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Marine mammal culling programs: Review of effects on predator and prey populations


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Culling is widely practised as a means to reduce predation effects of terrestrial carnivores, birds and marine mammals in many parts of the world. Of marine mammals, coastal pinniped species have usually been the target of culling programs, but dolphins and a large odontocete have also been culled. We reviewed the published literature on marine mammal culling programs to evaluate the extent of their efficacy as a fisheries management measure. Changes in species' distributions and abundance demonstrate that culling programs can be very effective at reducing predator density. Several conclusions from experimental studies of terrestrial mammals and birds may also apply to marine mammal control. Firstly, predator removal generally increases productivity and population size of target prey populations, but not always. Secondly, culling programs typically involve a large proportional reduction (>50%) in predator populations. Thirdly, the effects of culling are typically dependent on continued control, and in the absence of control the population rapidly returns to pre‐culling density. This underscores the need for predator removal to be a long‐term management strategy. Fourthly, culling predators often has non‐intuitive and unintended consequences for target species and for other predator and prey species. Marine mammal culling programs rarely have measurable objectives with respect to prey populations, and their success has not been evaluated. Culling marine mammals is controversial because of the following: (i) they are high‐profile charismatic megafauna; (ii) many populations are recovering from a period of over‐exploitation while others remain threatened or endangered; and (iii) the scientific evidence needed to justify a cull is usually highly uncertain. Marine mammal culling programs should be based on scientific analysis with stated and measurable objectives to be evaluated during planned follow‐up monitoring.
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Marine mammal culling programs: review of effects on
predator and prey populations
W. D. BOWEN* Population Ecology Division, Bedford Institute of Oceanography, Dartmouth, NS B2Y
4A2, Canada. E-mail:
Damian LIDGARD Biology Department, Dalhousie University, Halifax, NS B3H 4JI, Canada.
cetaceans, pinnipeds, predator control, seals
*Correspondence author.
Submitted: 3 August 2011
Returned for revision: 20 October 2012
Revision accepted: 26 April 2012
Editor: KH
1. Culling is widely practised as a means to reduce predation effects of terrestrial
carnivores, birds and marine mammals in many parts of the world. Of marine
mammals, coastal pinniped species have usually been the target of culling pro-
grams, but dolphins and a large odontocete have also been culled.
2. We reviewed the published literature on marine mammal culling programs to
evaluate the extent of their efficacy as a fisheries management measure.
3. Changes in species’ distributions and abundance demonstrate that culling pro-
grams can be very effective at reducing predator density.
4. Several conclusions from experimental studies of terrestrial mammals and
birds may also apply to marine mammal control. Firstly, predator removal gener-
ally increases productivity and population size of target prey populations, but not
always. Secondly, culling programs typically involve a large proportional reduction
(>50%) in predator populations.
5. Thirdly, the effects of culling are typically dependent on continued control, and
in the absence of control the population rapidly returns to pre-culling density.
This underscores the need for predator removal to be a long-term management
strategy. Fourthly, culling predators often has non-intuitive and unintended con-
sequences for target species and for other predator and prey species.
6. Marine mammal culling programs rarely have measurable objectives with
respect to prey populations, and their success has not been evaluated. Culling
marine mammals is controversial because of the following: (i) they are high-
profile charismatic megafauna; (ii) many populations are recovering from a
period of over-exploitation while others remain threatened or endangered; and
(iii) the scientific evidence needed to justify a cull is usually highly uncertain.
7. Marine mammal culling programs should be based on scientific analysis with
stated and measurable objectives to be evaluated during planned follow-up
Some 270 species comprise the Carnivora, a diverse group
of terrestrial and aquatic species representing 11 families of
mammals. Many of these species limit or regulate prey
species populations and their top-down effects alter the
structure and functioning of ecosystems, either through
control of herbivore populations or of medium-sized
predators when predation pressure from top predators is
reduced (Crooks & Soule 1999, Terborgh et al. 1999,
2001, Berger et al. 2001, Johnson et al. 2007). Because of
these effects and the prey species consumed (which are
often commercially valuable), carnivore–human conflicts
are common and widespread (Treves & Karanth 2003).
Although these conflicts have a long history (wolf Canis
lupus control to protect livestock dates back 2500 years in
Mammal Review ISSN 0305-1838
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Greece; Reynolds & Tapper 1996), expansion of human
activities combined with the recovery of over-exploited
animal populations has led to an increase in contact and
conflict between people and predators in many parts of the
world (Woodroffe & MacDonald 1995, Berger 2006).
Predation by marine mammals on fish and shellfish species
of commercial importance also has been a source of conflict
with fisheries for centuries (e.g. Merriam 1901, Worthington
1964, Fiscus 1979, Gulland 1987, Bearzi et al. 2004). Increas-
ing pressure on fisheries (Anonymous 2010) coupled with an
increase in the perception of conflict with marine mammals,
particularly pinnipeds, has renewed an interest in the use of
culling as a management tool to alleviate conflicts (e.g.
Anonymous 2011). The perceived conflict between harvesters
and marine mammals underlies recent calls for increased
whaling in the context of ecosystem management and the
‘whales eat fish’ argument (Gerber et al. 2009).
The objectives of this review are fourfold. The first is
briefly to review predator effects on prey populations. The
second is to evaluate the efficacy of marine mammal preda-
tor control (i.e. culling – killing animals for a specific
purpose, e.g. to reduce the perceived negative effects of a
predator on prey species of interest to humans) on prey
populations. In most cases, this amounts to determining if
culling predators increases the productivity and population
size of the target prey species. Although culling has been
used widely to control marine mammal species, the results
of those programs have not been comprehensively reviewed.
The third is to draw some lessons about predator control as
a management tool. Here, we draw on experimental studies
from the terrestrial and freshwater literature, as there are
few experimental studies of the efficacy of culling in marine
ecosystems. The fourth is to provide recommendations with
respect to culling marine mammals.
We reviewed the literature by querying the Web of Science
for articles containing the following topic expressions, pub-
lished during the period 1900–2010: vertebrate predator
control, vertebrate predator removal, predator–human con-
flict, predator–prey conflict, seal-fishery, cull* and seal pre-
dation. Additional primary literature and government
reports referenced in papers identified in the Web of Science
queries were also consulted.
Predator–prey background
Predator control is based on the assumption that predators
limit prey abundance and that a decrease in predators will
increase prey productivity or abundance. Many consider that
removing predators should increase prey populations, but
predator–prey interactions usually are far too complex to
assume this. For example, if prey are limited by habitat, then
removal of predators has little impact. The success of preda-
tor removal depends on what fraction of natural mortality is
caused by the predator and how other sources of natural
mortality interact. Thus, to manage predator and prey popu-
lations successfully, it is important to know if there is indeed
an impact of predators on prey and then to quantify this
impact (Graham et al. 2005). Sinclair et al. (1998) attempted
to predict the size of the prey population needed to overcome
predation effects and the degree of predator reduction
needed to allow prey populations to increase. Assuming that
predation was known to be the cause of the decline, Sinclair
et al. (1998) observed that to counteract predation effects
requires knowledge not only of the degree of predation but
also of the nature of the functional response, because the
stability of the interaction differs depending on the type of
functional response. Thus, three types of evidence from a
predator–prey interaction are relevant to management and
the decision to cull. Firstly, do per capita rates of change for
prey increase or decrease with declining prey densities?
Secondly, is predation depensatory or density dependent?
Thirdly, what is the magnitude of predation?
The view that predators can limit prey populations has
been a source of debate dating back to the middle of the last
century (Nicholson 1933, Andrewartha & Birch 1954).
