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The harbour porpoise (Phocoena phocoena L.) in the southern North Sea: Abundance, threats, research- and management proposals

Authors:

Abstract

The harbour porpoise (Phocoena phocoena) is the most numerous cetacean species in the North Sea. For reasons not well understood, it gradually disappeared from the southern North Sea during the 1950s, to make a spectacular return towards the end of the 20th century. The analysis of Belgian and Dutch sighting data, together with the results of research on the hundreds of animals washed ashore, yielded information on ecological aspects of the population, trends and threats. The recent increase in numbers in the southern North Sea is probably food related, and is believed to be due to an influx of porpoises from more northern waters. Strandings data seem to indicate that the influx consists for the main part of juveniles, with significantly more males than females. However, stranded pregnant females and numerous neonates indicate that some reproduction takes place in the southern North Sea. Currently, a clear seasonal pattern is apparent in the presence of porpoises. A peak in numbers in coastal waters of the southern North Sea is reached between February and April. In late spring a northward migration towards more offshore waters is observed, and by summer the number of porpoises in coastal waters has become low. In the Dutch Delta Area (Zeeland) a small resident population seems to have been established. Observations during 2007 and 2008 have indicated that the seasonal pattern might not be stable. Together with the return of the porpoise to the southern North Sea, a bycatch problem became apparent. Up to half of the stranded porpoises had been killed incidentally in fishing gear, a rate that justifies concerns. The main fishing gears responsible for the porpoise bycatch are gill- and tangle nets, considered otherwise as selective and relatively environmentally friendly. Next to a lack of data on the ecology of the porpoise, data are lacking on the true level of bycatch, and on the extent, and spatial and temporal distribution of relevant fishing methods. To obtain such data, research initiatives should be coordinated and standardised internationally. Basic research funds should be structural and be provided for a long time span. Currently protection initiatives are dispersed in many international nature conservation fora. Perhaps the best forum for the coordination of scientific research efforts in relation to porpoises in the North Sea would be ASCOBANS. For further developing measures, the most appropriate framework would be the European Community, given its competence in, and responsibilities for both fisheries and environmental matters. Also measures to prevent bycatch in recreational fisheries should be coordinated internationally. One of the most promising bycatch prevention measures is the use of pingers (acoustic alarms). However, many problems with their use remain, and currently they are not mandatory for most gill and tangle net fisheries in the southern North Sea. While currently only few Belgian and Dutch fishermen use gill- and tangle nets, this is gradually changing, due to environmental concerns of beamtrawling and especially the soaring gasoline prices (up to the end of 2008). Therefore it is likely that without effective protective measures, the porpoise bycatch in certain areas in the North Sea will increase. It is clear that disentangling the problems the harbour porpoise is facing, is a challenging task, given the combination of environmental, social, economical, political, legal and technical factors involved.
Abundance, threats and research- & management proposals
THE
HARBOUR
PORPOISE
in the southern North Sea
Project financed by IFAW (International Fund for Animal Welfare)
Jan Haelters1 & Kees (C.J.) Camphuysen2
1 Royal Bel gian Institute of Natural Sciences (R BINS), depart ment Management Unit of the Nor th Sea Mathemat ical Models (MUMM)
2 Royal Nether lands Institute for S ea Research (NIOZ )
Abundance, threats and research- & management proposals
THE
HARBOUR
PORPOISE
in the southern North Sea
3
THE HARBOUR PORPOISE
in the southern North Sea
Abundance, threats and research- & management proposals
Table of contents
Summary / Samenvatting / Résumé 4
1. Introduction 7
2. The harbour porpoise 8
3. Legal framework for the protection of the harbour porpoise 11
3.1 International fora 11
3.2 National legislation in Belgium and The Netherlands 14
4. The harbour porpoise in Belgian and Dutch waters 15
4.1 Before the 20th century 15
4.2 Research on the harbour porpoise in Belgium and The Netherlands during the 20th century 16
4.3 The occurrence of the harbour porpoise in Belgium and
