as indicating that habitat preferences of females could be “dictated by their
association with young calves”. The absence of milk in the stomachs of six calves by-caught
with their mothers in gill nets in the Bay of Fundy led Smith & Read (1992) to suggest
calves are unable to nurse while their mothers are actively foraging. It is also possible that,
rather than merely being a consequence of segregation, any differences in diet between the
sexes could be a mechanism to reduce competition.
Few differences were found between the diets of male and female porpoises in Scotland,
Denmark and Holland (Santos 1998). In Scotland, male porpoises ate more sepiolids and
had a higher overall prey diversity than females as well as there being some difference in
THE DIET OF HARBOUR PORPOISE IN THE NE ATLANTIC
367
prey size. In Denmark, female porpoises had significantly more prey in the stomach than
males. In Holland, female porpoises ate significantly more gobies than did male porpoises.
This higher prey diversity in Scottish male porpoises could indicate different feeding
grounds or less selectivity in the prey eaten. Female porpoises are significantly bigger and
heavier than males and the higher number of prey in females’ stomachs in Denmark could
reflect higher energetic needs. However, this result was not found for the Scottish and Dutch
porpoises. Aarefjord et al. (1995) found no significant differences in diet between seven
adult females and 48 adult males, although they recognised that the number of females
examined was very low. On the other hand, significant differences in the number of prey in
the stomach were found between male and female porpoises of 1-yr old or less, with males
eating more fish than females.
Outside the northeast Atlantic, results suggest an absence of dietary differences between
the sexes. No differences in the diet between sexes were found in a sample of 81 harbour
porpoises collected from eastern Canadian waters between 1969 and 1972 (Smith & Gaskin
1974) or in a sample of 100 harbour porpoises by-caught along the northern coast of Wash-
ington State (Gearin et al. 1994). Prey weight showed no significant differences between the
sexes in a sample of 138 harbour porpoises by-caught in the Gulf of St Lawrence, Canada,
in 1989 (Fontaine et al. 1994). Finally, no significant differences were seen in the diet of
males and non-lactating females in a sample of 95 harbour porpoises by-caught in the Gulf
of Maine (Gannon et al. 1998).
It should be noted that many studies do not distinguish between diets of lactating and
non-lactating females and it seems that differences between diets of males and females are
most likely to be seen when females are nursing calves.
Cause of death and diet variability
A basic problem with most recent studies on porpoise diets is that they are based on dead
animals. Differences in stomach contents from animals dying from different causes are diffi-
cult to relate to feeding strategies. Thus, if sick animals had a diet different from healthy
animals, does this difference indicate active changes in diet selection or simply a con-
sequence of reduced mobility leading to lower prey encounter rates? Certainly, the cause of
death represents a potential confounding factor and source of bias in dietary studies. If
nothing else, different components of the population are represented in proportion to the fre-
quency with which they die rather than their relative abundance in the living population. The
problems arising from the use of stranded specimens in dietary analysis have been exten-
sively discussed and reviewed elsewhere (e.g. Pierce & Boyle 1991, Sekiguchi et al. 1992).
Strandings of cetaceans can be considered an “opportunistic” resource, the composition of
which depends on many factors (wind and currents carrying the carcasses to the coast,
accessibility of coastal locations, state of preservation, etc.). With the increase in interac-
tions between marine mammals and fisheries, by-catches have become another source of
samples for dietary studies. In contrast to strandings, which could represent injured or ill
individuals, by-catches may provide samples of “healthy” animals (Kuiken et al. 1994a).
However, the use of by-caught individuals is not free of potential biases, notably the pos-
sible bias of the diet towards the target species of the fishery and associated species (Waring
et al. 1990) and the possible “net selection” of particular porpoise size and age classes. A
disproportionately high number of juvenile porpoises amongst by-catches was reported in
M.B. SANTOS & G.J. PIERCE
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Denmark (Clausen & Andersen 1988, Kinze 1994) and for the German fleet fishing in the
Baltic and the North Sea (Kock & Benke 1996). It has been suggested that avoidance of nets
could be related to experience, making young animals more vulnerable. Young animals
could try to explore and play with the nets and become entrapped and this fact could also put
females at risk if they try to rescue their calves (IWC 1994, Kinze 1994). Kinze also pointed
out that the existence of age-related segregation in harbour porpoises would make some
groups more vulnerable to by-catch. Gaskin & Blair (1977) found sub-adult males segregat-
ing from other groups in Canadian waters and staying closer to the coast and thus becoming
more frequently entrapped in nets.
A problem with identifying by-catches, as such, arises if by-caught animals are freed
from the nets (either by the fishermen or by other causes) and are found floating at sea or
stranded on the coast. The diagnosis of by-catch in these animals is at present a difficult
task. Carcasses are often too decomposed to allow any post-mortem study to be done and
some net types do not cause net marks on the skin – perhaps the clearest indication of by-
catch (Kuiken 1996). At present only a small percentage of cetaceans found stranded can be
diagnosed clearly as by-catches (Siebert et al. 1996).
In Scotland, harbour porpoises killed by bottlenose dolphins is another source of samples
available for analysis. However, these samples are also not free from potential biases. Ross
& Wilson (1996) observed that, although there were no significant differences in the number
of males and females killed, there was a bias towards porpoises of 100cm to 140cm, which
corresponded with juvenile or prepubertal animals between 1yr and 3yr of age. They also
noted that there is seasonal variation in the total number of strandings in the Moray Firth
area, with a peak value in June, and that the “injured porpoises represented a relatively con-
stant proportion of this number, such that within each month the number of injured por-
poises was significantly correlated with the number that died of other causes”.
In the northeast Atlantic, by-caught porpoises in Irish waters had eaten less clupeids and
whiting than stranded porpoises but both groups had a similar proportion of gadoids in the
diet (Rogan & Berrow 1996). However, the sample size in this study was small (nine and ten
animals, respectively).
Santos (1998) found significant differences in the diet between porpoises killed by bot-
tlenose dolphins and porpoises that died of other causes. Taking into consideration that the
seasonal distribution of the deaths caused by bottlenose dolphins was not homogeneous,
with more deaths occurring in the second and third quarters (spring and summer), the author
noted that it was not surprising that porpoises killed by dolphins had taken more sandeels
(since the importance of sandeels in the diet was also found to be significantly related to
season with sandeels eaten mainly in spring and summer). The differences in the importance
of the other species in the diet could be related not only to seasonal but also geographical
differences in abundance and/or availability. The sample of harbour porpoises killed by bot-
tlenose dolphins came mainly from the Moray Firth and the surrounding areas. A further
seven came from further south, near the Firth of Forth. In all cases harbour porpoises killed
by bottlenose dolphins came exclusively from the east coast of Scotland. Significant differ-
ences in the size of prey eaten, between porpoises killed by bottlenose dolphins and the
remaining porpoises, could also be explained by the seasonal distribution of the deaths
caused by dolphins. During the summer months, when more deaths take place, younger
sandeels and bigger whiting are taken by harbour porpoises in bigger numbers.
Differences in diet between by-caught and stranded animals have been found for other
cetacean species, for example, common dolphins, dusky dolphins (Lagenorhynchus obscu-
THE DIET OF HARBOUR PORPOISE IN THE NE ATLANTIC
369
rus) and Heaviside’s dolphins (Cephalorhynchus heavisidii) in South Africa (Sekiguchi et
al. 1992).
Where and how do porpoises feed?
Direct information on where porpoises feed comes from studies of distribution and diving
behaviour. Westgate et al. (1995) recorded diving behaviour of harbour porpoises in the Bay
of Fundy by attaching time-depth recorders. Typical dives were short and shallow (mean
duration 44s and mean depth 14m), although dives to depths of up to 226m (the maximum
water depth in the area) and durations up to 321s were recorded. Some evidence was found
that dives were less frequent, but deeper, at night. Most dives were characterised as “flat bot-
tomed”, with around one-third of the dive time being bottom time, which would be consis-
tent with foraging at the sea bed. Pierpoint et al. (1999) recorded porpoise echolocation
activity on the Welsh coast using acoustic data loggers and found that porpoise activity was
highest at night and during the ebb tide.
Goodson (1994), in the context of discussing by-catches in set gill nets, comments that
very little is known about the foraging strategies of harbour porpoises. However, records of
by-catches themselves provide evidence about where and how porpoises feed, some
information can be gleaned from records of surface observations, and relevant data are
emerging from recent studies on echolocation behaviour. There is little doubt that porpoises
often feed near the sea bottom, as indicated by several lines of evidence: the importance of
sandeels in the diet, the presence of sepiolids in the diet, the characteristics of the sonar
system and the fact that porpoises are often caught in bottom-set gill nets.
Surface observations
Harbour porpoises seem to be gregarious and schools consist normally of few animals (nine
in the Bay of Fundy, Gaskin et al. 1974) although aggregations of several hundred indi-
viduals have been reported in the literature (Fink 1959, Rae 1965). However, porpoises are
believed to hunt independently rather than in groups (Read 1999).
Pierpoint et al. (1994) observed porpoises in tidal races surfacing repeatedly at the same
location, always orientated so as to face into the tidal stream, which they interpreted as for-
aging activity. The presence of gulls scavenging at the water surface supports this interpreta-
tion and, although associations between individuals were temporary, groups of up to 10
porpoises were seen. Silva et al. (1999) observed porpoises from land on the Portuguese
coast during daylight hours. Numbers of sightings were highest at 09.00 and gradually
declined through the day. In relation to the tidal cycle, numbers were low at both low and
high tide. Maximum numbers were seen when tide height was 1m below the height at high
tide, but it is not stated whether this was during the ebb tide or flood tide.
Watson & Gaskin (1993) used surface observations to estimate dive times, which they
record as being between 35s and 4min for feeding porpoises in the Bay of Fundy.
M.B. SANTOS & G.J. PIERCE
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Echolocation in porpoises
Sturtivant et al. (1994) reviewed information on porpoise echolocation. Evans (1973) sug-
gested that sonar in porpoises is used mainly for target detection rather than target classifica-
tion owing to its monochromatic nature; Amundin et al. (1988) also noted that porpoise
clicks show a narrow-band width. Hatakeyama & Soeda (1990) recorded the clicks to be
mainly in the range 125–140kHz.
Prey searching involves the use of a narrow-beam, narrow-band high-frequency sonar
(with a peak frequency around 130kHz, Kastelein et al. 1999) believed by Goodson &
Sturtivant (1996) to have evolved for short-range foraging, particularly near to the sea
surface or the sea bottom. Kastelein et al. (1997a) also suggest that porpoise echolocation is
likely to be adapted for detecting prey on the sea bottom. Their experiments showed that
porpoises could detect steel and plastic discs buried up to 7cm deep in sand and the authors’
comment that similar objects should be detectable when buried at greater depths in a muddy
substratum because the substratum density would be closer to that of water.
The occurrence of very small prey such as bobtail squid (Sepiolidae, e.g. Sepiola
atlantica, the adults of which are no more than 2cm in length) naturally leads to questions
about how they are caught. Sepiolids are sit-and-wait predators which normally remain par-
tially buried in the substratum. They are probably detected by porpoises directing their sonar
into the substratum, and it is probably the acoustic signal from the hole rather than the
animal that allows them to be detected (D. Goodson, pers. comm.)
Inferences from dietary studies
Some further general inferences about feeding areas and the feeding niche can be made with
reference to the ecology of the prey species. A wide variety of prey has been found in the
stomachs of harbour porpoises (see previous sections), including pelagic, mesopelagic and
benthic species. Prey species found in the diet of harbour porpoises are mainly small school-
ing fish 400mm long, indeed in most cases 300mm (Read 1999). It has been suggested
that harbour porpoise teeth are used only to hold the prey but not to break it up into smaller
pieces and, therefore, porpoises are limited as to the size of prey they can consume.
In the northeast Atlantic, a mixture of mainly demersal species (whiting, cod, sandeels,
Trisopterus spp., gobies) has been cited as the main prey in most cases (Lick 1991a,b, Benke
& Siebert 1996, Martin 1996, Rogan & Berrow 1996, Santos 1998) although in some areas a
higher proportion of pelagic prey (mainly herring) has been recorded (e.g. Berggren 1996).
Aarefjord et al. (1995) found a predominance of pelagic species (herring, capelin) in the diet
of harbour porpoises by-caught in Norwegian waters, while benthic prey (cod, whiting,
sandeels, Pleuronectidae) predominated in the stomach contents of porpoises by-caught and
stranded in Sweden and Denmark. Outside the northeast Atlantic, pelagic species such as
herring, capelin, smelt (Smith & Gaskin 1974, Recchia & Read 1989, Fontaine et al. 1994,
Gannon et al. 1998) have been quoted as the main prey for harbour porpoises in the Bay of
Fundy, the Gulf of Maine and the Gulf of St Lawrence (eastern Canada). In the North
Pacific, clupeids (herring, capelin, sardine) were again recorded as the main prey for harbour
porpoises (Wilke & Kenyon 1952, Scheffer 1953, Fink 1959).
In Scottish waters, the two main prey types recorded in stomach contents during the
1990s were whiting and sandeels (Santos 1998) and it may be proposed that porpoises spend
THE DIET OF HARBOUR PORPOISE IN THE NE ATLANTIC
371
a substantial amount of time in areas frequented by these species. Whiting is a demersal
species living in shallow waters, usually from 39–200m over sandy or muddy grounds. It can
reach up to 70cm standard length, although normally the size is 30–40cm (Whitehead et al.