Population limitation occurs when factors, such as preda-
tion, reduce the rate of population growth to limit the
population below its carrying capacity (Sinclair & Pech
1996). There are two schools of thought on the effects of
predation. The ‘compensatory hypothesis’ states that preda-
tors consume the proportion of the prey population that
would have suffered natural mortality in the absence of
predation (Errington 1946, Jenkins et al. 1963). Under this
hypothesis, control of predators would not be expected to
result in larger or more productive prey populations. By
contrast, the ‘additive hypothesis’ states that predation
causes mortality above the level that would have occurred in
the absence of predation and thus can limit prey population
numbers (Valkama et al. 2005). Under this hypothesis,
predator removal can be expected to result in either
increased productivity or abundance of prey. Both hypoth-
eses have received support in studies of wild populations
(see, e.g. references in Holt et al. 2008). There is almost cer-
tainly a continuum of responses, with the compensatory
mortality hypothesis at one end of the continuum and the
additive mortality hypothesis at the other. Nevertheless, Sin-
clair and Pech (1996) have argued that completely additive
mortality and exact compensation are unlikely conditions.
Predators can exert density dependent, inverse density
dependent and more complex effects on prey populations
(Sinclair & Pech 1996). The total response of predators to
Marine mammal predator control W. D. Bowen and D. Lidgard
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changes in prey density is the product of two components –
the functional response and the numerical response – and
represents the total proportional mortality imposed by the
predator on its prey. At high prey density, both functional
and numerical responses reach an asymptote because of
handling time and satiation in the former and interference
in the latter. This means that the prey will experience
decreasing proportional mortality, i.e. predation is depensa-
tory at high prey density.
Effects of predators on prey populations depend on the
form of the functional response (Sinclair & Pech 1996). If
the predator functional response is type II, the proportional
mortality on prey is dispensatory at all prey densities. If the
functional response is type III, then the accelerating part of
the curve at low prey density has a density-dependent effect
on prey, whereas the decelerating part at high prey density is
depensatory. If prey density is low and the predation rate is
greater than recruitment, predators can drive prey extinct.
For this to happen, predators must have a type II functional
response, no density dependence in numerical response,
and predators must depend primarily on another prey.
Examples of this are reported for wolves driving caribou
Rangifer tarandus extinct while depending on moose Alces
alces (Seip 1992), and for small carnivores driving passerine
bird populations extinct (Terborgh 1992).
Culling marine mammals
Culling marine mammals ostensibly to protect fish stocks
has a long history (Smith 1995, Lavigne 2003, Bearzi et al.
2004) and has been undertaken in many parts of the world.
This history involves culls on eight species of pinnipeds and
a similar number of odontocete species in 14 countries
(Table 1). In Canada and the USA, culling programs were
conducted on both the east and west coasts, including in
Alaska, USA. With the exception of the selective cull of
Cape fur seals Arctocephalus pusillus pusillus in South Africa
to reduce predation on Cape gannets Morus capensis
(Makhado et al. 2009), conflicts with marine mammals are
about perceived economic loss and the belief that consump-
tion by marine mammals represents losses that would
otherwise be available to fisheries.
Three of 15 phocids and five of 17 otariid species have been
culled (Fig. 1). Grey seals Halichoerus grypus, harbour seals
Phoca vitulina and ringed seals Histriophoca fasciata have
been the targets of control of phocid species (Table 1).
These species and the five otariid species have coastal distri-
butions for at least part of the year, making them accessible
to hunters. The sole member of the Odobenidae, the walrus
Odobenus rosmarus, apparently has not been culled.
One of the first large-scale control programs was under-
taken in the Baltic Sea against grey and ringed seals
(Table 1) which were considered competitors with fisher-
men. Several bounty systems were introduced to reduce the
seal stocks and protect stocks of Atlantic cod Gadus morhua,
herring Clupea harengus and sprat Sprattus sprattus.Grey
seals were culled for 38 years beginning in 1889. This was
followed by decades of culls of ringed seals beginning in
1889 in Denmark (36 years), 1903 in Sweden (64 years) and
1909 in Finland (60 years; Harding & Harkonen 1999).
Bounties dramatically reduced the seals’ populations, by
perhaps 80% in the 1950s. This decrease was followed by a
second decline in the 1960s because of reduced reproductive
success caused by pollutants (Harding & Harkonen 1999).
There appears to have been no evaluation of the impacts of
the culls on prey populations.
During the same period, seals and sea lions were consid-
ered competitors with fisheries in western North America
(Table 1). In 1899, the California State Board of Fish
Commissioners authorized a two-year cull of California
sea lions Zalophus californianus on the grounds that sea
lions were ‘highly destructive of the salmon fishery’
(Oncorhynchus spp.; Merriam 1901). In British Columbia,
Canada, harbour seals were culled for almost 50 years,
beginning in 1914, despite the fact that the harbour seal
population had been decimated by a long period of com-
mercial harvesting that ended in 1914 (Olesiuk 2009).
Although more than 2900 harbour seals per year were
reported killed for bounty in most years between 1914 and
1963, at least as many kills probably went unreported (Bigg
1969). The cull held the population roughly stable, at
perhaps 40% of historical abundance, until another period
of commercial harvesting dramatically reduced the popula-
tion to about 10% of the estimated historical population
size (Olesiuk 2009). Steller sea lion Eumetopias jubatus
numbers were also controlled along the British Columbia
coast until their protection in 1970 (Table 1). The control
programs and commercial harvests reduced the population
to about one-quarter to one-third of historic levels (Bigg
1984, 1985). In order to protect coastal fisheries in Oregon,
USA, control programs were conducted against harbour
seals, northern sea lions Callorhinus ursinus and California
sea lions for seven years, beginning in 1925 and again
for a period of three decades ending in 1967. These culls
were thought to have reduced harbour seal numbers by
about 50%. Sea lions declined by an unknown amount.
Further north in Alaska, control of harbour seals and
Steller sea lions began in 1927 to attempt to protect Pacific
salmon Oncorhynchus spp. fisheries (Table 1). For almost
50 years, both culls and bounties were employed, but little
effort was given to determining by how much the seal
populations were reduced or how salmon populations
W. D. Bowen and D. Lidgard Marine mammal predator control
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Table 1. Culls conducted to reduce numbers of marine mammal predators to protect fisheries. From a Web of Science search for papers published in 1900–2010
Species Dates Location Target prey Predator reduction Prey response Source
Grey and ringed seal 1889–1927, 1941–77
Denmark; 1903–67
Sweden; 1909–18,
1924–75 Finland
Baltic Sea Cod, herring, sprat Populations reduced by
c. 80%
None at time, recent
evaluation using
ecosystem model
Harding & Harkonen
1999, Hansson et al.
Harbour seal 1891–1905; 1937–45
Maine; 1888–1962
Eastern USA Cod, mackerel Scomber
scombrus, other fishes,
salmon, lobster
Homarus americanus
Population was reduced
greatly; estimate
claimed for bounty
Unknown Lelli & Harris 2006, Lelli
et al. 2009
Harbour seal 1914–63 British Columbia, Canada Herring, salmon Population reduced 60%
from historical level
Unknown Olesiuk 2009
Harbour seal 1927–67 Alaska, USA Salmon in gill net fishery Bounty and cull, not clear
how much populations
were reduced
Unknown Lensink 1958
Harbour seal 1924–33; 1936–67 Columbia River, North
Salmon c. 50% reduction in
harbour seals
Unknown Pearson & Verts 1970
Harbour seal 1927–76 Nova Scotia, Canada Inshore fisheries Population was
substantially reduced
Unknown Boulva & McLaren 1979
Grey seal 1934; 1962–65 – pup
culls; 1978 – pups and
adult females
UK Salmon Population continued to
Unknown Harwood & Greenwood
1985, Bonner 1989
Grey seal 1967–83 Eastern Canada,
excluding Sable Island
Inshore fishing gear
and/or catch: cod,
salmon, mackerel,
herring, lobster
Population continued to
Unknown Mansfield & Beck 1977,
Zwanenburg et al.
Grey seal 1978–90 Gulf of St. Lawrence,
Not specified Culling pups – population
briefly stabilized
Unknown Lavigueur & Hammill
Grey seal 1980–90; 2003– Norway Coastal fisheries Bounty seems to have
stabilized population
Unknown Nilssen & Haug 2007
Grey and harbour
1982– Iceland Cod Grey seal reduced by c.