The Netherlands in the 20th and early 21st century 17
4.4 Increase in the number of porpoises in the southern North Sea
at the end of the 20th century: evidence from stranding records 18
4.5 Increase of the numbers of porpoises in the southern Nor th Sea
at the end of the 20th century: evidence from sightings 18
4.6 Possible reasons for the irregular occurrence of the harbour porpoise
in the southern North Sea during the 20th century 22
4.7 An analysis of strandings data 23
5. Bycatch of porpoises in Belgium and The Netherlands 27
5.1 Introduction 27
5.2 Fishing gears leading to bycatch 27
5.3 Gill and tangle net sheries in Belgium and The Netherlands 27
5.4 National and international legislation concerning sheries and the bycatch of cetaceans 31
5.5 The use of pingers 33
5.6 Assessing bycatch levels in Belgium and The Netherlands 35
5.7 Level of bycatch among stranded porpoises in Belgium and The Netherlands 42
6. Other threats 44
7. Recommendations 45
References 48
Notes to the text 56
THE HARBOUR PORPOISE
in the southern North Sea
4
Summary
The harbour porpoise (Phocoena
phocoena) is the most numerous
cetacean species in the North Sea. For
reasons not well understood, it gradually
disappeared from the southern North
Sea during the 1950s, to make a
spectacular return towards the end of
the 20th centur y. The analysis of Belgian
and Dutch sighting data, together with
the results of research on the hundreds
of animals washed ashore, yielded
information on ecological aspects of the
population, trends and threats.
The recent increase in numbers in
the southern North Sea is probably
food related, and is believed to be due
to an inux of porpoises from more
northern waters. Strandings data seem
to indicate that the inux consists for the
main part of juveniles, with signicantly
more males than females. However,
stranded pregnant females and
numerous neonates indicate that some
reproduction takes place in the southern
North Sea. Currently, a clear seasonal
pattern is apparent in the presence of
porpoises. A peak in numbers in coastal
waters of the southern Nor th Sea is
reached between February and April.
In late spring a nor thward migration
towards more offshore waters is
observed, and by summer the number of
porpoises in coastal waters has become
low. In the Dutch Delta Area (Zeeland)
a small resident population seems to
have been established. Observations
during 2007 and 2008 have indicated
that the seasonal pattern might not be
stable.
Together with the return of the porpoise
to the southern North Sea, a bycatch
problem became apparent. Up to half of
the stranded porpoises had been killed
incidentally in shing gear, a rate that
justies concerns. The main shing gears
responsible for the porpoise bycatch
are gill- and tangle nets, considered
otherwise as selective and relatively
environmentally friendly.
Next to a lack of data on the ecology
of the porpoise, data are lacking on the
true level of bycatch, and on the extent,
and spatial and temporal distribution
of relevant shing methods. To obtain
such data, research initiatives should
be coordinated and standardised
internationally. Basic research funds
should be structural and be provided for
a long time span.
Currently protection initiatives are
dispersed in many international nature
conservation fora. Perhaps the best
forum for the coordination of scientic
research efforts in relation to porpoises
in the North Sea would be ASCOBANS.
For fur ther developing measures, the
most appropriate framework would
be the European Community, given
its competence in, and responsibilities
for both sheries and environmental
matters. Also measures to prevent
bycatch in recreational sheries should
be coordinated internationally. One of
the most promising bycatch prevention
measures is the use of pingers (acoustic
alarms). However, many problems with
their use remain, and currently they are
not mandatory for most gill and tangle
net sheries in the southern North Sea.
While currently only few Belgian and
Dutch shermen use gill- and tangle
nets, this is gradually changing, due to
environmental concerns of beamtrawling
and especially the soaring gasoline prices
(up to the end of 2008). Therefore it is
likely that without effective protective
measures, the porpoise bycatch in
certain areas in the North Sea will
increase. It is clear that disentangling
the problems the harbour porpoise is
facing, is a challenging task, given the
combination of environmental, social,
economical, political, legal and technical
factors involved.
5
Samenvatting
De bruinvis (Phocoena phocoena) is
veruit de talrijkste walvisachtige in het
Noordzeegebied. Om niet volledig
begrepen redenen is de soort uit de
zuidelijke Noordzee verdwenen aan het
einde van de jaren vijftig van de 20e eeuw.