1989). Its distribution extends from northern Norway towards Iceland to the west and towards
the northern coasts of Portugal to the south. It is also present in the Mediterranean, Aegean,
Adriatic and Baltic Seas (Hislop 1972). Whiting mature at around 2yr of age and at a modal
length (for females) of 26cm. The spawning season is extended, beginning in February in the
southern North Sea and March in the northern North Sea and ending in June. Spawning nor-
mally takes place in waters less than 100m depth (Hislop 1984). During the first year of life,
whiting are found in shallow waters, concentrating in the central and southern North Sea and
in Scottish coastal waters. Most of the whiting taken by Scottish, Dutch and Danish harbour
porpoises in Santos (1998) were estimated to be 23cm in length and were therefore probably
younger than 2-yr old (Hislop 1984). Concentrations of 1-yr olds are found mainly in Scottish
coastal waters and in the central and southern North Sea (Hislop 1984), which would explain
the greater importance of whiting in Scottish and Dutch porpoise diets than in diets of por-
poises from more northern areas such as Norwegian Waters.
Sandeels are a group of demersal fishes which, in the northeast Atlantic, comprise three
main species with very similar otoliths, Ammodytes marinus, A. tobianus and Gymnam-
modytes semisquamatus. Two other species, Hyperoplus immaculatus and H. lanceolatus
(commonly called greater sandeels) attain a bigger size, which distinguishes them from the
common or lesser sandeels, and they are also less abundant. Sandeels receive their name
because of their unique way of life, spending the hours of darkness and most of the winter
buried in the sand (Macer 1966, Reay 1970, Langham 1971, Winslade 1974). Because of
this characteristic they are considered demersal, depending on a suitable bottom substratum
to burrow, but during their activity periods they lead a pelagic life (Storey 1993). Sandeels
feed on plankton and move throughout the water column during the day. Of the three main
species found in the Northeast Atlantic, Ammodytes marinus is the most common, also being
one of the commonest species on the continental shelf of northwest Europe and accounting
for 10–15% of the total fish biomass of the North Sea (Sparholt 1990).
Food consumption by harbour porpoises
Harbour porpoises have some unique characteristics among cetaceans. They are one of the
smallest cetaceans and most of their range is in cold waters. Their life history includes a
very short nursing period (usually less than 1yr), sexual maturity is attained at around 3yr of
age and there is a very short resting period between pregnancies (usually females give birth
each year), so that females are often pregnant and lactating at the same time (Read et al.
1997). Smeenk (pers. comm.) found that, in Dutch waters, most of the females do not give
birth every year, perhaps due to unfavourable food conditions or impaired health. Harbour
porpoise habitat and life history impose very high energetic demands. Furthermore, their
small size means that they cannot store much energy and this makes them more dependent
on a year-round proximity to food sources (Brodie 1995). According to Brodie, for harbour
porpoise this dependence has “the consequence that its distribution and nutritive condition
may more strongly reflect the distribution and energy density of its prey than for other
cetaceans”.
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Yasui & Gaskin (1986) estimated that the daily feeding rate of a non-lactating adult
harbour porpoise would be 3.5% of its total body weight per day, considerably lower than
values from previous studies based on food intake records from captive animals (e.g. 8.26%
quoted by Sergeant 1969, based on data from Andersen 1965). However, more recent
captive feeding studies tend to support the higher values. Kastelein et al. (1997b) recorded
food consumption in captivity for the species (based on six individuals) to be between 4%
and 9.5% of body weight. Lockyer et al. (2001) recorded food consumption to change sea-
sonally from 7% to 9.5% of body weight for two harbour porpoises in captivity.
Santos (1998) used the more conservative estimate of 3.5% and population size estimates
from Hammond et al. (1995) to calculate the amount of prey removed each year by harbour
porpoises in Scottish, Danish and Dutch waters. Her figures indicate that porpoises could be
removing significant amounts of several commercial fish species. For example, the estimated
consumption of whiting by porpoises surpasses the landings of this species for human con-
sumption in the North Sea. Thus, extrapolating from Scottish dietary data, harbour porpoises
off Scotland and the east coast of England (SCANS blocks C, D and J) could consume
around 14640t of whiting, 13800t of sandeels and 1000t of herring per year. Off the Danish
coast (SCANS blocks I and L), harbour porpoises could eat around 2880t of herring, 6660t
of cod and 6230t of viviparous blenny, while off the Dutch coast and west coast of Germany
(SCANS blocks H and Y) porpoises could eat around 1800t of whiting, 650t of cod and
300t of sandeels (assuming porpoises off the east coast of Germany to have a diet similar to
the combined diet of Danish and Dutch porpoises). Finally, using combined dietary data for
Scotland, Denmark and Holland, harbour porpoises in the central North Sea could eat
around 3900t of herring, 33400 t of whiting and 14000 t of sandeels.
It should be noted that confidence limits on all these estimates are wide: regardless of
accuracy, the precision available is low, reflecting the level of uncertainty associated with
sampling error, the regressions used to estimate size of fish prey eaten, the population size
and the energy requirements (Santos 1998, see Santos et al. 2001b for a similar calculation
for sperm whales).
Competition with other predators
Comparing the harbour porpoise with other predators in the northeast Atlantic, the predator
with the most similar average body size is the common (or harbour) seal, which has a range
in weight in UK waters of around 45–106kg in females and 55–130kg in males (Corbet &
Harris 1991). Several studies have been carried out on the diet of common seals in the
Moray Firth (e.g. Pierce et al. 1989, 1991a,b, Thompson et al. 1991, Tollit & Thompson
1996). In general, common seal diet is dominated by sandeels in summer and other species
such as gadoids (whiting, cod) and clupeids (herring, sprat) in winter. Thus, in recent
studies, diets of harbour porpoise and common seals are seen to follow a similar pattern,
which is consistent with both types of predators exploiting the same locally abundant
resources. Tollit & Thompson (1996) also found considerable interannual variation in the
diet of common seals in the Moray Firth between 1989 and 1992. In the Skagerrak and Kat-
tegat (May–September 1988), Härkönen (1988) found that the most important species in
common seal diet were cod, plaice, dab, lemon sole Microstomus kitt and sandeels. Härkö-
THE DIET OF HARBOUR PORPOISE IN THE NE ATLANTIC
373
nen & Heide-Jørgensen (1991) reported the main prey of common seals in the same area in
July–December 1989 to be mainly gadoids, particularly cod. Cod was also the most import-
ant species by weight in Danish harbour porpoise diets (Santos 1998), although viviparous
blennies were the second most important category and very few flatfishes were eaten.
Grey seals in the Moray Firth have a similar summer diet to common seals in the same
area (i.e. predominantly sandeels (Pierce et al. 1991a)). Studies elsewhere in Scotland have
shown that other types of fish, particularly gadoids, dominate the winter diet (e.g. Hammond
& Prime 1990, Pierce et al. 1990, Hammond et al. 1994). Again, broadly speaking, there are
similarities with harbour porpoise diet.
There are few data on bottlenose dolphin diet in Scottish waters, but cod, saithe and
whiting were the main prey in 10 stomachs (eight from the Moray Firth) examined by
Santos et al. (2001c). It has been speculated that bottlenose dolphins kill porpoises as a
result of food competition (Ross & Wilson 1996) but clearly more data are needed to test
this hypothesis.
Apart from parallels with seals, the high importance of sandeel in harbour porpoise diets
is shared with a large range of other predators, for example, whiting and cod (Daan 1989),
pleuronectids and salmonids (Storey 1993), Arctic tern Sterna paradisaea (Monaghan et al.
1989), puffins Fratercula arctica, guillemots Uria aalge, razorbills Alca torda (Harris &
Riddiford 1989), great skuas Catharacta skua (Furness & Hislop 1981), Arctic skuas Sterco-
rarius parasiticus, kittiwakes Rissa tridactyla (Furness 1987), shags Phalacrocorax aris-
totelis (Harris & Wanless 1991), minke whales Balaenoptera acutorostrata (Jonsgåard
1982) and humpback whales Megaptera novaengliae (Perkins et al. 1982).
Interactions with fisheries
Interactions of marine mammals with fisheries are of two general types, operational and bio-
logical (Harwood & Greenwood 1985). The former include fishery by-catch of porpoises
and the latter include predation by porpoises on fished species. Both types of interaction
arguably reflect diet choice, the latter more directly.
The results of most of the studies on harbour porpoise diets in the northeast Atlantic show
an overlap between the fish species consumed by porpoises and those targeted by fisheries.
This potential for competition was already noted at the beginning of last century in the North
Sea when direct observations of porpoises reputedly “chasing salmon” or “playing with
salmon” led fishermen and naturalists to believe in the possibility of porpoises competing
with local salmon fisheries (e.g. Macintyre 1934, Berry 1935). Concern over the status of the
Baltic salmon stock led Svärdson (1955) to propose:
The relation between porpoise and salmon can be and ought to be tested by an experi-
ment. The porpoise-hunting tradition . .. must be revived and as many of the migrating
porpoises as possible caught for some years, so as to see what happens to the salmon in
the Baltic. If the relation is once again positive a method has been found for conserving
in the Baltic a permanent salmon population more abundant than the present one.
In fact, studies from this period found no evidence of predation on salmon in porpoise
stomach contents (Rae 1965, 1973, Lindroth 1962).
M.B. SANTOS & G.J. PIERCE
374
The North Sea and adjacent areas (waters west of Scotland and the Skagerrak/Kattegat
area) have a long history of fishery exploitation. The types of fishery include pelagic and
demersal fisheries for human consumption, and industrial fisheries (where the catch is used
for reduction purposes). The pelagic fishery mainly targets herring, mackerel and horse
mackerel, while the demersal fisheries usually catch a mixture of roundfish species (e.g. cod,
haddock, whiting) and/or a mixture of flatfish species (plaice Pleuronectes platessa and sole)
with a by-catch of roundfish. The industrial fishery mainly takes sandeels, Norway pout and
sprat, although catches also include herring, haddock and whiting (Anonymous 2002a).
Whiting is the third most important species of commercial demersal fish in the North Sea.
Catches of this species in the North Sea increased during the 1950s and 1960s and reached a
maximum in 1969 with 200000t. After this date maximum landings started to decline and
reached an historical low in 1998. At present the stock is considered to be outside safe bio-
logical limits (Anonymous 2002a). In recent years, a significant part of the whiting landed is
taken as by-catch in the industrial fishery, mainly for Norway pout, and in addition large
quantities of whiting are being discarded in favour of higher priced species.
The sandeel Ammodytes marinus supports the largest single-species fishery in the North
Sea. Its distribution extends in the eastern North Atlantic from 74ºN to 49ºN (Channel
Islands and western English Channel), including eastern Greenland, Barents Sea and the
Baltic Sea (Whitehead et al. 1989). Sandeels are used for bait and food on a small scale in
many areas, but the major fisheries are for the production of fishmeal with between
600000–1100000t being taken from the North Sea each year (mainly by Denmark)
(Anonymous 2002a).
Santos (1998) estimated that harbour porpoises take more whiting than are landed by
fisheries in the North Sea, although the whiting taken by porpoises were mainly smaller than
those targeted by the fishery. In Scotland, just over 99% of the whiting consumed were
below the minimum landing size established for the species (27cm), compared with 99.5%
for Denmark and 70% for Holland. However, sizes of fishes may be underestimated from
measurements on otoliths because no correction was applied for otolith erosion. Wijnsma et
al. (1999) carried out in vitro digestions of fish otoliths to estimate consequences of otoliths
erosion for dietary studies on porpoises and suggested that the overall picture of diet compo-
sition for porpoises in Scottish waters was relatively robust to such errors.
Discards of smaller whiting and other species in the North Sea are at present a cause for
concern because the amount of whiting discarded is estimated to be equivalent to 60% of the
amount landed (Anonymous 2002a) and discards are similarly high for haddock. Discards
are known to be eaten by seabirds (Hudson & Furness 1989, Berghahn & Rösner 1992,
Furness et al. 1992) and it is interesting to speculate whether harbour porpoises might also
take advantage of this resource. Some evidence of feeding on discards exists for other
cetaceans, for example, killer whales (Couperus 1994).
Long-term trends in porpoise diet
Harbour porpoises in the northeast Atlantic may already have switched prey species follow-
ing the decline in herring stocks to a diet based on sandeels, whiting and other gadoid
species. The studies by Rae (1965, 1973) on harbour porpoise diet in Scotland between 1959
and 1971 showed clupeids (herring and sprat) to be the most frequent prey. Gadoids (mainly
THE DIET OF HARBOUR PORPOISE IN THE NE ATLANTIC
375
whiting) were found to be second in importance in the diet, while sandeels represented a
minor proportion (being identified in less than 8% of the stomachs). Although the fact that
most of the animals were obtained during the winter months could explain the lack of
sandeels in the diet, the importance of herring in the stomachs of harbour porpoises analysed
by Rae is clearly greater than in the diet of harbour porpoises in more recent studies.
The decline in herring stocks in the North Sea between the 1950s and 1970s is well docu-
mented (Cushing & Burd 1957, Burd 1978) and has been proposed as one of several hypoth-
esis (together with organochlorine pollution, noise pollution produced by increasing boat
traffic and by-catches in fishing gear) to explain the apparent decline in numbers of harbour
porpoises in the North Sea (e.g. Verwey 1975, Otterlind 1976, Van Bree 1977, Andersen &
Clausen 1983, Gaskin 1984, Kayes 1985, Smeenk 1987, IWC 1994, Kleivane et al. 1995).
The collapse of the herring stocks did indeed coincide with the apparent decline in harbour
porpoise numbers in the southern North Sea. However, herring was also overfished on the
Scottish coasts but there is no evidence of a parallel decline in porpoise numbers (Evans
1980). Furthermore, an earlier disappearance of Zuiderzee herring (a brackish water popu-
lation) in Dutch waters during the 1930s, was not accompanied by a decline in harbour por-
poise numbers in the area (Smeenk 1987). On the other hand, Camphuysen (1994) noted that
an increase in herring stocks in recent years had been followed by a slight increase in sight-
ings of harbour porpoises in the southeast North Sea.