50% and eliminated
from northeast coast;
harbour seal reduced
by 66%
No formal evaluation, cod
biomass fluctuated
without trend
Hauksson & Bogason
1997, Anonymous
Marine mammal predator control W. D. Bowen and D. Lidgard
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Grey and harbour
1993–2004; 2005– Scotland Salmon Harbour seal population
reduced by 25–55%,
grey seal no trend
Management plan to
evaluate success with
respect to objectives
agreed in 2005
Thompson et al. 2007,
Butler et al. 2008,
California sea lion c. 1897–99 California, USA Salmon Unknown Unknown Merriam 1901
California sea lion 2005– Oregon, Columbia River,
Salmon Selective on few
Steller sea lion 1913–16; 1923–39 British Columbia, Canada Salmon Local extinction Salmon stock went
extinct some time after
sea lions exterminated
Bigg 1985, McKinnell
et al. 2001, Olesiuk
pers. comm. 2010
Steller sea lion 1927–67 Alaska, USA Salmon in gill net fishery Bounty and cull, not clear
how much populations
were reduced
Unknown Lensink 1958
Cape fur seal 1921; 1928–29;
Tasmania, Australia Taken from seriously
depleted population
Unknown R. Kirkwood pers.
comm., Ling 1999
Northern and
California sea lion
1925–31 Oregon, USA Coastal commercial
Sea lions declined by
unknown amount
Unknown Pearson & Verts 1970
Cape fur seal 1993– Namibia Commercial fishes Mainly pups killed,
population declining
Cape fur seal 1993–2001; 2007 South Africa Cape gannet Targeted removal of 153
selected individuals
Seal predation mortality
reduced by c. 7%;
effects short term
David et al. 2003;
Makhado et al. 2009
Dolphins c. 1910–82 Japan Yellowtail, damage to
gear and catch
No evaluation of success
of culling; unknown
effects on dolphin
No evidence that cull had
any positive impact on
Kasuya 1985
Short-beaked common
dolphin, bottlenose
dolphin Tursiops
Historical times;
1940–60 (Croatia);
1930–60 (Italy)
Adriatic Sea Fisheries in general Dolphin populations
declined by unknown
No evaluation of effects
on fisheries
Bearzi et al. 2004
Killer whale 1969–80 Norway Herring 700 whales Stock failed to recover
until mid-1980s
Oien 1988, Dragesund
et al. 1997
W. D. Bowen and D. Lidgard Marine mammal predator control
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On the east coast of North America, bounties for harbour
seals began in the late 1880s in Maine and Massachusetts,
USA, and in about 1927 in Nova Scotia, Canada, to reduce
the number of seals ‘harassing’ and competing with fisher-
men (Table 1, Boulva 1973). What appears to be the longest
control program (74 years) took place in Massachusetts
from 1888 to 1962. In Massachusetts and Maine together,
beginning in 1891, it is estimated that between 72000 and
136000 seals were killed for the bounty (Lelli et al. 2009).
Although the extent of reduction is not known, it was clear
that the populations were greatly reduced. Further north in
Nova Scotia, culling did not commence until some 40 years
later, but continued for 50 years, ending in 1976. Although
aimed at harbour seals, the bounty included an unknown
proportion of grey seals until 1949 when the submission of
a jaw was required to claim a bounty. This long period of
hunting greatly reduced the harbour seal population,
although again quantitative estimates of the reduction are
not available. In each of the above jurisdictions, there
appears to have been no analysis of the benefit of these
long-standing culls on fish stocks.
In Norway, grey seals have been hunted along the coast
for centuries. Between 1980 and 1990, a culling program
was instigated along southern and central coastal areas
(Nilssen & Haug 2007). Additional grey seals were shot by
local hunters during this period, but the numbers are
unknown. Since 2003, in areas of particular conflict with
grey seals, Norwegian management authorities have used
hunting to control the grey seal population size by permit-
ting a quota of 20–30% of the estimated population,
assessed every 5 years. Although a time series of estimates is
not available, there was probably no significant difference in
population estimates in 1996–98 and in 2001–03; both
suggest a population of 4000–6000 seals. There seems to
have been no evaluation of the effects of controlling seals on
Grey seals have also been hunted for centuries in Iceland
(Hauksson 2007). The shooting of grey seals is allowed in all
areas except the west coast and a bounty program was initi-
ated in 1982 to address conflicts with fisheries. Grey seals
and harbour seals are thought to compete with fisheries and
to show a preference for cod (Hauksson & Bogason 1997),
although the authors of the study provide no evidence for
either conclusion other than that cod are eaten by seals.
Increased hunting beginning in 1990 resulted in a reduction
in the distribution of grey seals and their disappearance
along the northeast coast. Abundance estimates indicated
that the size of the grey seal population had declined by
about 3% per year from 1982 to 1990 and by 6% per year
from 1990 to 2002, due to the increased hunting effort. By
comparison, the harbour seal population was reduced by
66%. During this period of culling, cod spawning stock
biomass fluctuated without trend and average recruit-
ment declined slightly, suggesting no obvious population
response to the seal culls (Anonymous 2008).
Complaints about the effects of seals on fisheries in the
UK are thought to have begun in the 1920s, in connection
with damage done to Atlantic salmon Salmo salar popula-
tions by grey seals (Rae 1960). As a result, predator control
programs against grey seals were undertaken in the 1930s to
protect salmon (reviewed by Harwood & Greenwood 1985,
Bonner 1989). In the early 1960s, the grey seal population
1Arctocephalus pusillus doriferus
1880 20201920 1960 200019801900 1940
Grey and
Ringed seal
Harbour seal
Steller sea lion
California and
Northern sea lion
Cape fur seal
Maine, USA
Massachusetts, USA
British Columbia, Canada
Alaska, USA
Nova Scotia, Canada
Eastern Canada
California, USA
British Columbia, Canada
USA Oregon, USA
Oregon, USA
South Africa
Australian fur seal1
Tasmania, Australia
Fig. 1. Timeline and locations of pinniped cull programs.
Marine mammal predator control W. D. Bowen and D. Lidgard
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had increased to the point where control in Orkney (Scot-
land) and the Farne Islands (England) was proposed to
reduce the population to three-quarters of its current size
by killing moulted pups. Although culling began in 1963, by
1965 on the Farne Islands the National Trust, owner of the
Islands, decided that culling seals was not consistent with
the goals of the reserve and did not permit further killing.
Further complaints of damage to fisheries led to a new
control program, which would have reduced the Orkney
and the Outer Hebrides (Scotland) populations from 50000
in 1976 to 35000 in the end of 1982, through killing both
adult females and pups. However, public protest after the
first cull in 1978 on the Orkneys resulted in a return to pup
culling only, which had already proved inadequate to limit
population growth and had the further undesirable feature
that the effects of culling were not evident in pup produc-
tion figures until about 6 years after culling began
(Harwood & Prime 1978). Side effects from the adult female
cull were also evident. Firstly, some females failed to return
to breed and, secondly, some females that came ashore to
breed abandoned their pups when the colony was disturbed
(Summers & Harwood 1979). While these unintended
effects could be seen as increasing the effectiveness of the
cull, they also raise ethical issues in terms of responsible
management. There seems to have been no evaluation of
the effectiveness of this limited control program on fish
The largest sustained cull of a pinniped occurs in
Namibia, where since 1993 large numbers of Cape fur seal
pups have been killed during the breeding season. The cull
began during a period of declining fish stocks thought to be
partly caused by poor environmental conditions. Justified to
protect fish stocks, the number of fur seal pups culled
increased from c. 50000 in 1993 to 85000 in 2009. Annually
since 2006, an additional 6000 adult males have been culled.