Aan het einde van de 20e eeuw was er
echter sprake van een spectaculaire
terugkeer van de bruinvis in de
Nederlandse en Belgische kustwateren.
Een analyse van Belgische en Nederlandse
waarnemingsgegevens, gekoppeld aan
de resultaten van onderzoek van vele
honderden gestrande bruinvissen, heeft
belangrijke nieuwe inzichten opgeleverd
in de ecologische achtergronden van de
populatieontwikkelingen en bedreigingen
van deze zeezoogdiersoort.
De recente toename van de bruinvis in de
zuidelijke Noordzee is het gevolg van een
verschuiving van een deel van de populatie
vanuit de noordelijke Noordzee naar
het zuiden, en is vermoedelijk voedsel-
gerelateerd. Strandinggegevens laten
uitschijnen dat vooral juveniele dieren
migreren tot Belgische en Nederlandse
kustwateren, en signicant meer
mannetjes dan wijfjes. De regelmatige
strandingen van zwangere, adulte wijfjes
en de talrijke pasgeboren bruinvissen die
langs de kust aanspoelen laten echter
zien dat in de zuidelijke Noordzee ook
gereproduceerd wordt. Er werd de
laatste jaren een duidelijk seizoenpatroon
gevonden in de aanwezigheid van
bruinvissen. Van februari tot en met
april worden langs de kust de grootste
aantallen waargenomen. Daarna volgt
een migratie van de kust weg, en tegen
de zomer zijn de aantallen bruinvissen in
kustwateren zeer laag. Waarnemingen in
2007 en 2008 lijken echter uit te wijzen
dat dit seizoenpatroon niet stabiel is. In
het Nederlandse Deltagebied (Zeeland)
lijkt een kleine populatie jaarrond te
verblijven.
Tegelijk met de terugkeer van de bruinvis
naar de zuidelijke Noordzee werd het
probleem van bijvangsten actueel. Tot
ongeveer de helft van alle gevonden
bruinvissen bleek door incidentele
vangst om het leven gekomen te
zijn: een percentage dat bezorgdheid
over de effecten op populatieniveau
rechtvaardigt. Kieuw- en warnetten,
in andere opzichten een selectief en
relatief milieuvriendelijk vistuig, zijn
vermoedelijk verantwoordelijk voor de
meeste bijvangsten van bruinvissen in de
zuidelijke Noordzee.
Er ontbreken nog veel gegevens over
de ecologie van bruinvissen, over de
omvang van bijvangst, en over de
ruimtelijke en temporele spreiding van de
verschillende visserijtypes. Om dergelijke
gegevens te kunnen verzamelen, zou
een internationaal gecoördineerd en
gestandaardiseerd onderzoek moeten
worden opgezet. De daarvoor benodigde
fondsen zouden structureel, stabiel (over
een lange periode) en vooral onafhankelijk
van zowel de visserijindustrie als van
andere belangengroepen ter beschikking
moeten worden gesteld.
Initiatieven om de bruinvis te beschermen
zijn tegenwoordig van teveel verschillende
fora afkomstig. ASCOBANS lijkt ons de
meest geschikte overlegstructuur voor
de coördinatie van wetenschappelijke
onderzoeksprogramma’s betreffende
bruinvissen in de Noordzee. Effectieve
maatregelen om bijvangsten te
voorkomen of op zijn minst te
verminderen, kunnen daarentegen beter
vanuit de Europese Commissie genomen
worden, gezien hun competenties en
verantwoordelijkheden met betrekking
tot de visserij en het mariene milieu.
Ook maatregelen ter beteugeling van de
recreatieve staandwant visserij vereisen
een internationale aanpak.
Een veelbelovende methode om
bijvangsten te verminderen is het gebruik
van pingers (geluidsapparaatjes die
aan netten bevestigd kunnen worden).
Het gebruik van pingers is momenteel
meestal niet verplicht bij de kieuw- en
warnetvisserij in de zuidelijke Noordzee,
en problemen die effectief gebruik van
pingers in de weg staan zouden zo snel
mogelijk aangepakt en opgelost moeten
worden.