There has been speculation about the likelihood and consequences of porpoises switching
to other prey species if their main prey were depleted by overfishing, because many of the
prey species eaten by harbour porpoise are also commercially exploited (e.g. herring, sprat,
sardine, cod, whiting, sole, sandeels) (IWC 1996).
Fish stocks have shown considerable variation in abundance and distribution in the past,
some of which has been the result of over-exploitation. Well known cases include the
collapse of the North Sea herring stocks (Burd 1978, Corten 1990) and the massive
decrease of North Sea mackerel population in the early 1970s (Cushing 1980). Not only
have pelagic species shown large fluctuations in abundance, landings from the Shetland
sandeel fishery fell during the 1980s and a parallel decline in seabird breeding success fol-
lowed (Monaghan et al. 1989). Changes in the structure of fish communities due to fishery
exploitation have already been described for some areas (e.g. Celtic Sea, Pinnegar et al.
2002).
At present, many stocks in the North Sea are outside or close to safe biological limits,
with high fishing mortality that is believed to be unsustainable in the longer term and spawn-
ing stock biomasses below safe levels or declining towards critical levels. Over-exploitation
of herring for the human consumption fishery, together with considerable by-catches of
juveniles in the industrial fishery in the North Sea and Skagerrak/Kattegat area, caused a
rapid decline of the stock and in the 1990s emergency regulations were introduced to reduce
fishing mortality. The stock is at present believed to be inside safe biological limits (Anony-
mous 2002b) – although reported to be outside safe biological limits in 2001 (Anonymous
2001) – but stock rebuilding has been delayed by too optimistic assessments and misreport-
ing. The state of the sprat stock is not well known with large natural fluctuations in annual
stock biomass (Anonymous 2001). The North Sea component of the mackerel stock is still
severely depleted and considered to be in need of maximum protection (Anonymous 2002c).
For the species taken in the demersal fishery for human consumption, the stock of cod is
considered to be outside safe biological limits and there is concern that, if the rate of fishing
continues, the stock will collapse. For both haddock and whiting the North Sea stocks are
M.B. SANTOS & G.J. PIERCE
376
also considered to be outside safe biological limits, with spawning stock biomass reaching
an historical low in 1998 for whiting. The status of saithe stocks in the North Sea (including
the Skagerrak area) and the West of Scotland is also causing concern, having fluctuated
around safe biological limits in recent years. Of the flatfish species, plaice and North Sea
sole are considered to be outside safe biological limits with the historical minimum of
spawning stock biomass recorded in 1997 for plaice and in 1998 for sole (Anonymous
2002a).
Finally, for the industrial fishery, both Norway pout and sandeel stocks are considered to
be inside safe biological limits but recruitment for both species appear to be highly variable
and can influence the abundance of the species rapidly due to the short life-span (Anony-
mous 2002a).
Consequences of diet for individual health and population status
Possible causes for the decline of harbour porpoises in most European waters noted by
Smeenk (1987) included the decline in herring stocks and other factors such as organochlo-
rine pollution. Other threats to populations include fishery by-catch, habitat degradation
through pollution, disturbance by ship traffic and boats and coastal development (Read et al.
1997). Of these threats, by-catch is arguably the most serious, the fishery by-catch of
harbour porpoises in northeast Atlantic waters being regarded as unsustainable by the IWC
(IWC 1995).
As argued above, harbour porpoises in Scottish waters have apparently already switched
prey species following the decline in herring stocks to a diet based on sandeel, whiting and
other gadoid species. Switching from a prey with high calorific value such as herring (Murray
& Burt 1977) to one with lower calorific density could have long-term effects on survivorship
and productivity. Short-term effects have been described by Dudok van Heel (1962) who
observed that captive porpoises fed on young cod lost weight but this weight loss was halted
when the diet was changed to the same amount of herring. Evidence of physiological changes
in harbour seals, related to changes in diet composition, was found by Thompson et al.
(1997). Analysing haematological parameters of a harbour seal population, they found that in
years when herring and sprat dominated the diet, leukocyte counts were significantly higher
than in years when alternative prey dominated the diet. Evidence of widespread macrocytic
anaemia was also found in years when an alternative prey dominated the diet.
Diet is the route of entry of persistent organic pollutants and toxic elements. Of all the
different types of organochlorine compounds used, only two groups (especially resistant to
biodegradation) have entered the marine food chain in high concentrations and are present
in marine mammal tissues. These two groups are the DDTs (dichloro-diphenyl-
trichloroethanes, used as pesticides in agriculture until late 1970s) and PCBs (polychlori-
nated biphenyls, used mainly in the electricity industry) (Aguilar & Borrell 1995).
Aguilar & Borrell (1995) found that, although levels of DDT (and other contaminants
such as heavy metals) in tissues were low, levels of PCBs in harbour porpoises from the
eastern North Atlantic were high enough to cause concern about their possible effects on the
population. Organochlorine compounds are thought to depress reproductive performance
(Subramanian et al. 1987, Addison 1989) and the immune system (Wassermann et al. 1979,
Brouwer et al. 1989, Vos & Luster 1989, Swart et al. 1994, Ross et al. 1996) and have been
THE DIET OF HARBOUR PORPOISE IN THE NE ATLANTIC
377
shown in experimental studies to adversely affect mammalian reproduction (e.g. Merson &
Kirkpatrick 1976, Fuller & Hobson 1986, Reijnders 1986, Boon et al. 1987, Gray et al.
1998). The harbour porpoise was the first cetacean analysed for these compounds (Holden &
Marsden 1967).
Harbour porpoise, with its coastal habitat, its position at the top of the food chain and its
small body size (and high metabolic rate) could be especially affected by these pollutants.
Moreover, it has been suggested that the capacity of small cetaceans to metabolise certain
PCB congeners is very low compared with that of birds and terrestrial mammals (Tanabe et
al. 1988) and is possibly lower in harbour porpoises than in other odontocetes (Duinker et al.
1989). Jepson et al. (1999) found a significant association between elevated blubber chloro-
biphenyl concentrations and mortality due to infectious diseases in harbour porpoises from
England and Wales stranded and by-caught between 1990 and 1996. However, Kuiken and
co-authors found blubber chlorobiphenyl concentrations to be unrelated to adrenocortical
hyperplasia in porpoises stranded and by-caught in 1990 and 1991 (Kuiken et al. 1993) or
mortality due to infectious diseases in porpoises stranded and by-caught between 1989 and
1992 (Kuiken et al. 1994b). At present, while the proximate origin of PCBs in porpoise
blubber is clearly to be found in the prey species, it is not clear whether particular prey
species (or marine habitats) are responsible for this transfer. However, it may be noted that
in general the pattern of abundance of different PCB congeners in marine mammal blubber
differs markedly between fish eaters (such as the harbour porpoise) and cephalopod eaters
(Wells & Mckenzie 1994).
The coastal distribution of harbour porpoises makes them vulnerable to high levels of
incidental fishery mortality, particularly in bottom-set gill nets but also in other fishing gear,
e.g. salmon drift nets, pound nets for herring and salmon and mid-water trawls (e.g. Lindroth
1962, van Utrech 1978, Kinze 1990a, Northridge & Lankester 1992, Lowry & Teilmann
1994, Read 1999). Unlike dolphins, harbour porpoises seem likely to become entangled
when the nets are on the sea bottom (Read & Gaskin 1988). However, it is unclear whether
porpoises become entangled because they attempt to take fishes already caught in the nets or
if they are simply foraging in the area where nets are set and fail to detect the nets (see Tre-
genza et al. 1997). One possibility is that, when porpoises are echolocating fishes buried in
the substratum, objects in the water column (such as nets) are not detected.
Future research
Most recent results on porpoise diet derive from examination of stomach contents of
stranded animals. Inevitably this leads to an incomplete and potentially biased view of diet,
and makes it difficult to partition variation reliably. It is clear that there are regional, sea-
sonal, sex- and size-related differences in diet and there may well be individual differences
in food preferences. However, stomach contents of dead animals provide only a single snap-
shot of diet, with no possibility of repeated samples from the same animal. Furthermore, the
most recent meal is not necessarily representative of the typical diet, especially if the animal
was weakened by disease, but also if it was by-caught in a fishing net as a result of pursuing
a particular type of fish. Last and by no means least, stomach contents analysis is compli-
cated by the digestive erosion of prey tissues and hard parts (see review by Pierce & Boyle
1991).
M.B. SANTOS & G.J. PIERCE
378
In other marine mammals, notably seals (e.g. Iverson et al. 1997), analysis of fatty acid
composition of the blubber has allowed inferences to be made about the average diet compo-
sition of individuals over extended periods, both overcoming some of the biases of stomach
contents analysis and allowing good data to be collected from animals with empty stomachs.
There are both logistical and technical obstacles to be overcome before this method is
used routinely on porpoises. Quantitative interpretation of fatty acid profiles requires exten-
sive libraries of the fatty acid profiles of putative prey species. Calculating the most likely
diet composition requires considerable computing power: the higher the number of possible
prey species and the more fatty acids that are taken into account, the more possible combina-
tions of different proportions of prey types need to be screened. Finding the relative import-
ance of different prey in the diet is equivalent to solving a series of simultaneous equations,
with data on each fatty acid expressed as a separate equation, each of which has a term for
each prey species. For example, for three fatty acids and two prey species:
I1Ca,1 I2Ca,2 Ca,obs
I1Cb,1 I2Cb,2 Cb,obs
where: Iyimportance of prey type yin the diet
Cx,y concentration of fatty acidxin the body of prey species y
Cx,obs concentration of fatty acidxin porpoise blubber
In these equations, all the Cterms are known and it is necessary to find the set of values for I
values. Further complications are provided by variation in fatty acid profiles within prey
species (e.g. in relation to the prey life cycle and reproductive cycle or variation in the prey
species’ own diet) and differences between the overall dietary fatty acid profile and that of
the blubber (e.g. due to variation in assimilation, metabolism and de novo synthesis in differ-
ent fatty acids). Given variation in fatty acid profiles of individual prey species, the set of
simultaneous equations is unlikely to have an exact solution. The most probable solution
could be calculated using computer simulations incorporating known variability in prey fatty
acid profiles. The only realistic way of completely overcoming the latter problem would be
to derive correction factors based on captive feeding experiments involving animals on con-
trolled diets. However, while relatively straightforward for seals, this is unlikely to be feasi-
ble for porpoises, nor would it be ethically acceptable in some countries.
To follow the diet of an individual porpoise over its lifetime, repeated biopsy samples of
blubber could be taken. Individual identification could be confirmed from DNA analysis of
the tissue sample. However, fatty acids of recent dietary origin are concentrated in the lower
blubber, so that complete blubber cores would be needed, potentially providing a route for
infection and again raising ethical questions. Regardless of whether fatty acid analysis could
or should be extended to studies on live animals, it offers the most likely method for obtain-
ing good dietary data on this species.
For some large cetaceans, faecal analysis has proved to be viable, collecting material
from behind a swimming animal using a net. It seems unlikely that this would be success-
fully for an animal as small (and generally shy of human contact) as a porpoise. Attachment
of cameras (“crittercams”) has also been successful for pinnipeds and larger cetaceans and,
THE DIET OF HARBOUR PORPOISE IN THE NE ATLANTIC
379
given a sufficiently small package, it should be possible to obtain a porpoise-eye view of its
feeding activity. Other kinds of recording devices (e.g. time-depth recorders) have already
been successfully attached to porpoises (e.g. Westgate et al. 1995).
Major uncertainties about the ecological importance – and indeed the status – of porpoise
populations remain due to the lack of good data on population size, especially in European
waters. In the North Sea, the SCANS survey – based on a single month’s data collection in
1994 (Hammond et al. 1995) – still provides the most comprehensive picture of porpoise
distribution and abundance. Good data on individual energy requirements are also required
and could greatly affect present calculations of food consumption.
Acknowledgements
MBS was supported by CEC Project No EVK3-CT-2000-00027 (BIOCET).
References
Aarefjord, H., Bjørge, A., Kinze, C.C. & Lindstedt, I. 1995. Diet of the harbour porpoise, (Phocoena
phocoena), in Scandinavian waters. In Special Issue, 16: Biology of phocoenids, A. Bjørge & G.P.
Donovan (eds). Cambridge: International Whaling Commission, 211–222.
Ackman, R.G. & Lamothe, F. 1989. Marine mammals. In Marine biogenic lipids, fats and oils. Vol. 2:
Distribution of marine lipids, R.G. Ackmann (ed.). Boca Raton: CRC Press, 179–381.
Addison, R.F. 1989. Organochlorines and marine mammal reproduction. Canadian Journal of Fish-
eries and Aquatic Sciences 46, 360–368.
Aguilar, A. & Borrell, A. 1995. Pollution and harbour porpoises in the eastern North Atlantic: a
review. In Special Issue, 16: Biology of phocoenids, A. Bjørge & G.P. Donovan (eds). Cambridge:
International Whaling Commission, 231–242.
Amano, M. & Miyazaki, N. 1992. Geographic variation in skulls of the harbour porpoise, Phocoena
phocoena. Mammalia 56, 133–144.
Amundin, M., Kallin, E. & Kallin, S. 1988. The study of the sound production apparatus in the harbour
porpoise, Phocoena phocoena, and the jacobita, Cephalorhynchus commersoni, by means of serial
cryo-microtome sectioning and 3-D computer graphics. In Animal sonar: processes and perform-
ance, P. Nachtigall & P.W.G. Moore (eds). New York: Plenum Press, 61–66.