There appears to be no published scientific analysis of the
predation mortality caused by fur seals or how this mortal-
ity compares with other sources of natural mortality on
target fish stocks. Again, there appears to have been little
effort to evaluate the impact of the seal culls on fish stock
productivity, although productivity did increase with the
return of favourable environmental conditions in the late
1990s (
Pinniped culls have typically been non-selective and have
resulted in the killing of large numbers of animals.
However, recently several selective culls have been used to
reduce predation mortality caused by seals (Table 2). Cape
fur seals prey upon young Cape gannets. Between 1993 and
2001 and again in 2007, the targeted removal of individual
fur seals known to have eaten gannets successfully reduced
mortality. Selective removal of small numbers of individuals
has also been used since 2005 to reduce predation by
Californian sea lions on endangered populations of Pacific
salmon, including steelhead Oncorhynchus mykiss, on the
Columbia River, Oregon (Stansell et al. 2010).
Based on the published literature, it appears that in about
46% of the populations culled (often in multiple time
periods, see Table 1) the extent of the resulting population
decline is unknown (Table 2). Overall, 79% of culled popu-
lations declined, whereas the remainder are thought or
known to have increased or remained stable (Table 2).
There appear to have been two local extinctions: Steller sea
lions at a site in British Columbia and grey seals along the
northeast coast of Iceland.
Culling dolphins has an equally long or perhaps even
longer history than culling pinnipeds, having occurred in
the Adriatic Sea (Bearzi et al. 2004) and the Black Sea
(Mitchell 1975, Birkun 2002) in historical times. A culling
campaign against dolphins, mainly short-beaked common
dolphins Delphinus delphis, was launched in 1949 in
Croatia with the intent of eradicating them from the
Adriatic Sea (Table 1). Bounties to promote the killing
of dolphins were also paid in Italy from the early 1930s.
Although the number of animals killed is poorly docu-
mented, dolphin populations are thought to have declined
by an unknown amount.
Dolphins were viewed as competitors in the yellowtail
Seriola quinqueradiata fishery in Japan, so fishermen were
paid to cull five species from about 1910 to 1982 (Table 1).
As there were no species-specific estimates of the size of
these dolphin populations, it was not possible to determine
the impact of culls on the dolphin species, or their effect on
fishery interactions, with any confidence (Kasuya 1985).
Apparently, the only large odontocete species to have
been culled is the killer whale Orcinus orca (Table 1). When
the Norwegian spring-spawning herring population col-
lapsed in the late 1960s, mainly as a result of massive over-
fishing, the government organized a hunt of resident killer
whales which were known to eat herring. Over 700 whales
Table 2. Pinniped population trends at the end of culling programs
listed in Table 1
Population status at end of cull Number of populations
Increasing 2
Stable 3
Decline >75% 3
Decline >25% <75% 3
Decline of unknown extent 11
Local extinction 2
Total* 24
*Excluding two selective culls involving small numbers of Cape fur seals
and California sea lions.
W. D. Bowen and D. Lidgard Marine mammal predator control
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were killed between 1969 and 1980 (Oien 1988). During this
period, the herring stock remained at very low levels and
showed no signs of recovery until the mid-1980s (Drage-
sund et al. 1997). Killer whale predation appears not to have
been considered further as a limiting factor on herring stock
dynamics, although northern minke whale Balaenoptera
acutorostrata predation has been investigated (Tjelmelan &
Lindstrøm 2005).
Predicting the effects of predator removal
on prey populations and ecosystems
Although predators have frequently been culled to alleviate
conflict with terrestrial and marine mammals, little atten-
tion has been given to evaluating the success of culls. Deter-
mining the benefit to a fishery or hunt of having reduced a
predator population is confounded by several aspects of
predator–prey ecology (DeMaster & Sisson 1992). Prey
species usually have more than one predator and thus gains
from culling may be offset by changes in the functional,
aggregative or numerical responses of other predators.
Predators are rarely dependent on a single prey and this
could result in either greater or lesser impact depending on
the functional response of the predator. In the marine envi-
ronment, recruitment to fish populations is highly variable
and difficult to predict but has a large impact on prey abun-
dance. This last point led participants of the Benguela
Ecology Programme Workshop to conclude that compari-
son of fishery yields before and after a seal cull ‘would
almost certainly not provide a reliable indication of its
effect’ (Butterworth & Harwood 1991). Fish usually are the
dominant predator in aquatic ecosystems that have been
studied (e.g. Bax 1991, Trites et al. 1997) and so reducing
populations of marine mammals is likely to produce only
marginal increases in yield that could be difficult to detect
(Gulland 1987, DeMaster & Sisson 1992). Finally, because
of the inherent indeterminacy of outcomes of multispecies
manipulations (Yodzis 1988), an experimental approach to
evaluating the benefit of a cull is often impracticable.
As indicated above, the magnitude of declines is often not
known for marine mammal populations that have been
culled. Thus, it should not be surprising to learn that the
more difficult goal of evaluating the benefit to prey popula-
tions has rarely been attempted. When an evaluation was
done, it was generally not part of the control program itself.
For example, despite the long history of non-sustainable
harvesting and culling of both harbour seals and Steller sea
lions in British Columbia, only recently have analyses shed
light on the probable impacts of previous harbour seal
culls on target prey populations (P. Olesuik pers. comm.).
Harbour seals in the Strait of Georgia, British Columbia,
feed mainly on Pacific hake Meluccius productus and Pacific
herring Clupea pallasii. Herring stock assessments indicate
that herring survival rates declined as harbour seal preda-
tion increased. Selective predation by seals may also explain
an observed decline in the mean weight at age of herring
and size at age of hake, as seal populations increased. In the
1980s, hake consumed mainly juvenile hake and juvenile
herring; however, the smaller hake now feed on euphausiids.
It has been hypothesized that reduced cannibalism by older
hake has led to improved recruitment and higher numerical
abundance of young hake. Thus, hake biomass has
remained stable. Overall predation on herring appears not
to have changed much, but seals have displaced hake as the
primary herring predator. Recruitment rates of herring have
also increased, and there is a positive relationship between
seal abundance and herring recruitment (presumably
because seals are removing large hake which feed on juve-
nile herring). Given these interactions, the previous culling
of harbour seals from this system may have had complex
consequences for prey population dynamics, but those con-
sequences were not explicitly evaluated.
The potential consequences of seal culling in the Baltic
Sea on target fish populations over the last century have
been recently evaluated using ecosystem models to compare
the effects of different scenarios of seal abundance, fishing
pressure and nutrient loads on fish production (Hansson
et al. 2007). Hansson and Rudstam (1990) hypothesized
that the increase in Baltic Sea fish catches during the past
century has been made possible by the increased productiv-
ity resulting from eutrophication. Thus, fish production has
been influenced by a combination of eutrophication (which
has both positive and negative effects), decreased seal preda-
tion and increased fishing. To account for these interacting
factors, an ecosystem model was constructed. Status quo
scenarios of c. 9000 seals were compared with a historical
population of 100000 seals, which authors considered con-
servative as estimated historical abundances in the Baltic
ranged from c. 200000 ringed seals and 100000 grey seals
(Harding & Harkonen 1999) to upwards of 450000 ringed
seals and 200000 grey seals (Kokko et al. 1999). Increasing
seal abundance in the model predicted a 30% drop in
Atlantic cod abundance, a lesser decline in herring and an
increase in Baltic sprat Sprattus sprattus balticus abundance.
However, if the fishery was managed according to a precau-
tionary approach, the model predicted that seal populations
could be as large as they were a century ago, and the stocks
of cod and herring would still be as high as or higher than
they were in the reference period of 1996–2000. Hansson
et al. (2007) concluded that a decrease in benthic produc-
tion had greater negative effects on cod than an increase
in seals.