Hoewel staand want in de Belgische
en Nederlandse commerciële visserij
nog maar een bescheiden plaats
inneemt, zijn er duidelijke veranderingen
waarneembaar. Onder druk van de
brandstofprijzen (tot eind 2008), maar
ook gezien de milieubezwaren ten aanzien
van boomkorvisserij, stappen steeds
meer vissers over op passief, selectief
vistuig. Wanneer deze verandering niet
kritisch wordt gevolgd en begeleid, dan
staan er in de toekomst nog grotere
problemen met bijvangsten te wachten.
Het is duidelijk dat een integrale oplossing
van het probleem van bijvangsten van
bruinvissen een grote uitdaging genoemd
kan worden, gezien de combinatie van
samenhangende factoren zoals wetgeving,
milieu, klimaat, en sociale, economische,
politieke en visserijtechnische aspecten.
THE HARBOUR PORPOISE
in the southern North Sea
6
su
En mer du Nord, le cétacé le plus
abondant est le marsouin, Phocoena
phocoena. Pour des raisons qui ne sont
pas complètement élucidées, cette
espèce avait graduellement disparu de
la partie sud de la mer du Nord dans
les années 1950 pour y opérer un retour
spectaculaire vers la n du 20ème siècle.
Une analyse des données d’observation
provenant de Belgique et des Pays-Bas
et les résultats de l’examen de plusieurs
centaines d’animaux échoués sur la
côte, nous renseignent sur des aspects
écologiques de cette population, sur les
tendances constatées et sur les menaces
auxquelles elle est exposée.
La récente augmentation de cette
espèce dans la partie sud de la mer du
Nord est probablement liée à des causes
alimentaires et provoquée par l’arrivée
d’individus venant du nord. L’apport
consiste principalement en juvéniles,
les mâles étant signicativement plus
nombreux que les femelles. Toutefois,
l’échouage de femelles gravides et la
présence de nombreux nouveaux-nés
indiquent que la reproduction n’est pas
absente de la partie sud de la mer du
Nord. La présence du marsouin suit
clairement un schéma saisonnier. Les
comptages atteignent un maximum
dans les eaux côtières entre février et
avril. A la n du printemps on observe
un déplacement vers le nord et la haute
mer et une fois l’été venu les marsouins
deviennent rares dans les eaux côtières.
Une petite population résidentielle
semble s’être établie dans la zone du
Delta aux Pays-Bas. Des observations
faites en 2007 et 2008 indiquent que ce
schéma saisonnier pourrait être instable.
Avec le retour du marsouin dans la partie
sud de la mer du Nord, un problème
de prise accessoire est apparu. Près
de la moitié des marsouins rejetés sur
le littoral sont morts accidentellement
dans des engins de pêche, un taux qui a
de quoi inquiéter. On considère que les
trémails et les lets maillants, des types
d’engin de pêche par ailleurs relativement
respectueux de l’environnement, ont
été responsables de la plupart des prises
accessoires enregistrées.
Outre un manque de données sur
l’écologie du marsouin est apparu
un besoin urgent de données
supplémentaires sur le taux réel de prise
accessoire et sur la distribution spatiale
et temporelle des procédés de pêche
concernés. Pour obtenir ces données
il faudrait coordonner et standardiser
des initiatives de recherche à l’échelon
international. A la base, les fonds de
recherche devraient être structurels,
stables et indépendants de l’industrie
de la pêche et des pêcheurs. Pour le
moment, les initiatives de protection
sont dispersées entre de nombreuses
organisations. Pour coordonner les
effor ts scientiques en mer du Nord,
le forum le plus adéquat serait
probablement ASCOBANS. Pour
mettre au point des mesures correctives,
le cadre le plus approprié devrait être la
Commission Européenne, compte tenu
de sa compétence tant en matière de
pêche qu’en matière d’environnement.
Il faut aussi que des mesures de
prévention des prises accessoires soient
prises à l’échelon international pour la
pêche récréative.
Une des mesures de prévention des
prises accessoires les plus prometteuses
est l’usage de pingers. Il reste cependant
de nombreux problèmes à résoudre
pour leur utilisation et, à l’heure actuelle,
leur usage n’est pas obligatoire dans la
plupar t des pêcheries à trémail et let
maillant dans la par tie sud de la mer du
Nord.