Andersen, L.W. 1993. The population structure of the harbour porpoise, Phocoena phocoena, in
Danish waters and part of the North Atlantic. Marine Biology 116, 1–7.
Andersen, S. 1965. L’alimentation du marsouin (Phocoena phocoena L.) en captivité. Vie Milieu,
Series A, 16, 799–810.
Andersen, S. & Clausen, B. 1983. Bycatches of the harbour porpoise, Phocoena phocoena, in Danish
fisheries 1980–1981, and evidence for overexploitation. International Whaling Commission,
SC/35/SM14.
Anonymous 2001. Report of the herring assessment working group for the area South of 62°N. Inter-
national Council for the Exploration of the Sea, C.M. 2001/ACFM: 12, Copenhagen, Denmark.
Anonymous 2002a. Report of the working group on the assessment of demersal stocks in the North
Sea and Skagerrak. International Council for the Exploration of the Sea, C.M. 2002/ACFM: 1,
Copenhagen, Denmark.
Anonymous 2002b. Report of the herring assessment working group for the area South of 62°N. Inter-
national Council for the Exploration of the Sea, C.M. 2002/ACFM: 12, Copenhagen, Denmark.
M.B. SANTOS & G.J. PIERCE
380
Anonymous 2002c. Report of the working group on the assessment of mackerel, horse mackerel,
sardine and anchovy. International Council for the Exploration of the Sea, C.M. 2002/ACFM: 6,
Copenhagen, Denmark.
ASCOBANS 1994. Agreement on the conservation of small cetaceans of the Baltic and North Seas.
Report from the First Meeting of the Parties, Stockholm, 26–28 September 1994. ASCOBANS Sec-
retariat, Cambridge, UK.
Baird, R.W. & Hooker, S.K. 2000. Ingestion of plastic and unusual prey by a juvenile harbour por-
poise. Marine Pollution Bulletin 40, 719–720.
Benke, H. & Siebert, U. 1996. The current status of harbour porpoises (Phocoena phocoena) in
German waters. International Whaling Commission, SC/47/SM49, Cambridge, UK.
Berggren, P. 1996. A preliminary assessment of the status of harbour porpoises (Phocoena phocoena)
in the Swedish Skagerrak, Kattegat and Baltic Seas. International Whaling Commission,
SC/47/SM50, Cambridge, UK.
Berghahn, R. & Rösner, H.-U. 1992. A method to quantify feeding of seabirds on discard from the
shrimp fishery in the North Sea. Netherlands Journal of Sea Research 28, 347–350.
Berrow, S.D., Long, S.C., McGarry, A.T., Pollard, D., Rogan, E. & Lockyer, C. 1998. Radionuclides
(137Cs and 40K) in harbour porpoise Phocoena phocoena from British and Irish coastal waters.
Marine Pollution Bulletin 36, 569–576.
Berry, J. 1935. British mammals and birds as enemies of the Atlantic salmon (Salmo salar). Reports of
the Avon Biological Research 1934–35, 31–64.
Bjørge, A. & Donovan, G.P. (eds) 1995. Special Issue, 16: Biology of phocoenids. Cambridge: Inter-
national Whaling Commission, 552 pp.
Bjørge, A. & Øien, N. 1995. Distribution and abundance of harbour porpoise, Phocoena phocoena, in
Norwegian waters. In Special Issue, 16: Biology of phocoenids, A. Bjørge & G.P. Donovan (eds).
Cambridge: International Whaling Commission, 89–98.
Boon, J.P., Reijnders, P.J.H., Dols, J., Wensvoort, P. & Hillebrand, M.T. J. 1987. The kinetics of indi-
vidual polychlorinated biphenyl congeners in female harbour seals (Phoca vitulina), with evidence
for structure-related metabolism. Aquatic Toxicology 10, 307–324.
Börjesson, P. & Berggren, P. 1996. Seasonal variation in diet of harbour porpoises (Phocoena pho-
coena) from the Kattegat and Skagerrak Seas. In European research on cetaceans – 10, P.G.H.
Evans (ed.). Cambridge: European Cetacean Society, 261 only.
Börjesson, P. & Berggren, P. 1997. Morphometric comparisons of skulls of harbour porpoises (Phocoena
phocoena) from the Baltic, Kattegat, and Skagerrak seas. Canadian Journal of Zoology 75, 280–287.
Brodie, P.F. 1995. The Bay of Fundy/Gulf or Maine harbour porpoise (Phocoena phocoena): some
considerations regarding species interactions, energetics, density dependence and bycatch. In
Special Issue, 16: Biology of phocoenids, A. Bjørge & G.P. Donovan (eds). Cambridge: Inter-
national Whaling Commission, 181–187.
Brouwer, A., Reijnders, P.J.H. & Koeman, J.H. 1989. Polychlorinated biphenyl (PCB)-contaminated
fish induces vitamin A and thyroid hormone deficiency in the common seal (Phoca vitulina).
Aquatic Toxicology 15, 99–106.
Burd, A.C. 1978. Long-term changes in North Sea herring stocks. Rapports et Procès-verbaux des
Réunions, Conseil International pour l’Exploration de la Mer 172, 137–153.
Camphuysen, C.J. 1994. The harbour porpoise, Phocoena phocoena in the southern North Sea. II: A
come-back in Dutch coastal waters? Lutra 37, 54–61.
Camphuysen, C.J. & Leopold, M.F. 1993. The harbour porpoise, Phocoena phocoena in the southern
North Sea, particularly the Dutch sector. Lutra 36, 1–24.
Clausen, B. & Andersen, S. 1988. Evaluation of bycatch and health status of the harbour porpoise
(Phocoena phocoena) in Danish waters. Danish Review of Game Biology 13, 1–20.
Corbet, G.B. & Harris, S. (eds) 1991. The handbook of British mammals. London: Blackwell, 3rd edn.
Corten, A. 1990. Long-term trends in pelagic fish stocks of the North Sea and adjacent waters and their
possible connection to hydrographic changes. Netherlands Journal of Sea Research 25, 227–235.
THE DIET OF HARBOUR PORPOISE IN THE NE ATLANTIC
381
Couperus, A.S. 1994. Killer whales (Orcinus orca) scavenging on discards of freezer trawlers north
east of the Shetland islands. Aquatic Mammals 20, 47–51.
Cushing, D.H. 1980. The decline of the herring stocks and the gadoid outburst. Journal du Conseil
pour l’Exploration Internationale de la Mer 39, 70–81.
Cushing, D.H. & Burd, A.C. 1957. On the herring of the southern North Sea. Fishery Investigations,
London, Series 2 20, 1–31.
Daan, N. (ed.) 1989. Data base report of the stomach sampling project 1981. ICES Cooperative
Research Report, No. 164, Copenhagen, Denmark.
Darling, F.F. 1947. Natural history in the Highlands and Islands. London: Collins.
Desportes, G. 1985. La nutrition des odontocetes en Atlantique nord-est (cotes Francaises – iles
Feroe). PhD thesis, Université de Poitiers, France.
Dewhurst, H.W. 1834. The natural history of the order Cetacea. London: Dewhurst.
Donovan, G.P. & Bjørge, A. 1995. Harbour porpoises in the North Atlantic: edited extract from the
Report of the IWC Scientific Committee, Dublin 1995. In Special Issue, 16: Biology of phocoenids,
A. Bjørge & G.P. Donovan (eds). Cambridge: International Whaling Commission, 3–25.
Dudok van Heel, W.H. 1962. Experiments with Phocoena phocoena L. Netherlands Journal of Sea
Research 1, 427–458.
Duinker, J.C., Hillebrand, M.T.J., Zeinstra, T. & Boon, J.P. 1989. Individual chlorinated biphenyls
and pesticides in tissues of some cetaceans species from the North Sea and the Atlantic Ocean;
tissue distribution and biotransformation. Aquatic Mammals 15, 95–124.
Dunnet, G.M. 1996. Aquatic predators and their prey: the take-home messages. In Aquatic predators
and their prey, S.P.R. Greenstreet & M.L. Tasker (eds). Oxford: Fishing News Books, Blackwell
Science, 184–186.
Evans, P.G.H. 1980. Cetaceans in British waters. Mammal Review 10, 1–52.
Evans, P.G.H. & Borges, L. 1995. Associations between porpoises, seabirds, and their prey in south-
east Shetland, N. Scotland. In European research on cetaceans – 9, P.G.H. Evans (ed.). Cam-
bridge: European Cetacean Society, 173–178.
Evans, W.E. 1973. Echolocation by marine delphinids and one species of freshwater dolphin. Journal
of the Acoustical Society of America 54, 191–199.
Evans, W.E. 1975. Distribution, differentiation of populations, and other aspects of the natural history
of Delphinus delphis Linnaeus in the northeast Pacific Ocean. PhD thesis, University of California,
Los Angeles, California.
Fink, B.D. 1959. Observations of porpoise predation on a school of Pacific sardines. California Fish
and Game 45, 216–217.
Fontaine, P.M., Hammill, P.O., Barrette, C. & Kingsley, M.C. 1994. Summer diet of the harbour por-
poise (Phocoena phocoena) in the estuary and the northern Gulf of St Lawrence. Canadian Journal
of Fisheries and Aquatic Sciences 51, 172–178.
Fraser, F.C. 1946. Report on Cetacea stranded on the British coasts from 1933 to 1937. British
Museum (Natural History) No. 12, London.
Fuller, G.B. & Hobson, W.C. 1986. Effects of PCBs on reproduction in mammals. In Vol. 2: PCBs
and the environment. Florida: CRC Press, 101–125.
Furness, R.W. 1987. The skuas. Calton: T. & A.D. Poyser.
Furness, R.W., Ensor, K. & Hudson, A.V. 1992. The use of fishery waste by gull populations around
the British Isles. Ardea 80, 105–113.
Furness, R.W. & Hislop, J.R.G. 1981. Diets and feeding ecology of great skuas Catharacta skua
during the breeding season in Shetland. Journal of Zoology, London 195, 1–23.
Gannon, D.P., Craddock, J.E. & Read, A.J. 1998. Autumn food habits of harbour porpoises, Pho-
coena phocoena, in the Gulf of Maine. Fishery Bulletin 96, 428–437.
Gaskin, D.E. 1977. Harbour porpoise Phocoena phocoena (L.) in the western approaches to the Bay of
Fundy 1969–75. Reports of the International Whaling Commission 27, 487–492.
Gaskin, D.E. 1984. The harbour porpoise, Phocoena phocoena (L.): regional populations, status, and
M.B. SANTOS & G.J. PIERCE
382
information on direct and indirect catches. Reports of the International Whaling Commission 34,
569–586.
Gaskin, D.E., Arnold, P.W. & Blair, B.A. 1974. Phocoena phocoena. Mammalian Species 42, 1–8.
Gaskin, D.E. & Blair, B.A. 1977. Age determination of harbour porpoise, Phocoena phocoena (L.), in
the western North Atlantic. Canadian Journal of Zoology 55, 18–30.
Gaskin, D.E. & Watson, A.P. 1985. The harbor porpoise, Phocoena phocoena, in Fish Harbor, New
Brunswick, Canada: occupancy, distribution, and movements. Fishery Bulletin 83, 427–442.
Gaskin, D.E., Yumamoto, S. & Kawamura, A. 1993. Harbour porpoise, Phocoena phocoena (L.), in
the coastal waters of northern Japan. Fishery Bulletin 91, 440–454.
Gearin, P.J., Melin, S.R., DeLong, R.L., Kajimura, H. & Johnson, M.A. 1994. Harbour porpoise
interactions with a Chinook salmon set-net fishery in Washington State. In Special Issue, 15: Gill-
nets and cetaceans, W.F. Perrin et al. (eds). Cambridge: International Whaling Commission,
427–438.
Goodson, A.D. 1994. Bottom-set gillnets: problems of perception for dolphins and porpoises. In Euro-
pean research on cetaceans – 8, P.G.H. Evans (ed.). Cambridge: European Cetacean Society,
54–57.
Goodson, A.D. & Sturtivant, C.R. 1996. Sonar characteristics of the harbour porpoise (Phocoena pho-
coena): source levels and spectrum. ICES Journal of Marine Science 53, 465–472.
Gray, L.E., Ostby, J., Wolf, C., Lambright, C. & Kelce, W.R. 1998. The value of mechanistic studies
in laboratory animals for the prediction of reproductive effects in wild life. Environmental Toxicol-
ogy and Chemistry 17, 109–118.
Hammond, P.S., Benke, H., Berggren, P., Borchers, D.L., Buckland, S.T., Collet, A., Heide-
Jørgensen, M.-P., Heimlich-Boran, S., Hiby, A.R., Leopold, M.F. & Øien, N. 1995. Distribution
and abundance of the harbour porpoise and other small cetaceans in the North Sea and adjacent
waters. Final Report to the EU, Life 92-2/UK/027.
Hammond, P.S., Hall, A.J. & Prime, J.H. 1994. The diet of grey seals around Orkney and other island
and mainland sites in north-eastern Scotland. Journal of Applied Ecology 31, 340–350.
Hammond, P.S. & Prime, J.H. 1990. The diet of British grey seals (Halichoerus grypus). Canadian
Bulletin of Fisheries and Aquatic Sciences 222, 243–254.
Hardy, A.C. 1959. The open sea: II. Fish and fisheries. London: Collins.
Härkönen, T.J. 1988. Food-habitat relationships of harbour seals and black cormorants in the Skager-
rak and Kattegat. Journal of Zoology, London 214, 673–681.