The most thoroughly analysed example of a proposed
marine mammal cull is in the Benguela ecosystem off the
southwest coast of South Africa (Butterworth & Harwood
1991). Here, interactions between Cape fur seals and hake
Marine mammal predator control W. D. Bowen and D. Lidgard
8Mammal Review •• (2012) ••–•• © 2012 The Authors. Mammal Review © 2012 Mammal Society/Blackwell Publishing
fisheries have been well studied with respect to the probable
benefits of culling. Two species of hake are involved, one
inhabiting shallow water (Merluccius capensis) and the other
inhabiting deep water (Merluccius paradoxus); the former
predates on small individuals of the latter. Multispecies
modelling results showed that a reduction in numbers of
fur seals is likely to reduce the abundance of hake, since
fewer seals would result in more shallow-water hake, a main
food of fur seals, and thus more predation on deep-water
hake (Punt & Butterworth 1995). Using a 29-species
food-web model of the same seal-hake system, Yodzis (1998)
concluded also that seal culling may have non-intuitive
consequences, whereby the removal of an upper-trophic
level predator may lead to increases in another species, and
that culling was more likely to be detrimental to total yield
from all exploited species than it was to be beneficial.
Cetaceans are also seen as competitors with fisheries,
and governments of whaling countries have advocated the
culling of whales to allow over-exploited fish stocks to
recover and to increase fishery yields (e.g. Komatsu &
Misaki 2001). Gerber et al. (2009) examined the potential
increase in biomass of commercially important fish stocks
that might result if the number of whales in Northwest
African and Caribbean ecosystems were reduced as a fisher-
ies management action. Although the authors acknowl-
edged that the data were often scarce, their ecosystem
modelling results suggested that in the tropical ecosystems,
with a wide range of assumptions on whale abundance,
feeding rates and fish biomass, even the complete removal
of baleen whales would not lead to an appreciable increase
in biomass of commercially exploited fish. There appears to
be little overlap between fisheries and whale consumption
in terms of prey types, and fisheries remove far more fish
biomass than whales consume. By contrast, reductions in
fishing mortality were predicted to have large positive
effects on fish biomass in these ecosystems.
Insights from other taxa
As the case studies above indicate, the few evaluations of the
efficacy of culling marine mammals to increase prey popu-
lations that have been done were based on retrospective
modelling. Although modelling is valuable and often the
only possible approach, experimental studies are needed to
understand fully the costs and benefits of culling. We briefly
review experimental studies in selected terrestrial and fresh-
water systems to see what has been learned about the
efficacy of culling in these better-studied systems.
Globally, predator culling continues to be the most com-
monly used tool for increasing game abundance for hunters
(Reynolds & Tapper 1996, Cote & Sutherland 1997). Holt
et al. (2008) used 40 published studies in a meta-analysis to
quantify the effects of predators on the abundance of their
prey. They found that predator removal caused a 1.6-fold
increase in the abundance of prey, but that there was signifi-
cant heterogeneity in the prey response to culling; cases in
which culling was effective tended to involve mammalian
or multiple predators. Fletcher et al. (2010) conducted an
8-year culling experiment in northern England in which a
43% reduction in red fox Vulpes vulpes abundance and a
78% reduction in carrion crow Corvus corone abundance
led to threefold increases in breeding success of all five
ground-nesting bird species studied.
Several meta-analytical studies have been used to
evaluate the effectiveness of predator removal to enhance
bird populations. Cote and Sutherland (1997) found that
predator control was generally effective in increasing
hatching success and post-breeding population size, but
was not reliably effective in increasing the breeding popu-
lation. These authors also noted that predator removal did
not have long-lasting effects, and if it was not maintained
the benefits rapidly disappeared. Updating the work of
Cote and Sutherland (1997), Smith et al. (2010) summa-
rized the results of 83 predator removal studies to conserve
vulnerable bird populations from six continents. In most
(63%) of the studies analysed, predator removal and
control areas were compared; in the remainder, bird popu-
lations before and after predator removal were compared.
Predator control increased hatching and fledging success
but did not have a significant positive effect on post-
breeding population size. Smith et al. (2010) also found
significant heterogeneity in the population response to
predator removal and that larger increases in bird popula-
tions were achieved by culling all predator species rather
than a subset, as removing all predator species excluded
mesopredator release. Once predator removal was discon-
tinued, any positive effects on prey populations soon
disappeared, underscoring the need for a long-term man-
agement strategy (Tapper et al. 1982, Coté & Sutherland
1997, Smith et al. 2010).
Several populations of double-crested cormorants Phala-
crocorax auritus have been experimentally culled, resulting
in an increase in fish stocks (e.g. Dorr et al. 2010, Fielder
2010, Johnson et al. 2010). Schnieder et al. (1999) con-
ducted an intensive field study to determine the impact of
double-crested cormorants on smallmouth bass Micropterus
dolomieu and other fisheries in the eastern basin of Lake
Ontario, North America. The results indicated a high rate of
predation by double-crested cormorants on smallmouth
bass. A five-year control program and further research
showed that predation on target fish species declined as
cormorant numbers also declined (Burnett et al. 2002,
Lantry et al. 2002, Farquhar et al. 2004). On the basis of
these results, the program was continued, and since 2003 the
abundance of cormorants and their predation on fish in Lake
Ontario has continued to decline (Johnson et al. 2010).
W. D. Bowen and D. Lidgard Marine mammal predator control
9Mammal Review •• (2012) ••–•• © 2012 The Authors. Mammal Review © 2012 Mammal Society/Blackwell Publishing
Culls are usually conducted on the assumption that all
predators are equally likely to impact prey species, but
there is increasing evidence that this may not be the case
in a variety of taxa (fish – Svanbäck & Persson 2004,
mammals – Estes et al. 2003, and birds – Guillemette &
Brousseau 2001, Oro et al. 2005). Yellow-legged gulls Larus
michahellis prey upon both breeding and immature storm
petrels Oceanites oceanicus and the resulting mortality is
additive to other causes of mortality (e.g. Oro et al. 2005).
Selective culling of only 16 individual gulls led to a mean
reduction of 65% in the number of storm petrel remains
found locally in gull pellets (Sanz-Aguilar et al. 2009).
Experimental results showed that predation by gulls
affected negatively both adult annual survival probability
and breeding success of petrels, and that after removing
specialist gulls, adult survival probabilities and breeding
success of storm petrels greatly and rapidly increased (by
16% and 23%, respectively).
Culling predators may have unintended consequences
for the target prey populations, depending on the nature
and complexity of the interactions among predators and
prey species (e.g. Summers & Harwood 1979, Punt &
Butterworth 1995, Yodzis 1998). Culling upper trophic level
predators can also result in mesopredator release. For
example, the removal of coyotes Canis latrans may result in
increased predation pressure by smaller (meso) predators
such as foxes Urocyon cinereoargenteus, skunks Mephitis
mephitis and domestic cats Felis catus, and the reduction or
local extinction of avian prey (e.g. Crooks & Soule 1999).
Similarly, the removal of dingos Canis lupus dingo ledtoan
increase in red fox numbers on the removal side of a dingo
exclusion fence in Australia (Letnic & Koch 2010). These
and the above examples serve to illustrate that unintended
consequences of culling top predators are perhaps not
uncommon (Paine et al. 1998, Scheffer et al. 2001, Worm &
Myers 2003).
Many terrestrial and aquatic ecosystems have been severely
changed by commercial exploitation of living resources,
habitat destruction and the effects of invasive species
(Jackson et al. 2001, Lotze & Worm 2009). A common con-
sequence of those changes is that some species become less
abundant. Where one of the threats is predation, the ques-
tion arises as to whether to control the predator to reverse
the decline in the prey population or to remove anthropo-
genic threats and hope that natural processes will return the
ecosystem towards a more desirable state (Lessard et al.
2005). Implicit in the let nature do it’ approach of no inter-
vention is the concept that if left alone the system will
return to some former more desirable state. However, there
is considerable evidence that ecosystems do not generally
have a single characteristic state (Scheffer et al. 2001) and
therefore we may be misguided in thinking that passive
management actions will be sufficient (Lessard et al. 2005).