Bien qu’actuellement peu de pêcheurs
belges et hollandais utilisent des trémails
ou lets maillants, cette situation
est en train de changer en raison du
prix du carburant et des objections
environnementales quant à l’usage du
chalut à perche. Il est par conséquent
probable qu’en l’absence de réelles
mesures de protection le problème de
la prise accessoire du marsouin dans
les régions côtières de la mer du Nord
aille en s’aggravant. En découdre avec
les problèmes auxquels le marsouin fait
actuellement face constitue évidemment
un dé, vu la combinaison de facteurs
environnementaux, climatiques, sociaux,
économiques, politiques, juridiques et
techniques dont il faut tenir compte.
7
The occurrence of the harbour porpoise
in the southern North Sea has been
irregular during the 20th century. The
fairly sudden reappearance of the
species in coastal waters of the southern
North Sea in the 1990s resulted in a
renewed research effort. This effort was
dedicated to the ecology, life history
and causes of death (Addink et al.,
1995; Leopold & Camphuysen, 2006;
Debacker et al., 2002; Jauniaux et al.,
2002a), but attention was also paid to
pollutant loads in the tissues of these top
predators (Chu et al., 2003; Das et al.,
2004; Evans et al., 2007; Van de Vijver et
al., 2004; Weijs et al., 2009a; b). Also since
1991 some stranded porpoises were
successfully rehabilitated and research
on anatomy, physiology, biomechanics,
behaviour and acoustics was conducted
in captive settings (Nachtigall et al., 1995;
Read et al., 1997).
Together with the increase of the
number of porpoises, an impor tant
threat for this vulnerable species became
apparent. Many of the corpses washed
ashore appeared to be animals that had
suffocated in shing gear, a problem
which was already well known in other
parts of the North Sea (Benke, 1994;
Berggren, 1994; Kinze, 1994; Carlström
& Berggren, 1997; Vinther, 1999; Vinther
& Larsen, 2004).
This repor t intends to review the
current situation regarding the status
of, and threats to the harbour porpoise
in the southern North Sea. The North
Sea Conservation Plan for the harbour
porpoise, as is being developed by
ASCOBANS (Reijnders et al., 2009;
see also Eisfeld & Kock, 2006), includes
similar information as provided in this
repor t. The focus on the report before
you lies with the historical abundance
of the porpoise in Belgian and Dutch
waters, on recent trends in numbers
of strandings and sightings, and on the
related specic conser vation problems in
these waters. However, the conclusions
and recommendations could be of value
for other parts of the species’ range.
1. Introduction
THE HARBOUR PORPOISE
in the southern North Sea
8
Description
The harbour porpoise (Phocoena
phocoena) is a toothed whale
(Odontoceti). With a maximum length
of only 1.8 m, it is one of the smallest
cetaceans in existence (Rice, 1998).
Porpoises are characterised by small
spatulate (chisel) teeth, a relatively
indistinct, triangular dorsal n, and a
blunt (rounded) head. The dorsal side
of the harbour porpoise is brownish to
dark grey, while the ventral parts are
bright white. A gradual colour change
is prominent on the sides of the animal.
Flippers and tail are blackish to dark grey
(Fig. 1 and 2).
Distribution
The harbour porpoise is found in the
cool and temperate coastal waters of
both the North-Atlantic and Pacic
Oceans. Throughout its range, it is
generally limited to the continental shelf.
Within European and adjacent waters,
the species is widespread and locally
abundant in Icelandic and Norwegian
waters, the North Sea, the Skagerrak,
the Kattegat, around the British Isles, the
eastern Channel and the Atlantic coast
of France. Smaller densities occur within
the Baltic Sea, and off Portugal and the
Atlantic coast of Spain. Sparse records in
the eastern part of the Mediterranean
are mostly linked to the isolated Black
Sea population (Birkun & Frantzis,
2006; Frantzis et al., 2001; Fontaine et
al., 2007; Güçlüsoy, 2007; Rosel et al.,
2003), although some strandings suggest
the presence of a small local stock
(Fernández-Casado et al., 2000; Frantzis
et al., 2003). An isolated population
exists off West Africa (Boisseau et
al.,2007).