Härkönen, T.J. & Heide-Jørgensen, M.-P. 1991. The harbour seal Phoca vitulina as a predator in the
Skagerrak. Ophelia 3, 191–207.
Harmer, S.F. 1927. Report on Cetacea stranded on the British coasts from 1913 to 1926. British
Museum (Natural History) No. 10, London.
Harris, M.P. & Riddiford, N.J. 1989. The food of some young seabirds on Fair Isle in 1986–1988.
Scottish Birds 15, 119–125.
Harris, M.P. & Wanless, S. 1991. The importance of the lesser sandeel Ammodytes marinus in the diet
of the shag Phalacrocorax aristotelis. Ornis Scandinavica 22, 375–382.
Harwood, J. & Greenwood, J.J.D. 1985. Competition between British grey seals and fisheries. In Marine
mammals and fisheries, J.R. Beddington et al. (eds). London: George Allen & Unwin, 153–169.
Hatakeyama, Y. & Soeda, H. 1990. Studies on echolocation of porpoises taken in salmon gillnet fish-
eries. In Sensory abilities of cetaceans, laboratory and field evidence, J.A. Thomas & R.A.
Kastelein (eds). New York: Plenum Press, 269–281.
Heide-Jørgensen, M.-P., Mosbech, A., Teilmann, J., Benke, H. & Schultz, W. 1992. Harbour porpoise
(Phocoena phocoena) densities obtained from aerial surveys north of Fyn in the bay of Kiel.
Ophelia 35, 133–146.
Heide-Jørgensen, M.-P., Teilmann, J., Benke, H. & Wulf, J. 1993. Abundance and distribution of
harbour porpoises Phocoena phocoena in selected areas of the western Baltic and the North Sea.
Helgoländer Meeresuntersuchungen 47, 335–346.
THE DIET OF HARBOUR PORPOISE IN THE NE ATLANTIC
383
Hislop, J.R.G. 1972. Some aspects of the biology of whiting, Merlangius merlangus (L.). PhD thesis,
University of Aberdeen, Aberdeen, Scotland.
Hislop, J.R.G. 1984. A comparison of the reproductive tactics and strategies of cod, haddock, whiting
and Norway pout in the North Sea. In Fish reproduction: strategies and tactics, G.W. Potts & R.J.
Wootton (eds). London: Academic Press, 311–329.
Hislop, J.R.G. 1996. Changes in North Sea gadoid stocks. ICES Journal of Marine Science 53,
1146–1156.
Hobson, K.A. 1990. Stable isotope analysis of marbled murrelets: evidence for freshwater feeding and
determination of trophic level. Condor 92, 897–903.
Hohn, A.A. & Brownell, R.L. 1990. Harbour porpoise in central Californian waters: life history and
incidental catches. International Whaling Commission, SC/42/SM47, Cambridge, UK.
Holden, A.V. & Marsden, K. 1967. Organochlorine pesticides in seals and porpoises. Nature 216,
1274–1276.
Hooker, S.K., Iverson, S.J., Ostrom, P. & Smith, S.C. 2001. Diet of northern bottlenose whales
inferred from fatty-acid and stable-isotope analyses of biopsy samples. Canadian Journal of
Zoology 79, 1442–1454.
Hudson, A.V. & Furness, R.W. 1989. The behaviour of seabirds foraging at fishing boats around Shet-
land. Ibis 131, 225–237.
Iverson, S.J., Frost, K.J. & Lowry, L.F. 1997. Fatty acid signatures reveal fine scale structure of forag-
ing distribution of harbor seals and their prey in Prince William Sound, Alaska. Marine Ecology
Progress Series 151, 255–271.
Iverson, S.J., Oftedal, O.T., Bowen, W.D., Boness, D.J. & Sampugna, J. 1995. Prenatal and postnatal
transfer of fatty acids from mother to pup in the hooded seal. Journal of Comparative Physiology, B
165, 1–12.
IWC 1991. Report of the scientific committee. Annex G. Report of the sub-committee on small
cetaceans. Reports of the International Whaling Commission 41, 172–188.
IWC 1992. Report of the scientific committee. Annex G. Report of the sub-committee on small
cetaceans. Reports of the International Whaling Commission 42, 178–234.
IWC 1994. Report of the workshop on mortality of cetaceans in passive fishing nets and traps. In
Special Issue, 15: Gillnets and cetaceans, W.F. Perrin et al. (eds). Cambridge: International
Whaling Commission, 1–71.
IWC 1995. Report of the scientific committee. Annex G. Report of the sub-committee on small
cetaceans. Reports of the International Whaling Commission 45, 165–186.
IWC 1996. Report of the scientific committee. Annex H. Report of the sub-committee on small
cetaceans. Reports of the International Whaling Commission 46, 160–179.
Jepson, P.D., Bennett, P.M., Allchin, C.R., Law, R.J., Kuiken, T., Baker, J.R., Rogan, E. & Kirk-
wood, J.K. 1999. Investigating potential associations between chronic exposure to polychlorinated
biphenyls and infectious disease mortality in harbour porpoises from England and Wales. The
Science of the Total Environment 243/244, 339–348.
Jonsgåard, A. 1982. The food of minke whales (Balaenoptera acutorostrata) in the northern North
Atlantic waters. Reports of the International Whaling Commission 32, 259–262.
Kastelein, R.A., Au, W.W.L., Rippe, H.T. & Schooneman, N.M. 1999. Target detection by an
echolocating harbour porpoise (Phocoena phocoena). Journal of the Acoustical Society of America
105, 2493–2498.
Kastelein, R.A., Hardeman, J. & Boer, H. 1997b. Food consumption and body weight of harbour por-
poises (Phocoena phocoena). In The biology of harbour porpoise, A.J. Read et al. (eds). Woerden,
The Netherlands: De Spil Publishers, 217–233.
Kastelein, R.A. & Lavaleije, M.S.S. 1992. Foreign bodies in the stomachs of a female harbour por-
poise (Phocoena phocoena) from the North Sea. Aquatic Mammals 18, 40–46.
Kastelein, R.A., Schooneman, N.M., Au, W.W.L., Verboom, W.C. & Vaughan, N. 1997a. The
ability of a harbour porpoise (Phocoena phocoena) to discriminate between objects buried in the
M.B. SANTOS & G.J. PIERCE
384
sand. In The biology of harbour porpoise, A.J. Read et al. (eds). Woerden, The Netherlands: De
Spil Publishers, 329–342.
Kayes, R.J. 1985. The decline of porpoises and dolphins in the southern North Sea: a current status
report. Greenpeace International, Political Ecology Research Group, Oxford.
Kinze, C.C. 1985. Intraspecific variation in Baltic and North Sea harbour porpoises (Phocoena pho-
coena (L., 1758)). Videnskabelige Meddelelser fra Dansk Naturhistorie Forening, 146, 63–74.
Kinze, C.C. 1989. On the reproduction, diet and parasitic burden of harbour porpoise (Phocoena pho-
coena) in the west Greenland waters. In European research on cetaceans – 3, P.G.H. Evans (ed.).
Cambridge: European Cetacean Society, 91–95.
Kinze, C.C. 1990a. Cetacean mortality in passive fishing nets and traps in the Baltic Sea: a review.
International Whaling Commission, SC/O90/G25, Cambridge, UK.
Kinze, C.C. 1990b. Marsvineundersøgelser 1989, afsluttende rapport. Zoologisk Museum, Copen-
hagen, Denmark.
Kinze, C.C. 1994. Incidental catch of harbour porpoises (Phocoena phocoena) in Danish waters,
1986–1989. In Special Issue, 15: Gillnets and cetaceans, W.F. Perrin et al. (eds). Cambridge: Inter-
national Whaling Commission, 183–187.
Kinze, C.C. 1995. Exploitation of harbour porpoises (Phocoena phocoena) in Danish waters: a histor-
ical review. In Special Issue, 16: Biology of phocoenids, A. Bjørge & G.P. Donovan (eds). Cam-
bridge: International Whaling Commission, 141–153.
Kirsch, P.E., Iverson, S.J. & Bowen, W.D. 2000. Effects of diet on body composition and blubber
fatty acids in captive harp seals (Phoca groenlandica). Physiological and Biochemical Zoology 73,
45–59.
Kirsch, P.E., Iverson, S.J., Bowen, W.D., Kerr, S.R. & Ackman, R.G. 1998. Dietary effects on the
fatty acid signature of whole Atlantic cod (Gadus morhua). Canadian Journal of Fisheries and
Aquatic Sciences 55, 1378–1386.
Kleivane, L., Skaare, J.U., Bjorke, A., Ruiter, E. de & Reijnders, P.J.H. 1995. Organochlorine pesti-
cide residue and PCBs in harbour porpoise (Phocoena phocoena) incidentally caught in Scandin-
avian waters. Environmental Pollution 89, 137–146.
Kock, K.H. & Benke, H. 1996. On the by-catch of harbour porpoise (Phocoena phocoena) in German
fisheries in the Baltic and the North Sea. Archives of Fishery and Marine Research 44, 95–114.
Koopman, H.N. 1998. Topographical distribution of the blubber of harbour porpoises (Phocoena pho-
coena). Journal of Mammalogy 79, 260–270.
Koopman, H.N., Iverson, S.J. & Gaskin, D.E. 1996. Stratification and age-related differences in
blubber fatty acids of the male harbour porpoise (Phocoena phocoena). Journal of Comparative
Physiology, B 165, 628–639.
Kuiken, T. 1996. Review of the criteria for the diagnosis of by-catch in cetaceans. In Newsletter 26
(Special Issue): Diagnosis of by-catch in cetaceans, Proceedings of the Second ECS Workshop on
Cetacean Pathology, T. Kuiken (ed.). Saskatoon, Canada: European Cetacean Society, 38–43.
Kuiken, T., Bennett, P.M., Allchin, C.R., Kirkwood, J.K., Baker, J.R., Lockyer, C.H., Walton, M. J.
& Sheldrick, M.C. 1994b. PCBs, cause of death and body condition in harbour porpoises (Pho-
coena phocoena) from British waters. Aquatic Toxicology 28, 13–28.
Kuiken, T., Hoefle, U., Bennett, P.M., Allchin, C.R., Kirkwood, J.K., Baker, J.R., Appleby, E.C.,
Lockyer, C.H., Walton, M.J. & Sheldrick, M.C. 1993. Adrenocortical hyperplasia, disease and
chlorinated hydrocarbons in the harbour porpoise (Phocoena phocoena). Marine Pollution Bulletin
26, 440–446.
Kuiken, T., Simpson, V.R., Allchin, C.R., Bennett, P.M., Codd, G.A., Harris, E.A., Howes, G. J.,
Kennedy, S., Kirkwood, J.K., Law, R.J., Merrett, N.R. & Phillips, S. 1994a. Mass mortality of
common dolphins (Delphinus delphis) in south west England due to incidental capture in fishing
gear. Veterinary Record 134, 81–89.
Langham, L.P.E. 1971. The distribution and abundance of larval sand-eels (Ammodytidae) in Scottish
waters. Journal of the Marine Biological Association of the United Kingdom 51, 697–707.
THE DIET OF HARBOUR PORPOISE IN THE NE ATLANTIC
385
Lick, R.R. 1991a. Untersuchungen zu Lebenszyklus (Krebse – Fische – marine Säuger) und Gefrierre-
sistentz anisakider Nematoden in Nord- und Ostsee [Investigations concerning the life cycle (crus-
taceans – fish – marine mammals) and freezing tolerance of anisakine nematodes in the North Sea
and the Baltic Sea]. PhD thesis, University of Kiel, Kiel, Germany.
Lick, R.R. 1991b. Parasites from the digestive tract and food analysis of harbour porpoise (Phocoena
phocoena) from German waters. In European research on cetaceans – 5, P.G.H. Evans (ed.). Cam-
bridge: European Cetacean Society, 65–68.
Lindroth, A. 1962. Baltic salmon fluctuations 2: porpoise and salmon. Reports of the Institute of Fresh-
water Research of Drottningholm 44, 105–112.
Lockyer, C. 1995. Investigation of aspects of the life history of the harbour porpoise, Phocoena pho-
coena, in British waters. In Special Issue, 16: Biology of phocoenids, A. Bjørge & G.P. Donovan
(eds). Cambridge: International Whaling Commission, 189–197.
Lockyer, C. 1999. Application of a new method to investigate population structure in the harbour por-
poise, Phocoena phocoena, with special reference to the North and Baltic Seas. Journal of
Cetacean Research and Management 1, 297–304.
Lockyer, C., Desportes, G., Anderson, K., Labberté, S. & Siebert, U. 2001. Monitoring growth of
harbour porpoise (Phocoena phocoena) in human care. International Council for the Exploration of
the Sea, C.M. 2001/J:29, Copenhagen, Denmark.
Lowry, N. & Teilmann, J. 1994. Bycatch and bycatch reduction of the harbour porpoise (Phocoena
phocoena) in Danish waters. In Special Issue, 15: Gillnets and cetaceans, W.F. Perrin et al. (eds).
Cambridge: International Whaling Commission, 203–209.
Macer, C.T. 1966. Sandeels (Ammodytidae) in the southwestern North Sea: their biology and fishery.
Fishery Investigations, London, Series 2 24, 1–55.
Macintyre, D. 1934. Sea enemies of salmon. Salmon and Trout Magazine 74, 38–42.
Malinga, M. & Kuklik, I. 1996. Food consumption of harbour porpoises (Phocoena phocoena) in
Polish waters of the Baltic Sea. In European research on cetaceans – 10, P.G.H. Evans (ed.). Cam-
bridge: European Cetacean Society, 260 only.
Marchessaux, D. 1980. A review of the current knowledge of the cetaceans in the eastern Mediter-
ranean Sea. Vie Marina 2, 59–64.