Thus, ecosystem management may require active food-web
manipulation when protection from further human threats
appears likely to fail. This underscores the importance of
determining the sources of mortality of the prey species of
concern (Lessard et al. 2005).
Culling has been widely practised to reduce marine
mammal populations. The perceived conflict with fisheries
has as much to do with fisheries practices and economics as
it has to do with the consumption of fish by native marine
mammals. Nevertheless, marine mammals are perceived as
taking too large a fraction of those fish left in the water
by fishermen. Several authors (Jackson et al. 2001, Lavigne
2003, Roberts 2007) have reminded us that the world’s
oceans were home to much larger populations of both fish
and marine mammals than they are today. So it is certainly
not clear that large populations of marine mammals pre-
clude large and productive fish populations.
Although it is widely practised, there is little evidence that
culling marine mammals to increase the abundance of tar-
geted prey species has been effective. A common feature of
marine mammal culls is the lack of explicit and measurable
objectives with respect to target populations and usually
with respect to the reduction in the size of the marine
mammal population to be achieved. Even so, Lavigne
(2003) argued that we should not expect calls for culling
marine mammals to abate in the near future, as there
appears to be little correlation between the overall size of a
marine mammal population and the conflict it generates
with fishermen. For example, although still a small fraction
of historic population size, the recovering grey seal popula-
tion in the Baltic Sea is once again considered a threat, and
the Swedish government has issued licenses to shoot several
hundred grey seals in response to complaints by fishermen
(Königson et al. 2009). Similarly, grey seals were considered
pests in Eastern Canada early in the last century when the
population size was small (Scott 1968). Due perhaps to their
coastal occurrence, it appears that the extent to which grey
and harbour seals are considered pests is largely indepen-
dent of total population size.
Several conclusions can be drawn from experimental
studies on other taxa. Firstly, predator removal can increase
the productivity and population size of target prey popula-
tions but does not always do so. Secondly, these studies
typically have involved large proportional reductions in
predator populations, presumably to increase effect size and
the statistical power to detect a significant effect. Thirdly,
the effects of culling are typically dependent on continued
control, and in the absence of control the benefits rapidly
disappear. Based on a review of predator control experi-
ments to enhance ungulate densities in North America, the
Marine mammal predator control W. D. Bowen and D. Lidgard
10 Mammal Review •• (2012) ••–•• © 2012 The Authors. Mammal Review © 2012 Mammal Society/Blackwell Publishing
National Research Council concluded that to be effective,
control must be both intense and frequent and that there is
no factual basis for assuming that short-term control will
have long-term effects (Anonymous 1997). Fourthly, culling
predators often has non-intuitive and unintended conse-
quences for target species and for other predator and prey
species. Therefore, follow-up monitoring should be a
planned component of a culling program, to evaluate the
success of culling with respect to objectives and to docu-
ment unintended consequences.
The decision to use predator control is a management
decision and not a scientific one. This decision should be
based on the best available science, but even when the scien-
tific evidence is strong, other factors, such as ethical consid-
erations and the likelihood of unintended consequences,
influence policy decisions (Reynolds & Tapper 1996).
DeMaster and Sisson (1992) argued that control programs
should only be considered if the magnitude of the increased
fishery yield can be estimated with its associated uncertainty
and if the cost of management is likely to be significantly
less than the minimum economic benefit to the fishery.
These and other ideas, such as clearly stated objectives, are
also embodied in criteria for culling marine mammals
developed by the Scientific Advisory Committee of the
Marine Mammal Action Plan (Anonymous 1999).
Although culling has often generated debate, culling
marine mammals is particularly controversial for at least
three reasons. The first is that they are regarded as charis-
matic megafauna and have a high profile in many parts of
the world. The second is that many populations of marine
mammals are still recovering or have only recently recov-
ered from a long period of over-exploitation, while others
remain threatened or endangered, raising conservation con-
cerns. The third is that the scientific evidence needed to
justify a cull is usually highly uncertain (Yodzis 2001).
The authors are grateful for comments on an earlier draft of
the manuscript by participants of a workshop on Atlantic
cod-seal interactions, Susan Heaslip, and for the helpful
comments of three anonymous reviewers. Financial support
for the work was provided by the Department of Fisheries
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... Since a 2018 fatality attributed to a white shark in Wellfleet, MA, calls for a seal cull have intensified. Scientists and environmentalists maintain culls would be ecologically damaging (Bowen & Lidgard, 2013;Morissette et al., 2012), unnecessary (Morissette et al., 2012), inhumane (de Vere et al., 2018), unacceptable to the public (Cummings et al., 2019;Jackman et al., 2018), and a threat to tourism (Parsons, 2003). They also point to the lack of evidence that culls reduce conflicts (Bowen & Lidgard, 2013;de Vere et al., 2018) or increase valued fish stocks (Costalago et al., 2019). ...
... Scientists and environmentalists maintain culls would be ecologically damaging (Bowen & Lidgard, 2013;Morissette et al., 2012), unnecessary (Morissette et al., 2012), inhumane (de Vere et al., 2018), unacceptable to the public (Cummings et al., 2019;Jackman et al., 2018), and a threat to tourism (Parsons, 2003). They also point to the lack of evidence that culls reduce conflicts (Bowen & Lidgard, 2013;de Vere et al., 2018) or increase valued fish stocks (Costalago et al., 2019). ...
... In the late 1800s, Gilpin (1874) speaks of herds of only 20 or 30 seals on Sable Island, and in the early 1950s, they were rare throughout eastern Canada (Fisher 1955;Lavigueur and Hammill 1993;Bowen 2011). Government-sponsored culls and a bounty program may have slowed grey seal recovery in the 20 th century (Bowen and Lidgard 2012), but over the last five decades the Canadian grey seal population has been estimated to have increased from approximately 15,000 animals in the early 1960s to approximately 400,000 by 2017 (Mohn and Bowen 1996;Rossi et al 2021). ...
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Here we introduce a new integrated population model (IPM) to provide harvest advice for theGulf of St. Lawrence (Gulf), Coastal Nova Scotia (CNS) and Sable Island grey seal herds, and compare model outputs with estimates from a deterministic model used in previous assessments. The IPM was fit to the pup production estimates for the Scotian Shelf (CNS and Sable Island combined) and the Gulf. As with the previous assessment model, the new model was fit to both pup production and pregnancy rates, and includes an index for ice-related pup mortality in the Gulf. The new model includes both density-dependent and density-independent pup mortality, and fits to sighting histories of individually marked seals at the breeding colony onSable Island to estimate sex- and age-specific survival and recruitment to the breeding colony.The model estimated that total pup production increased slightly from 92,300 (95%CI = 86,700–100,100) in 2016 to 99,300 (90,900–107,700) in 2021, while total abundance increased slightly from 339,400 (317,900–361,500) in 2016 to 366,400 (317,800–409,400) in2021. The rate of growth of the population has continued to slow, declining from approximately4% during the last assessment period, to 1.5% per year between 2016 and 2021. The updated population estimate from the previously accepted deterministic population model was 363,600(298,700–450,000) for 2021, which is very similar to the estimate of abundance generated using the IPM. Although the population continues to grow, the current estimate is below that presented during the 2016 assessment. The difference is due to changes in the structure of the new population model and higher estimates of juvenile mortality produced by the model fit to the2021 pup production estimates. Additional information on juvenile survival and how it responds to changes in abundance (density-dependent) and environmental (density independent)variation is needed as it represents a significant gap to our understanding of the dynamics of this population and of large marine mammals in general. Total allowable removals depend on age structure of the harvest and whether the harvests are conducted in winter at the breeding colonies, or at other times of the year when animals from all herds are mixed. Using an integrated model incorporates many of the inputs in a unified framework that allows for uncertainty to be propagated throughout the analyses.