Two dedicated abundance surveys
(SCANS I and SCANS II) concluded that
porpoises numbered around a quarter
of a million in the whole of the North
Sea (Hammond et al., 2002; SCANS II,
2008).
Reproduction
Porpoises are sexually mature at an
age of 3 to 4 (males) and 4 to 5 years
(females), and longevity is around 20
years. They reach sexual maturity at a
body length of around 1.35 m in males
and 1.40 m in females (Karakosta et
al., 1999; Lockyer, 1995a; b; Lockyer et
al., 2001; Van Deinse, 1925), although
geographical differences exist. Body size
and age at rst reproduction could also
vary in time, possibly as a consequence
of changed feeding conditions,
environmental factors or population
sizes, as has been noticed in the Bay of
Fundy, Canada (Read & Gaskin, 1990).
In the southern North Sea porpoises
are mostly born between May and
August, after a gestation period of 10
2. The harbour porpoise
Figure 1: The harbour porpoise is a small,
inconspicuous cetacean which in most cases will
avoid human presence.
9
to 11 months. A peak in births occurs in
June and July, as derived from strandings
of neonates and stillborn animals
(Addink et al., 1995). At birth porpoises
measure 0.7 to 0.8 m (Van Deinse,
1925). The lactation period lasts around
8 months. Females give bir th to one
young every year or, more likely, every
two years, which means this species
only reproduces very slowly (Fisher &
Harrison, 1970).
Social behaviour
Porpoises are inconspicuous animals.
They will generally ee from motorised
vessels, in contrast to several other
cetacean species. They are rarely found
in association with other cetaceans.
Porpoises generally do not form large
groups: usually they remain solitary or
form groups of 2 to 4 animals. In rich
feeding areas and during migration,
larger, but fairly loose congregations of
porpoises may be observed (Baptist &
Witte, 1996; Jefferson et al., 1993).
Feeding behaviour
Porpoises do not seem to specialise
on particular prey species, but can be
considered opportunistic feeders. Their
diet consists of many different species of
sh, cephalopods, crustaceans and even
polychaetes (Leopold & Camphuysen,
2006; Santos, 1998). It varies according
to area (geographical patterns in prey
distribution), season, and age of the
porpoise. Immature porpoises in the
southern North Sea were recently
found to focus on small demersal sh
(especially gobies Gobiidae), while
larger porpoises mainly feed on gadoids,
clupeids (sprat Sprattus sprattus and
herring Clupea harengus) and sandeels
(Ammody tidae), mostly smaller than 30
cm in length.
From recent diet studies in porpoises
stranded in The Netherlands, a
suggested historical link with the
seasonal occurrence of clupeids in
nearshore waters (Dudok van Heel,
1960; Rae, 1965; 1973; Ter Pelkwijk, 1937;
Verwey, 1975a; b; Verwey et al., 1947)
does not seem to be substantiated.
However, as Santos & Pierce (2003)
suggested, harbour porpoises in the
northeast Atlantic may have switched
to a diet based on sandeels, whiting
(Merlangius merlangus) and other sh
species following the decline in herring
stocks.
Unlike white-beaked dolphins
(Lagenorhynchus albirostris), North Sea
porpoises can frequently be observed
feeding close inshore, sometimes even
in the shallow waters of the surf zone.
The shallow southern part of the Nor th
Sea, and especially its coastal waters, can
be very turbid. There is little doubt that
porpoises can nd their food in these
murky waters by no other means than
echolocation (Kastelein et al., 1999).
They can even detect prey buried in
the sediment (Kastelein et al., 1997).
Fairly little is known about the hunting
techniques of harbour porpoises in the
southern North Sea. Verwey (1975a;
b) made a description of the feeding
behaviour of harbour porpoises in very
shallow parts of the Dutch coastal waters
and the western Wadden Sea: With
dorsal ns constantly exposed, animals
worked their way through the shallowest
Figure 2: A stranded dead por poise: for many
people the only chance to see a porpoise nearby.