Martin, A.R. 1996. The diet of harbour porpoises (Phocoena phocoena) in British waters.
International Whaling Commission, SC/47/SM48, Cambridge, UK.
Matthews, L.H. 1952. British mammals. London: Collins.
Merson, M.H. & Kirkpatrick, R.L. 1976. Reproductive performance of captive white-footed mice fed
a PCB. Bulletin of Environmental Contamination and Toxicology 16, 392–398.
Millais, J.G. 1906. Mammals of Great Britain and Ireland. London: Longmans Green.
Miyazaki, N., Amano, M. & Fujise, Y. 1987. Growth and skull morphology of the harbour porpoises
in the Japanese waters. Memoirs of the Natural Science Museum of Tokyo 20, 137–146.
Monaghan, P., Uttley, J.D., Burns, M.D., Thaine, C. & Blackwood, J. 1989. The relationship between
food supply, reproductive effort and breeding success in Arctic terns Sterna paradisaea. Journal of
Animal Ecology 58, 261–274.
Murray, J. & Burt, J.R. 1977. The composition of fish. Torry Advisory Note No. 38, MAFF, Aberdeen.
Northridge, S.P. & Lankester, K. 1992. Sightings of the harbour porpoise in the North Sea and some
notes on interactions with the fisheries. International Whaling Commission, SC/42/SM46, Cam-
bridge, UK.
Northridge, S.P., Tasker, M.L., Webb, A. & Williams, J.M. 1995. Distribution and relative abundance
of harbour porpoises (Phocoena phocoena L.), white-beaked dolphins (Lagenorhynchus albirostris
Gray) and minke whales (Balaenoptera acutorostrata Lacepède) around the British Isles. ICES
Journal of Marine Science 52, 55–66.
Otterlind, G. 1976. The harbour porpoise (Phocoena phocoena) endangered in Swedish waters. Inter-
national Council for the Exploration of the Sea, C.M. 1976/N:16, Copenhagen, Denmark.
Palka, D.L., Read, A.J., Westgate, A.J. & Johnston, D.W. 1996. Summary of current knowledge of
M.B. SANTOS & G.J. PIERCE
386
harbour porpoises in US and Canadian Atlantic waters. Reports of the International Whaling Com-
mission 46, 559–565.
Pauly, D., Trites, A.W., Capuli, E. & Christensen, V. 1998. Diet composition and trophic levels of
marine mammals. ICES Journal of Marine Science 55, 467–481.
Perkins, J.S., Bryant, P.J., Nichols, G. & Patten, D.R. 1982. Humpback whales (Megaptera novaean-
gliae) off the west coast of Greenland. Canadian Journal of Zoology 60, 2921–2930.
Pierce, G.J. & Boyle, P.R. 1991. A review of methods for diet analysis in piscivorous marine
mammals. Oceanography and Marine Biology: an Annual Review 29, 409–486.
Pierce, G.J., Boyle, P.R., Diack., J.S.W. & Clark, I. 1990. Sandeels in the diets of seals: application
of novel and conventional methods of analysis to faeces from seals in the Moray Firth area of Scot-
land. Journal of the Marine Biological Association of the United Kingdom 70, 829–840.
Pierce, G.J., Diak, J.S.W. & Boyle, P.R. 1989. Digestive tract contents of seals in the Moray Firth
area of Scotland. Journal of Fish Biology 35 (Suppl. A), 341–343.
Pierce, G.J., Miller, A., Thompson, P.M. & Hislop, J.R.G. 1991a. Prey remains in grey seal (Hali-
choerus grypus) faeces from the Moray Firth, north-east Scotland. Journal of Zoology, London 224,
337–341.
Pierce, G.J., Thompson, P.M., Miller, A., Diack, J.S.W., Miller, D. & Boyle, P.R. 1991b. Seasonal
variation in the diet of common seals (Phoca vitulina) in the Moray Firth area of Scotland. Journal
of Zoology, London 223, 641–652.
Pierpoint, C., Earl, S. & Baines, M. 1994. Observations of harbour porpoise in West Wales, 1993. In Euro-
pean research on cetaceans – 8, P.G.H. Evans (ed.). Cambridge: European Cetacean Society, 75–78.
Pierpoint, C.J.L., Baines, M.E., Earl, S.J., Harris, R. & Tregenza, N. 1999. Night-life of harbour por-
poise. In European research on cetaceans – 13, P.G.H. Evans et al. (eds). Cambridge: European
Cetacean Society, 70–71.
Pinnegar, J.K., Jennings, S., O’Brien, C.M. & Polunin, N.V.C. 2002. Long-term changes in the
trophic level of the Celtic Sea fish community and fish market place distribution. Journal of
Applied Ecology 39, 377–390.
Pulliam, H.R. 1974. On the theory of optimal diets. American Naturalist 108, 59–74.
Rae, B.B. 1965. The food of the common porpoise (Phocoena phocoena). Journal of Zoology, London
146, 114–122.
Rae, B.B. 1973. Additional notes on the food of the common porpoise (Phocoena phocoena). Journal
of Zoology, London 169, 127–131.
Read, A.J. 1999. The harbour porpoise – Phocoena phocoena (Linnaeus, 1758). In Handbook of
marine mammals Vol. 6: The second book of dolphins and the porpoises, S.H. Ridgway & R. Har-
rison (eds). London: Academic Press, 323–355.
Read, A.J. & Gaskin, D.E. 1988. Incidental catch of harbour porpoises by gill nets. Journal of Wildlife
Management 52, 517–523.
Read, A.J. & Gaskin, D.E. 1990. Changes in growth and reproduction of harbour porpoises, Phocoena
phocoena, from the Bay of Fundy. Canadian Journal of Fisheries and Aquatic Sciences 47,
2158–2163.
Read, A.J. & Hohn, A.A. 1995. Life in the fast lane: the life history of harbour porpoises from the
Gulf of Maine. Marine Mammal Science 11, 423–440.
Read, A.J. & Westgate, A.J. 1997. Monitoring the movements of harbour porpoises (Phocoena pho-
coena) with satellite telemetry. Marine Biology 130, 315–322.
Read, A.J., Wiepkema, P.R. & Nachtigall, P.E. 1997. The harbour porpoise (Phocoena phocoena). In
The biology of the harbour porpoise, A.J. Read et al. (eds). Woerden, The Netherlands: De Spil
Publishers, 3–6.
Reay, P.J. 1970. Synopsis of biological data on north Atlantic sandeels of the genus Ammodytes (A.
tobianus, A. dubius, A. americanus and A. marinus). FAO Fisheries Synopsis 82.
Recchia, C.A. & Read, A.J. 1989. Stomach contents of harbour porpoises, Phocoena phocoena (L.),
from the Bay of Fundy. Canadian Journal of Zoology 67, 2140–2146.
THE DIET OF HARBOUR PORPOISE IN THE NE ATLANTIC
387
Reijnders, P.J.H. 1986. Reproductive failure in common seals feeding on fish from polluted coastal
waters. Nature 324, 456–457.
Rogan, E. & Berrow, S.D. 1996. A review of harbour porpoises, Phocoena phocoena, in Irish waters.
Reports of the International Whaling Commission 46, 595–605.
Rosel, P.E., Dizon, A.E. & Haygood, M.G. 1995. Variability of the mitochondrial control region in
populations of the harbour porpoise, Phocoena phocoena, on interoceanic and regional scales.
Canadian Journal of Fisheries and Aquatic Sciences 52, 1210–1219.
Rosel, P.E., France, S.C., Wang, J.Y. & Kocher, T.D. 1999. Genetic structure of harbour porpoise
Phocoena phocoena populations in the northwest Atlantic based on mitochondrial and nuclear
markers. Molecular Ecology 8, S41–S54.
Ross, G.J.B. 1984. The smaller cetaceans of the south east coast of southern Africa. Annals of the
Cape Provincial Museums (Natural History) 15, 173–410.
Ross, H.M. & Wilson, B. 1996. Violent interactions between bottlenose dolphins and harbour por-
poises. Proceedings of the Royal Society of London, B 263, 283–286.
Ross, P.S., De Swart, R.L., Timmerman, H.H., Reijnders, P.J.H., Vos, J. G., Van Loveren, H. &
Osterhaus, A.D.M.E. 1996. Suppression of natural killer cell activity in harbour seals (Phoca vit-
ulina) fed Baltic Sea herring. Aquatic Toxicology 34, 71–84.
Santos, M.B. 1998. Feeding ecology of harbour porpoises, common and bottlenose dolphins and
sperm whales in the northeast Atlantic. PhD thesis, University of Aberdeen, Aberdeen, Scotland.
Santos, M.B., Clarke, M.R. & Pierce, G.J. 2001b. Assessing the importance of cephalopods in the
diets of marine mammals and other top predators. Fisheries Research, 52, 121–139.
Santos, M.B., Pierce, G.J., Reid, R.J., Patterson, I.A.P., Ross, H. M. & Mente, E. 2001c. Stomach
contents of bottlenose dolphins (Tursiops truncatus) in Scottish waters. Journal of the Marine Bio-
logical Association of the United Kingdom 81, 873–878.
Santos, M.B., Spencer, N., Pierce, G.J., Lockyer, C., Reid, R.J., Patterson, I.A.P. & Edwards, W.
2001a. Life history parameters of harbour porpoise (Phocoena phocoena) in Scottish waters. 15th
Annual Conference of the European Cetacean Society, Rome, Italy.
Scheffer, V.B. 1953. Measurements and stomachs contents of 11 delphinids from the northeast Pacific.
Murrelet 34, 27–30.
Scott, T. 1903. Some further observations on the food of fishes, with a note on the food observed in the
stomach of a common porpoise. Report of the Fishery Board for Scotland 21, 218–227.
Sekiguchi, K. 1987. Occurrence and behaviour of harbour porpoises (Phocoena phocoena) at Pajaro
Dunes, Monterey Bay, California. MSc thesis, San Jose State University, California.
Sekiguchi, K., Klages, N.T.W. & Best, P.B. 1992. Comparative analysis of the diets of smaller odontocete
cetaceans along the coast of Southern Africa. South African Journal of Marine Science 12, 843–861.
Sequeira, M. 1996. Harbour porpoises, Phocoena phocoena, in Portuguese waters. Reports of the
International Whaling Commission 46, 583–586.
Sergeant, D.E. 1969. Feeding rates of Cetacea. Fiskeridirektorates Skrifter, Serie Havundersøkelser
15, 246–258.
Sergeant, D.E. & Fisher, H.D. 1957. The smaller Cetacea of eastern Canadian waters. Journal of the
Fisheries Research Board of Canada 14, 83–115.
Siebert, U., Benke, H., Frese, K., Pirro, F. & Lick, R. 1996. Postmortem examination of by-catches
from German fisheries and of suspected by-catches found on the coast of Germany. In Newsletter
26 (Special Issue): Diagnosis of by-catch in cetaceans, Proceedings of the Second ECS Workshop
on Cetacean Pathology, T. Kuiken (ed.). Saskatoon, Canada: European Cetacean Society, 27–30.
Silva, M.A., Sequeira, M., Prieto, R. and Alexandre, B. 1999. Observations of harbour porpoises
(Phocoena phocoena) on the northern coast of Portugal. In European research on cetaceans – 13,
P.G.H. Evans et al. (eds). Cambridge: European Cetacean Society, 267–269.
Slijper, E.J. 1962. Whales. London: Hutchinson.
Smeenk, C. 1987. The harbour porpoise Phocoena phocoena (L., 1758) in the Netherlands: stranding
records and decline. Lutra 30, 77–90.
M.B. SANTOS & G.J. PIERCE
388
Smeenk, C., Leopold, M.F. & Addink, M.J. 1992. Note on the harbour porpoise Phocoena phocoena
in Mauritania, West Africa. Lutra 35, 98–104.
Smith, G.J.D. & Gaskin, D.E. 1974. The diet of harbour porpoises (Phocoena phocoena (L.)) in
coastal waters of Eastern Canada, with special reference to the Bay of Fundy. Canadian Journal of
Zoology 52, 777–782.
Smith, R.J. & Read, A.J. 1992. Consumption of euphausiids by harbour porpoises (Phocoena pho-
coena) calves in the Bay of Fundy. Canadian Journal of Zoology 70, 1629–1632.
Southwell, T. 1881. The seals and whales of the British seas. London: Jarrold.
Sparholt, H. 1990. An estimate of the total biomass of fish in the North Sea. ICES Journal of Marine
Science 46, 200–210.
Stephen, A.C. 1926. Common porpoise stranded at Granton. Scottish Naturalist 1926, 46.
Storey, G. 1993. The trophic ecology of the sandeel Ammodytes tobianus (L.), and other planktivorous
fish species from Aberdeen Bay. PhD thesis, University of Aberdeen, Aberdeen.
Sturtivant, C.R., Datta, S. & Goodson, D.S. 1994. A review of echolocation research on the harbour
porpoise Phocoena phocoena and the common dolphin Delphinus delphis. In European research
on cetaceans – 8, P.G.H. Evans (ed.). Cambridge: European Cetacean Society, 164–168.
Subramanian, A.N., Tanabe, S., Tatsukawa, R., Saito, S. & Miyazaki, N. 1987. Reduction in the
testosterone levels by PCBs and DDE in Dall’s porpoises of northwestern North Pacific. Marine
Pollution Bulletin 18, 643–646.
Svärdson, G. 1955. Salmon stock fluctuations in the Baltic Sea. Reports of the Institute of Freshwater
Research of Drottningholm 36, 226–262.