... In the past years, reviews on the impacts of climate change drivers on the intertidal zone saw an increasing interest, evidenced by numerous previous reviews centered around ocean warming and acidification as singlestressor or multi-stressor impacts on the morphology (e.g., Byrne and Przeslawski, 2013;Cattano et al., 2018;Noor and Das, 2019), physiology (e.g., Kroeker et al., 2014;Ducker and Falkenberg, 2020), behavior (e.g., Clements and Hunt, 2015;Cattano et al., 2018;Clements and Comeau, 2019;Draper and Weissburg, 2019;Noor and Das, 2019) and ecology (e.g., Dupont et al., 2010aDupont et al., , 2010bKroeker et al., 2010;Hendriks et al., 2010;Węsławski et al., 2011;Harvey et al., 2013;Steneck and Wahle, 2013;Bass et al., 2021) of marine organisms. There is also interest in reviewing marine predator-prey interactions beyond climate change-driven impacts (e.g., Bowen and Lidgard, 2013;Scott et al., 2012;Weis and Candelmo, 2012;Daewel et al., 2014;Kubicka et al., 2017). As predation has an extensive importance in modulating the community structure of intertidal zones, we can only achieve a better holistic vision of climate change effects by not ignoring their influence on such interaction. ...
The effect of ocean warming and acidification on predator-prey interactions in the intertidal zone is a topic of growing concern for the scientific community. In this review, we aim to describe how scientists have explored the topic via research weaving, a combination of a systematic review, and a bibliometric approach. We assess articles published in the last decade exploring the impact of both stressors on predation in the intertidal zone, via experimental or observational techniques. Several methods were used to delve into how climate change-induced stress affected intertidal predation, as the study design leaned toward single-based driver trials to the detriment of a multi-driver approach. Mollusks, echinoderms, and crustaceans have been extensively used as model organisms , with little published data on other invertebrates, vertebrates, and algae taxa. Moreover, there is a strong web of co-authoring across institutions and countries from the Northern Hemisphere, that can skew our understanding towards temperate environments. Therefore, institutions and countries should increase participation in the southern hemisphere networking, assessing the problems under a global outlook. Our review also addresses the various impacts of ocean acidification, warming, or their interaction with predation-related variables, affecting organisms from the genetic to a broader ecological scope, such as animal behaviour or interspecific interactions. Finally, we argue that the numerous synonyms used in keywording articles in the field, possibly hurting future reviews in the area, as we provide different keyword standardizations. Our findings can help guide upcoming approaches to the topic by assessing what has been already done and revealing gaps in emerging themes, like a strong skew towards single-driver (specially acidification) lab experiments of northern hemisphere organisms and a lack of field multi-stressor experiments.
... However, beyond the ethical and biodiversity related issues, the efficacy of such practice is debated. Indeed, the analyses conducted by Bowen and Lidgard (2013) on actual data showed that predator removal is not always effective in increasing the productivity of target prey populations. On the contrary, they may have unintended consequences for target species, other predator and prey species. ...
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Abstract The expected increase in global food demand, as a consequence of a rising and wealthier world population, and an awareness of the limits and drawbacks of modern agriculture, has resulted in a growing attention to the potential of the seas and oceans to produce more food. The capture production of presently exploited marine fish stocks and other species has more or less reached its maximum and can only be slightly improved by better management. This leaves four alternative options open to increase marine food production: (1) manipulating the entire food web structure via removal of high trophic level species to allow an increasing exploitation of low trophic level species, (2) harvesting so far unexploited stocks, such as various fish species from the mesopelagic zone of the ocean or the larger zooplankton species from polar regions, (3) low‐trophic mariculture of seaweeds and herbivorous animals, and (4) restoration of impoverished coastal ecosystems or artificially increasing productivity by ecological engineering. In this paper, we discuss these four options and pay attention to missing scientific knowledge needed to assess their sustainability. To assess sustainability, it is a prerequisite to establish robust definitions and assessments of the biological carrying capacity of the systems, but it is also necessary to evaluate broader socio‐economic and governance sustainability.
... diversionary feeding, fisheries management) (Furness 2021;Russell et al., 2016). For such approaches to be effective, a robust understanding of the predator-prey relationship and the life histories of the species involved is essential (Bowen and Lidgard 2013). The removal of or reduction in predation pressure will only result in the recovery of the prey population when conditions meet the "additive hypothesis" described by Valkama et al. (2005), where predation causes excess mortality compared to a scenario where the predator was not present. ...
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The management of predator-prey conflicts can be a key aspect of species conservation. For management approaches to be effective, a robust understanding of the predator-prey relationship is needed, particularly when both predator and prey are species of conservation concern. On the Isle of May, Firth of Forth, Scotland, numbers of breeding Great Black-backed Gulls Larus marinus, a generalist predator, have been increasing since the 1980s, which has led to increasing numbers of sympatrically breeding Atlantic Puffins Fratercula arctica being predated during the breeding season. This may have consequences for species management on the Isle of May and impact assessments of offshore windfarms in the wider Firth of Forth area. We used population viability analysis to quantify under what predation pressure the Atlantic Puffin population may decline and become locally extinct over a three-generation period. The predation level empirically estimated in 2017 (1120 Puffins per year) was not sufficient to drive a decline in the Puffin population. Rather, an increase to approximately 3000 Puffins per year would be required to cause a population decline, and >4000 to drive the population to quasi-extinction within 66 years. We discuss the likelihood of such a scenario being reached on the Isle of May, and we recommend that where predator-prey conflicts occur, predation-driven mortality should be regularly quantified to inform conservation management and population viability analyses associated with impact assessments.
... culling; Westerberg 2010). It is unclear, however, whether this measure would effectively reduce interactions, since there appears to be little correlation between the overall size of a pinniped population and the conflict it generates with fishers (Bowen & Lidgard 2013). Furthermore, killing sea lions is not an option for many of the countries in South America, since all marine mammals are protected under national laws (see below). ...
1. The main issue affecting the conservation of most pinniped species has been identified as interactions with fisheries and aquaculture. In South American waters in particular, this problem has not been well evaluated. Consequently, there is an urgent need for research and conservation measures to address the problem, particularly for species of conservation concern. 2. In this study, we reviewed published and unpublished research and observations from the last 25 years on operational and biological interactions between pinnipeds and fisheries and salmonid aquaculture activities in South American waters, and the onservation and management implications of these interactions in the near future. 3. Two species of pinnipeds are primarily involved in biological and operational interactions with fisheries and aquaculture in South America: the South American sea lion Otaria flavescens and the South American fur seal Arctocephalus australis. Although phocids are present in South American waters, there is no published research on conflicts between phocids and human activities. 4. Interactions between fisheries and pinnipeds have been reported by many countries. In South American waters, interactions mostly involve male and subadult male South American sea lions. However, despite the high frequency of interactions, the estimated economic losses to fisheries and aquaculture related to sea lion depredation are generally low. 5. Incidental capture and mortality of pinnipeds has been reported, especially in artisanal gillnet fisheries and in industrial purse seine and trawl fisheries. Mortality of sea lions in salmonid aquaculture has been reported, but the magnitude of the problem is unknown. 6. In South America, limited progress has been made to incorporate mitigation measures, such as time-area closures, acoustic deterrent devices, and modifications to fishing gear, into fisheries and aquaculture. This is likely to be due to our limited understanding of ecosystem complexity, the costs of modifying fishing gear or incorporating acoustic deterrent devices, and the scarcity of fishing controls. We suggest that strategies for effectively reducing conflicts between pinnipeds, fisheries, and aquaculture should involve all stakeholders in participative research to facilitate co-management.
... The implementation of the ecosystem approach to fisheries management requires that more information on biological interactions, production, consumption and energy transfer among trophic levels in marine ecosystems are collected and considered when conducting fish stock assessments (Christensen and Pauly, 1995;Pauly et al., 2002;Link 2018;Rudd et al., 2018). Of the biological interactions, predator-prey interactions are known to affect food web structure and function, including direct effects on prey populations and indirect trophic cascades (Estes et al., 1998;Lazzaro et al., 2009;Bowen and Lidgard, 2012;Brose et al., 2019). The feeding strategy and natural prey preferences varies during the life stages of a fish and it has been established that diet compositions of predatory fish largely reflect prey temporal abundance (Overholtz et al., 2000;Link and Garrison, 2002;Ng et al., 2021) and spatial distribution (Fahrig et al., 1993;Link, 2004;Eriksen et al., 2018). ...