Swart, R. de L., Ross, P.S., Vedder, L.J., Timmerman, H.H., Heisterkamp, S., Loveren, H.V., Vos,
J.G., Reijnders, P.J.H. & Osterhaus, A.D.M. E. 1994. Impairment of immune function in harbour
seals (Phoca vitulina) feeding on fish from polluted waters. Ambio 23, 155–159.
Tanabe, S., Watanabe, S., Kan, H. & Tatsukawa, R. 1988. Capacity and mode of PCB metabolism in
small cetaceans. Marine Mammal Science 4, 103–124.
Taylor, B.L. & Dawson, P.K. 1984. Seasonal changes in density and behaviour of harbour porpoise
(Phocoena phocoena) affecting census methodology in Glacier Bay National Park, Alaska. Reports
of the International Whaling Commission 34, 479–483.
Teilmann, J. & Dietz, R. 1996. Status of the harbour porpoises (Phocoena phocoena) in Greenland.
International Whaling Commission, SC/47/SM44, Cambridge, UK.
Thompson, P.M., Pierce, G.J., Hislop, J.R.G., Miller, D. & Diack, J.S. W. 1991. Winter foraging by
common seals (Phoca vitulina) in relation to food availability in the inner Moray Firth, N.E. Scot-
land. Journal of Animal Ecology 60, 283–294.
Thompson, P.M., Tollit, D.J., Corpe, H.M., Reid, R.J. & Ross, H.M. 1997. Changes in haematologi-
cal parameters in relation to prey switching in a wild population of harbour seals. Functional
Ecology 11, 743–750.
Tiedemann, R., Harder, J., Gmeiner, C. & Haase, E. 1996. Mitochondrial DNA sequence patterns of
harbour porpoises (Phocoena phocoena) from the North and the Baltic Sea. Zeitschrift für
Säugetierkunde 61, 104–111.
Tiezen, L.L. 1978. Carbon isotope fractionation in biological material. Nature 276, 97–98.
Tolley, K.A. & Heldal, H.E. 2002. Inferring ecological separation from regional differences in radioactive
caesium in harbour porpoises (Phocoena phocoena). Marine Ecology Progress Series 228, 301–309.
Tolley, K.A., Rosel, P.E., Walton, M., Bjørge, A. & Øien, N. 1999. Genetic population structure of
harbour porpoises (Phocoena phocoena) in the North Sea and Norwegian waters. Journal of
Cetacean Research and Management 1, 265–274.
Tolley, K.A., Víkingsson, G.A. & Rosel, P.E. 2001. Mitochondrial DNA sequence variation and phy-
logeographic patterns in harbour porpoises Phocoena phocoena from the North Atlantic. Conserva-
tion Genetics 2, 349–361.
Tollit, D.J. & Thompson, P.M. 1996. Seasonal and between-year variations in the diet of harbour seals
in the Moray Firth, Scotland. Canadian Journal of Zoology 74, 1110–1121.
THE DIET OF HARBOUR PORPOISE IN THE NE ATLANTIC
389
Tomilin, A.G. 1957. Vol. IX: Cetacea. In Mammals of the U.S.S.R. and adjacent countries, V.G.
Geptner (ed.). Moscow: Izdat. Akad. Nauk SSSR. (English Translation, 1967, Jerusalem: Israel
Program for Scientific Translations).
Tregenza, N.J.C., Berrow, S.D., Leaper, R. & Hammond, P.S. 1997. Harbour porpoise, Phocoena pho-
coena L., by-catch in set gill nets in the Celtic Sea. ICES Journal of Marine Science 54, 896–904.
van Beneden, P.J. 1889. Histoire naturelle des delphinides des mers d’Europe. Mémoirs du Course de
l’Academie Royal Belge 43, 1–253.
van Bree, P.J.H. 1977. On former and recent strandings of cetaceans on the coast of the Netherlands.
Zeitschrift fur Saugetierkunde 42, 101–107.
van Utrech, W.L. 1978. Age and growth in Phocoena phocoena Linnaeus, 1758 (Cetacea, Odontoceti)
from the North Sea. Bijdragen tot de Dierkunde 48, 16–28.
Verwey, J. 1975. The cetaceans Phocoena phocoena and Tursiops truncatus in the Marsdiep area
(Dutch Wadden Sea) in the years 1931–1973. Nederlands Instituut voor Onderzoek der Zee, Publi-
caties en verslagen, 1975 17, 1–153.
Vinther, M. 1999. Bycatches of harbour porpoises (Phocoena phocoena L.) in Danish set-net fisheries.
Journal of Cetacean Research and Management 1, 123–135.
Vos, J.G. & Luster, M.I. 1989. Immune alterations. In Halogenated biphenyls, terphenyls, naph-
thalenes, dibenzodioxins and related products, R.D. Kimbrough & A.A. Jensen (eds). Amsterdam:
Elsevier, 295–322.
Walton, M.J. 1997. The population structure of harbour porpoises Phocoena phocoena in the seas
around the UK and adjacent waters. Proceedings of the Royal Society of London, B 264, 89–94.
Wang, J.Y., Gaskin, D.E. & White, B.N. 1996. Mitochondrial DNA analysis of harbour porpoise,
Phocoena phocoena, subpopulations in North American waters. Canadian Journal of Fisheries and
Aquatic Sciences 53, 1632–1645.
Wang, J.Y. & Berggren, P. 1997. Mitochondrial DNA analysis of harbour porpoises (Phocoena pho-
coena) in the Baltic Sea, the Kattegat–Skagerrak Seas and off the west coast of Norway. Marine
Biology 127, 531–537.
Wang, J.Y., Gaskin, D.E. & White, B.N. 1996. Mitochondrial DNA analysis of harbour porpoise,
Phocoena phoconea, subpopulations in North American waters. Canadian Journal of Fisheries and
Aquatic Sciences 5, 1632–1645.
Waring, G.T., Gerrior, P., Payne, P.M., Parry, B.L. & Nicolas, J.R. 1990. Incidental take of marine
mammals in foreign fishery activities off the northeast United States, 1977–88. Fishery Bulletin 88,
347–360.
Wassermann, M., Wassermann, D., Gershon, Z. & Zellermayer, L. 1979. Effects of organochlorine
insecticides on body defense systems. In Biological effects of pesticides in mammalian systems,
H.F. Kraybill (ed.). New York: Annals of the New York Academy of Sciences, 393–401.
Watson, A.P. & Gaskin, D.E. 1983. Observations on the ventilation cycle of the harbour porpoise
Phocoena phocoena in coastal waters of the Bay of Fundy. Canadian Journal of Zoology 61,
126–132.
Watson, L. 1985. Whales of the world. London: Hutchinson.
Wells, D.E. & McKenzie, C. 1994. Techniques for pattern recognition of organochlorine residues in
sea mammals from Scottish coastal waters. International Council for the Exploration of the Sea,
C.M. 1994/EN:10, Copenhagen, Denmark.
Westgate, A.J., Read, A.J., Berggren, P., Koopman, H.N. & Gaskin, D.E. 1995. Diving behaviour of
harbour porpoises, Phocoena phocoena. Canadian Journal of Fisheries and Aquatic Sciences 52,
1064–1073.
Westgate, A.J. & Tolley, K.A. 1999. Geographical differences in organochlorine contaminants in
harbour porpoises Phocoena phocoena from the western North Atlantic. Marine Ecology Progress
Series 177, 255–268.
Whitehead, P.J.P., Bauchot, M.-L., Hureau, J.-C., Nielsen, J. & Tortonese, E. (eds) 1989. Fishes of the
north-eastern Atlantic and the Mediterranean. UNESCO, Paris.
M.B. SANTOS & G.J. PIERCE
390
Whymper, F. 1883. The fisheries of the world. An illustrated and descriptive record of the inter-
national fisheries exhibition. London: Cassell.
Wijnsma, G., Pierce, G.J. & Santos, M.B. 1999. Assessment of errors in cetacean diet analysis: in
vitro digestion of otoliths. Journal of the Marine Biological Association of the United Kingdom 79,
573–575.
Wilke, F. & Kenyon, K.W. 1952. Notes on the food of fur seal, sea-lion, and harbour porpoise.
Journal of Wildlife Management 16, 396–397.
Winslade, P. 1974. Behavioural studies on the lesser sandeel Ammodytes marinus (Raitt). II. The effect
of light intensity on activity. Journal of Fish Biology 6, 577–586.
Wright, P.J. 1996. Is there a conflict between sandeel fisheries and seabirds? A case study at Shetland.
In Aquatic predators and their prey, S.P. R. Greenstreet & M. L. Tasker (eds). Oxford: Fishing
News Books, Blackwell Science, 154–165.
Yasui, W.C. & Gaskin, D.E. 1986. Energy budget of a small cetacean, the harbour porpoise, Pho-
coena phocoena (L.). Ophelia 25, 183–197.
Yurick, D.B. 1977. Populations, subpopulations and zoogeography of the harbour porpoise, Phocoena
phocoena (L.). MSc thesis, University of Guelph, Guelph, Ontario.
Yurick, D.B. & Gaskin, D.E. 1987. Morphometric and meristic comparisons of skulls of harbour por-
poise Phocoena phocoena (L.) from the North Atlantic and North Pacific. Ophelia 27, 53–75.
... Harbour porpoises are wide-ranging highly mobile animals (Read & Westgate 1997) and the most abundant cetacean species in the North Sea (Hammond et al. 2002(Hammond et al. , 2013(Hammond et al. , 2017. The diet of harbour porpoises consists of a wide variety of fish and cephalopod species and varies regionally; however, only a few prey types dominate the diet in any one area (Santos & Pierce 2003;Santos et al. 2004). In Scottish waters, historical studies have indicated whiting and sandeels dominate porpoise diets (Santos & Pierce 2003). ...
... The diet of harbour porpoises consists of a wide variety of fish and cephalopod species and varies regionally; however, only a few prey types dominate the diet in any one area (Santos & Pierce 2003;Santos et al. 2004). In Scottish waters, historical studies have indicated whiting and sandeels dominate porpoise diets (Santos & Pierce 2003). In Dutch coastal waters, porpoises tend to consume predominantly gadoids such as cod and whiting, gobies (Family Gobiidae), sandeels, and clupeids like European sprat and Atlantic herring (Leopold 2015). ...
Technical Report
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This project provides a first attempt to describe the spatiotemporal energetic availability of different prey species to harbour porpoises (Phocoena phocoena) in the North Sea. Harbour porpoises are very abundant in the North Sea and their diet consists of a variety of prey species. Harbour porpoises are listed on Annex II of the EU Habitats Directive and accordingly Special Areas of Conservation (SAC) are designated for this species, one of them being in the southern North Sea. The distribution of porpoises is thought to be prey driven but our understanding of prey availability, particularly in the context of the Southern North Sea SAC, is currently limited. The need to assess and potentially manage activities within the SAC is the context for initiating this work. To compare our prey estimates with knowledge of porpoise distribution we predicted prey availability for the most recent two years that North Sea-wide cetacean surveys were carried out (2005 and 2016). A cleaned dataset of the International Bottom Trawl Survey (NS-IBTS) was used to create density surface models using Generalised Additive Models for the different prey species. Soap filters were used to avoid smoothing across boundary features. Relative gear efficiency factors per prey species and size class data were used to correct for catchability and biomass values were converted to energetic content using energy density values from the literature. Energy maps were produced for Atlantic cod (Gadus morhua), whiting (Merlangius merlangus), European sprat (Sprattus sprattus), Atlantic herring (Clupea harengus) and sandeels (Family Ammodytidae). The modelled prey distribution maps fit well with previously described spatial patterns for the fish species. Overall, it appears that the energy available was higher in summer and was also higher in 2016 in comparison to 2005, especially in the southern and northwestern North Sea. For both the Southern North Sea SAC and in the wider North Sea, the main energetic contributions to the overall energy density were from whiting and sandeels. During the winter, European sprat also added considerably to the overall energy density while in summer, Atlantic herring added a substantial amount of energy. Overall, large amounts of prey energy are predicted to be available both within and outside the SAC boundary. Based on five of the reported main prey species of harbour porpoise overall mean estimates of total energy available in the North Sea ranged between 21,610 (winter)-30,764 megajoule (MJ) per km 2 (summer) in 2005 and 34,661 (winter)-76,938 MJ per km 2 (summer) in 2016. Reviews of harbour porpoise daily energy requirements varied between 9-31 MJ per day. However, the energy predicted may not correlate to the actual available energy for porpoises given the role of other marine predators and the fishing industry present in the North Sea.
... Previous distribution modelling in the Moray Firth, north-east Scotland (part of the current study area; Fig. 1) found a relatively high density of animals in the Smith Bank, a shallow (30-40 m) sand bank in the middle of the Firth (Brookes et al., 2013;Williamson et al., 2016). This is likely because the Smith Bank provides suitable habitat for sandeels (Ammodytidae) and whiting, Merlangius merlangus (Hopkins, 2011), two of the most common prey species for porpoise in this area (Santos and Pierce, 2003). Knowledge of porpoise distribution along the East coast of Scotland, however, is lacking. ...
Article
Full-text available
Species Distribution Models (SDMs) are used regularly to develop management strategies, but many modelling methods ignore the spatial nature of data. To address this, we compared fine-scale spatial distribution predictions of harbour porpoise (Phocoena phocoena) using empirical aerial-video-survey data collected along the east coast of Scotland in August and September 2010 and 2014. Incorporating environmental covariates that cover habitat preferences and prey proxies, we used a traditional (and commonly implemented) Generalized Additive Model (GAM), and two Hierarchical Bayesian Modelling (HBM) approaches using Integrated Nested Laplace Approximation (INLA) model-fitting methodology. One HBM-INLA modelled gridded space (similar to the GAM), and the other dealt more explicitly in continuous space using a Log-Gaussian Cox Process (LGCP). Overall, predicted distributions in the three models were similar; however, HBMs had twice the level of certainty, showed much finer-scale patterns in porpoise distribution, and identified some areas of high relative density that were not apparent in the GAM. Spatial differences were due to how the two methods accounted for autocorrelation, spatial clustering of animals, and differences between modelling in discrete vs. continuous space; consequently, methods for spatial analyses likely depend on scale at which results, and certainty, are needed. For large-scale analysis (>5–10 km resolution, e.g. initial impact assessment), there was little difference between results; however, insights into fine-scale (<1 km) distribution of porpoise from the HBM model using LGCP, while more computationally costly, offered potential benefits for refining conservation management or mitigation measures within offshore developments or protected areas.
... Feeding studies based on stomach contents in the North Atlantic have shown harbour porpoises are opportunistic, eating mainly fish but also cephalopods and crustaceans (Santos and Pierce 2003), although their prey may be chosen based on calorific value (Spitz et al. 2012). As they are generalist feeders, their diet may reflect prey availability in an ecosystem (Christensen and Richardson 2008). ...
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Individuals of different sex or age can vary in their resource use due to differences in behaviour, life history, energetic need, or size. Harbour porpoises are small cetaceans that rely on a constant prey supply to survive. Here, we use bone collagen carbon (δ 13 C) and nitrogen (δ 15 N) isotope compositions to elucidate sex and size differences in the foraging ecology of harbour porpoises from West Greenland. In this region, populations have a unique offshore, deep-water ecology. Female harbour porpoises are larger than males and we find that females have a higher trophic level than males, and δ 15 N positively correlates with size for females only. This indicates that size may matter in the ability of females to handle larger prey and/or dive deeper to catch higher trophic level prey. These results suggest that females, which also feed their calves, may be under different ecological constraints than males. We also analysed the harbour porpoise data with comparable stable isotope data from Greenland populations of belugas and narwhals. Consistent with their small body size, and a diet consisting primarily of capelin, we find that harbour porpoises have a lower trophic level than belugas and narwhals. Furthermore, harbour porpoises have the largest ecological niche of the three species, which is in accordance with tagging studies indicating they have a wide range in shelf and deep offshore waters of the sub-arctic and North Atlantic.
... In the southwestern part of the German North Sea, increasing densities of harbor porpoises have been reported over the last decade (Peschko et al., 2016). Harbor porpoises are top-predators and in the North Sea they mainly feed on small pelagic and demersal fish, such as clupeids, sand eels, roundfish, gobies, gadoids, and flatfish, with seasonal and agedependent variations (Leopold, 2015;Santos & Pierce, 2003). Due to their energy requirements, harbor porpoises must forage nearly continuously, which makes them particularly vulnerable to environmental disturbances (Wisniewska et al., 2016; but see also Hoekendijk et al., 2018 andWisniewska et al., 2018). ...
Article
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Harbor porpoises (Cetacea) are present in the North Sea throughout the year but periodically enter adjacent estuaries, which due to human activities are among the planet's most threatened aquatic systems. However, the occurrence of harbor porpoises in estuaries has rarely been studied. In this work, harbor porpoise occurrence at two stations in the anthropogenically modified Ems Estuary (Germany, Netherlands) was modeled using a machine learning approach (Random Forest) that drew on 8 years of acoustic monitoring data with C-PODs together with environmental data. Harbor porpoises were present year-round at both stations. According to the models, their detection was mainly explained by season, tide, and noise level, with the highest detection probabilities in spring, at high tide, and at low noise levels. The seasonal and tide-dependent occurrence of harbor porpoises coincided with prey availability. Presumed feeding activity was detected in 47% of all harbor-porpoise-positive 10 min blocks and indicated the importance of the estuary as a regular feeding area. The elevated noise levels detected at one station were attributed to tidal-induced currents and sediment movements. The results of this study can help to improve estuarine management through measures that include conducting dredging and disposal activities when harbor porpoise occurrence is less likely.
... Because echolocation clicks are highly directional and quiet, foraging behaviors such as bottom grubbing are less likely to be recorded by C-PODs (Akamatsu et al., 2005;Schaffeld et al., 2016). Sandeels are important prey for porpoises in the MF during summer (Santos & Pierce, 2003;Santos et al., 2004). Smith Bank had the highest probability of porpoise occurrence (Figure 2), and also provides good habitat for sandeels (Holland et al., 2005;Hopkins, 2011;Wright et al., 2000); however, a low P F was recorded in this area ( Figure 3 and Table 3). ...
Article
Full-text available
Understanding spatiotemporally varying animal distributions can inform ecological understanding of species' behavior (e.g., foraging and predator/prey interactions) and support development of management and conservation measures. Data from an array of echolocation-click detectors (C-PODs) were analyzed using Bayesian spatiotemporal modeling to investigate spatial and temporal variation in occurrence and foraging activity of harbor porpoises (Phocoena phocoena) and how this variation was influenced by daylight and presence of bottlenose dolphins (Tursiops truncatus). The probability of occurrence of porpoises was highest on an offshore sandbank, where the proportion of detections with foraging clicks was relatively low. The porpoises' overall distribution shifted throughout the summer and autumn, likely influenced by seasonal prey availability. Probability of porpoise occurrence was lowest in areas close to the coast, where dolphin detections were highest and declined prior to dolphin detection, leading potentially to avoidance of spatiotemporal overlap between porpoises and dolphins. Increased understanding of porpoises' seasonal distribution, key foraging areas, and their relationship with competitors can shed light on management options and potential interactions with offshore industries.
... They are opportunistic feeders, feeding mostly on the continental shelf, often targeting demersal or benthic 30 species (ex. Santos & Pierce, 2003; but see Nielsen et al. 2018). Coined the 'aquatics shrews' of the sea, prey availability has been shown to be an important driver of porpoise movements (Johnston et al., 2005;Sveegaard et al., 2012;Wisniewska et al., 2016) and local densities (Hammond et al., 2013;Marubini et al., 2009;Waggitt et al., 2018). ...
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Understanding a species response to past environmental changes can help forecast how they will cope with ongoing climate changes. Harbor porpoises are widely distributed in the North Atlantic and were deeply impacted by the Pleistocene changes with the split of three sub-species. Despite major impacts of fisheries on natural populations, little is known about population connectivity and dispersal, how they reacted to the Pleistocene changes and how they will evolve in the future. Here, we used phylogenetics, population genetics, and predictive habitat modelling to investigate population structure and phylogeographic history of the North Atlantic porpoises. A total of 925 porpoises were characterized at 10 microsatellite loci and one-quarter of the mitogenome (mtDNA). A highly divergent mtDNA lineage was uncovered in one porpoise off Western Greenland, suggesting that a cryptic group may occur and could belong to a recently discovered mesopelagic ecotype off Greenland. Aside from it and the southern sub-species, spatial genetic variation showed that porpoises from both sides of the North Atlantic form a continuous system belonging to the same subspecies (Phocoena phocoena phoceona). Yet, we identified important departures from random mating and restricted intergenerational dispersal forming a highly significant isolation-by-distance (IBD) at both mtDNA and nuclear markers. A ten times stronger IBD at mtDNA compared to nuclear loci supported previous evidence of female philopatry. Together with the lack of spatial trends in genetic diversity, this IBD suggests that migration-drift equilibrium has been reached, erasing any genetic signal of a leading-edge effect that accompanied the predicted recolonization of the northern habitats freed from Pleistocene ice. These results illuminate the processes shaping porpoise population structure and provide a framework for designing conservation strategies and forecasting future population evolution.
... eyond! 12! years! of! age! (Read,! 1999).! In! Scandinavian! waters! the! mating! season! is! between! June! and! August! and! after! approximately!ten!months!the!female!gives!birth!to!a!single!calf,!which!is!then!nursed!for! up!to!nine!months! (Börjesson!&!Read,!2003).!Adult!harbour!porpoises!feed!primarily!on! small! benthic! and! pelagic! fish! (Santos! &! Pierce,! 2003).! It! is! considered! likely! that! the! distribution! and! abundance! of! harbour! porpoises! is! strongly! influenced! by! the! availability!and!distribution!of!their!prey!(e.g. !&!Booth!et!al.,!2013,! but!this!has!not!yet!been!confirmed.! Like! other! toothed! whales! harbour! porpoises! communicate,! navigate! and! hunt! with! help! ...
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The harbour porpoise (Phocoena phocoena) is the only cetacean regularly found in Swedish waters. Little is known about its movements and behaviour. Around the peninsula of Kullaberg, harbour porpoises are regularly found in relatively large numbers and they are considered to use the area for both feeding and reproduction. Along the northern coast of Kullaberg six passive acoustic data loggers; Cetacean-Porpoise Detectors (C-PODs, Chelonia Ltd., Mousehole, UK) were deployed from April to August, 2014, to investigate the acoustic activity of harbour porpoises in these waters. In addition, opportunistic visual boat based observations were collected during a total of 27.5 hours between June and August, 2014. Data from a citizen science project in the same area and during the same time period was also used. A gradient of decreasing acoustic activity was found along the coast, ranging from extremely high activity at the tip of the peninsula to very few detections 8 km further east. During several periods of data collection, there was a very pronounced diurnal pattern in the acoustic activity closer to the tip of the peninsula, with detections peaking around midnight. There were also large effects on the acoustic activity caused by the lunar cycle. The visual observations confirm high porpoise abundance around Kullaberg, with ratios of calves to adult individuals supporting existing ideas of Kullaberg as a nursing area. This data provides detailed insights into the activities of harbour porpoises in an area that may be of great importance for the protection of this threatened species in Swedish waters.
... Harbour porpoises are believed to have a very high energy demand due to their large body surface to volume ratio and their high metabolism, and thus prey availability is of high importance to them (Kastelein, 1998). Some information on the diet of harbour porpoises in the North Sea is available through the analyses of stranded and bycaught animals (Börjesson et al., 2003;Santos & Pierce 2003;Santos et al., 2004;Jansen et al., 2012). How porpoises use their sonar to find and catch their prey has been described in captivity (e.g. ...
Technical Report
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This report provides an assessment of the feasibility of using a Passive Acoustic Monitoring (PAM) network on the Dutch Continental Shelf (DCS) to monitor harbour porpoises. It provides a description of what static PAM is currently able to do. It gives an overview of the technical and logistical requirements when using PAM at sea and the analytical limitations when using PAM to monitor harbour porpoises.
... This includes Secondly, harbour porpoises are vulnerable to noise pollution because their hunting and communication are largely dependent on acoustic signals. Thirdly, they are vulnerable to fishing activities because they drown due to accidental capture in fishing gear [9] and because harbour porpoises partly depend on fish species that are also targeted by human fisheries [10]. ...
Article
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Harbour porpoises (Phocoena phocoena) in the North Sea live in an environment heavily impacted by humans, the consequences of which are a concern for their health. Autopsies carried out on stranded harbour porpoises provide an opportunity to assess health problems in this species. We performed 61 autopsies on live-stranded harbour porpoises, which died following admission to a rehabilitation centre between 2003 and 2016. The animals had stranded on the Dutch (n = 52) and adjacent coasts of Belgium (n = 2) and Germany (n = 7). We assigned probable causes for stranding based on clinical and pathological criteria. Cause of stranding was associated in the majority of cases with pathologies in multiple organs (n = 29) compared to animals with pathologies in a single organ (n = 18). Our results show that the three most probable causes of stranding were pneumonia (n = 35), separation of calves from their mother (n = 10), and aspergillosis (n = 9). Pneumonia as a consequence of pulmonary nematode infection occurred in 19 animals. Pneumonia was significantly associated with infection with Pseudalius inflexus, Halocercus sp., and Torynurus convolutus but not with Stenurus minor infection. Half of the bacterial pneumonias (6/12) could not be associated with nematode infection. Conclusions from this study are that aspergillosis is an important probable cause for stranding, while parasitic infection is not a necessary prerequisite for bacterial pneumonia, and approximately half of the animals (29/61) probably stranded due to multiple causes. An important implication of the observed high prevalence of aspergillosis is that these harbour porpoises suffered from reduced immunocompetence.
Article
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Understanding species responses to past environmental changes can help forecast how they will cope with ongoing climate changes. Harbor porpoises are widely distributed in the North Atlantic and were deeply impacted by the Pleistocene changes with the split of three sub‐species. Despite major impacts of fisheries on natural populations, little is known about population connectivity and dispersal, how they reacted to the Pleistocene changes and how they will evolve in the future. Here, we used phylogenetics, population genetics, and predictive habitat modelling to investigate population structure and phylogeographic history of the North Atlantic porpoises. A total of 925 porpoises were characterized at 10 microsatellite loci and one‐quarter of the mitogenome (mtDNA). A highly divergent mtDNA lineage was uncovered in one porpoise off Western Greenland, suggesting that a cryptic group may occur and could belong to a recently discovered mesopelagic ecotype off Greenland. Aside from it and the southern sub‐species, spatial genetic variation showed that porpoises from both sides of the North Atlantic form a continuous system belonging to the same subspecies (Phocoena phocoena phocoena). Yet, we identified important departures from random mating and restricted dispersal forming a highly significant isolation‐by‐distance (IBD) at both mtDNA and nuclear markers. A ten times stronger IBD at mtDNA compared to nuclear loci supported previous evidence of female philopatry. Together with the lack of spatial trends in genetic diversity, this IBD suggests that migration‐drift equilibrium has been reached, erasing any genetic signal of a leading‐edge effect that accompanied the predicted recolonization of the northern habitats freed from Pleistocene ice. These results illuminate the processes shaping porpoise population structure and provide a framework for designing conservation strategies and forecasting future population evolution.
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