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The abundance of top predators in the southern Gulf of St. Lawrence, Canada, has fluctuated dramatically in recent decades. The associated increase in predation and its effect on the lack of recovery of many fish stocks in the system generates the need for a better understanding of predator-prey relationships and the implementation of an ecosystem approach to fisheries management. This study used stomach content analysis to further describe the diet of Atlantic bluefin tuna in the southern Gulf of St. Lawrence. Teleost fish largely dominated the stomach contents in all years. Previous studies established that Atlantic herring was the main component of the diet by weight, whereas herring was almost absent from the diet in this study. A shift in the diet of Atlantic bluefin tuna has been observed, as it now feeds almost exclusively on Atlantic mackerel. The estimated daily meal varied between years, ranging from 1026 g per day in 2019 to 2360 g per day in 2018. Daily meals and daily rations were calculated and showed substantial year-to-year variation.
... For these protected predators and protected prey to be managed effectively, or if conservation managers are to evaluate the suitability of lethal control as a management option, it is first crucial to know if the oystercatchers impact toheroa populations at Ripiro Beach and to what degree. Essentially, the predator-prey dynamics of these two species should be understood before any management actions are taken (Bowen and Lidgard 2013). ...
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In northern New Zealand, there is speculation that predation by a recovering shorebird, the South Island Pied Oystercatcher (Haematopus finschi), has caused the continuing decline of an endemic surf clam, the toheroa (Paphies ventricosa). Consequently, there have been requests by the public for these protected birds to be culled to protect and restore the toheroa. To better inform management and decision-making for these species we set out to understand the impact of oystercatcher predation on toheroa at Ripiro Beach in New Zealand. A study was conducted to establish the spatio-temporal overlap of toheroa and oystercatcher distributions, determine oystercatcher prey selection, and assess the impacts of oystercatcher predation on toheroa population size structure. Monthly surveys over a 12-month period indicated limited spatial overlap between the two species, and that most toheroa beds on Ripiro Beach are not subjected to oystercatcher predation. In areas where oystercatchers and toheroa co-occurred, oystercatchers consumed a variety of prey including polychaetes, small and medium-sized toheroa and a non-threatened surf clam (P. subtriangulata). While oystercatchers do feed on toheroa (and may be capable of locally reducing abundance of small toheroa in a population), feeding on larger individuals (>50 mm in length) was not observed. These results suggest that culling of oystercatchers would be of no benefit for the recovery of toheroa populations. It is likely that less obvious factors are impeding the recovery of toheroa in New Zealand. This study highlights the importance of understanding predator-prey interactions before implementing management actions such as lethal control.
Pinniped populations around the world increased rapidly after hunting and culling during the nineteenth and twentieth centuries ended. Some believe that pinnipeds are now preventing the recovery of certain fish populations, and that controlling pinniped population abundance using lethal measures such as harvesting or by non‐lethal means like contraception could recover fish populations. It is unclear, however, how effective and how long it would take for such methods of population control to bring numbers of pinnipeds down to target levels. We used sex‐ and age‐structured population models to estimate how quickly harbor seal ( Phoca vitulina ) abundance in British Columbia, Canada, could be reduced by 50%, through combinations of lethal removals and sterilization of adult females. Models were fit to seal abundance, demographic, and harvest data collected between 1879 and 2014. Simulation modeling suggests reliance on contraception exclusively is unlikely to reduce the current harbor seal population (numbering ~100,000) by 50% within 25 years, and would result in more variable outcomes, compared to lethal removals. Contraception could be combined with harvesting to maintain a target abundance of harbor seals (although captive studies with harbor seals are needed to confirm the efficacy of contraception). Our simulation modeling approach provides a useful framework to assess how non‐lethal measures could be integrated into policies that promote active population control of harbor seal numbers.
A R T I C L E I N F O ey ords Multispecies livelihoods Wildlife-human conflict Ecosystem-based fisheries management Posthumanism Animal ethics A B S T R A C T The sustainability paradigm in fisheries management is held legitimate to ensure food security and the ability for future generations. Conventional management principles and scientific methods may objectify data and nonhuman subjects in these practices. This paper introduces posthuman philosophies to Ecosystem-Based Fisheries Management (EBFM), and it explore the case of the Columbia sea lion in the Pacific Northwest United States. We offer a multispecies livelihoods perspective as an applied approach to wildlife-human conflict by adhering to the biocultural hierarchies of the region. An era of ecological rehabilitation should focus on coadaptation between human and nonhuman peoples, including that of the Columbia sea lion. Our findings are supported by a 12-month multispecies ethnographic design. They provide robust insight to the phenomenology of salmon-sea lion-human relations. A posthuman multispecies livelihoods approach offers a way forward for fisheries governance and contends that the rights, welfare, and agency of nonhuman individuals and their habitats must be foregrounded.
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Grey seals (Halichoerus grypus Fabricius, 1791) are distributed all around the Icelandic coast. The majority of the population breeds on the west- and northwest shores, with a second high density in the breeding distribution on the southeast coast of Iceland. During the last 5 decades the Icelandic grey seals have dispersed from the west- to the northwest-, the north- and the northeast-coast. The breeding period occurs from the middle of September to early November, with a maximum in mid October. The time of peak pupping shows some variation, beginning earlier along the west coast and later in the north and southeast. Seven aerial surveys to estimate pup production in Iceland were flown during October to November during the period from 1980 to 2004. Pup counts of the Icelandic grey seal, at all breeding sites combined, have been decreasing annually by about 3% (±1% s.e.), during the period 1982-2002. During the period 1990-2002, this downward trend doubled to about 6% annually. The abundance of the grey seal around Iceland in the year 2002 was estimated to be 4,100 to 5,900 animals. This is higher than estimates of around 2,000 animals during the 1960s, but much less than the estimated population of 8,000 to 11,500 in 1982.
In this paper, we discuss the merit (or lack, thereof) of controlling pinniped populations for the purpose of enhancing fish stocks. Inherent in this approach is the assumption that predation by pinnipeds limits net production of at least some fish populations and, that any net surplus in production, caused by a culling program, can be effectively utilized in commercial harvests. Four generally accepted ecological relationships work against the success of culling pinniped herds to enhance fishery production. First, prey species almost always have more than one predator. Second, pinniped species rarely are dependent on only one species of prey. Third, the recruitment rate of most fish stocks is highly variable, and this is one of the most likely factors determining stock abundance. And fourth, fish, as a predatory group, consume more fish than do other predators (e.g., seabirds, cetaceans, and pinnipeds). Two examples of control programs and their effects on local fisheries are discussed. The following information is necessary to evaluate the biological merits of a pinniped control program (excluding ethical considerations and public sentiment): (1) kind, size, and amount of target species taken by pinniped species (by age, sex, and area), by other predators, by commercial fisheries, and by recreational fisheries and how knowledge of these takes will be effected by the management program; (2) standing stock, trends in stock size, and the relationship between net production and standing stock of the target species; (3) population size, trends in abundance, status, and net production of pinniped species; (4) expected increase in yield (and confidence levels) resulting from cull, and value of this increase in net production to the fishery and the general public; (5) cost of the control program; (6) proposed number (by age and sex classes) of pinnipeds to be culled each year, and the duration of the cull in years; and, (7) long-term effect of the cull on the pinniped population. It is unlikely that information on points (1), (3), and (4) will ever be known with reasonable confidence. Statistically designed removal experiments may be the only method of determining the merit of a control program. Unfortunately, it will be difficult to generalize results from one such experiment to other areas, species, or fisheries.