THE HARBOUR PORPOISE
in the southern North Sea
10
waters near the shore”. In all situations,
the number of porpoises present during
a feeding bout in shallow inshore areas
was fairly small, half a dozen animals at
most, with not the slightest indication of
a co-ordinated drive hunt.
Off the Dutch coast, some areas are clearly
favoured, such as the outer Marsdiep
area (western Wadden Sea), the edges
of some deeper waters in the western
Oosterschelde off Burghsluis, and waters
around the piers of IJmuiden. An attractive
local feeding opportunity is the most likely
explanation for the frequent presence of
porpoises in these areas. Recent sightings
of apparently feeding animals also highlight
the importance of nearshore river plume
fronts and tidal eddies as feeding grounds
(C.J. Camphuysen, unpublished data).
Along most of the sandy coasts of Belgium
and The Netherlands, however, the feeding
behaviour is generally much harder to study
and, hence, the possible signicance of
these waters cannot easily be determined.
In offshore waters of the southern
North Sea, co-ordinated drive hunts
for pelagic sh are frequently seen,
often characterised by associated
seabirds: northern gannets (Morus
bassanus), black-legged kittiwakes (Rissa
tridactyla) and several tern species
(Sternidae; C.J. Camphuysen, personal
observation). It has been impossible to
identify the targeted prey under these
conditions, but these drive hunts are
typically conducted by small numbers of
porpoises (2 to 4 animals, occasionally
more) working in a concerted action and
swimming at high speed while driving
prey sh towards the surface. Potential
prey candidates under these conditions
are sandeels, clupeids, and perhaps even
mackerel (Scomber scombrus) and horse
mackerel (Trachurus trachurus): shoaling
sh in the water column, rather than
more dispersed demersal sh species.
It should be noted that pelagic shing
behaviour by cetaceans is obviously
considerably easier to spot than any
activities on the sea oor.
The frequent local co-occurrence of high
densities of harbour porpoises and divers
(Gaviidae) in the southern North Sea
(Camphuysen, 2004b; Camphuysen &
Leopold, 1998; Leopold, 1996) and the
remarkable similarities in the diet of these
two types of predators (M.F. Leopold pers.
comm.; Leopold, 2001) indicate similar
demersal foraging habits in this area.
Natural enemies
Natural enemies of the harbour porpoise
in the North Sea are large sharks, killer
whales (Orcinus orca) and common
bottlenose dolphins (Tursiops truncatus),
but none of these species commonly
occur in Belgian and Dutch waters.
Several of the (very few) killer whales
found stranded in The Netherlands were
found to have the remains of harbour
porpoises in their stomach (Camphuysen
& Peet, 2006; Van Dieren, 1931; van
Laar, 1963). From a detailed analysis of
the sightings data presented by Verwey
(1975a; b) in the Marsdiep area, it is
obvious that porpoises were relatively
rare in spring, when bottlenose dolphins
entered the area for their annual hunt for
Zuiderzeeharing (a herring race). From
ship-based observations in the North Sea
at large (including the southern North
Sea), it is obvious that dolphins (mostly
white-beaked dolphins) and harbour
porpoises do not normally co-occur in
substantial numbers in the same areas at
the same time (ESAS, unpublished data).
11
3. Legal framework for the protection of the
harbour porpoise
Harbour porpoises, as all cetaceans in
the North Sea, are legally protected by
Belgium and The Netherlands by means
of international and national legislation.
This means that intentional killing,
intentional disturbance, and trading or
collecting animals or parts of them is
illegal.
3.1 International fora
Cetaceans are very (or even the most)
popular marine animals, in particular
because of their intelligence, their
impressive size and their elegance.
However, some species are very
vulnerable, or even threatened with
extinction. For these reasons cetaceans
were among the rst animals to have
been legally protected. Since most
cetaceans are wide-ranging, often
displaying migration patterns, protection
measures are to be co-ordinated on an
international level.
There is a variety of international
conventions, agreements and action
plans dealing with the protection and
conservation of cetaceans (Trouwborst
& Dotinga, 2008). They provide a
framework for their parties to adopt
national implementing legislation.
The following instruments will not be
discussed in detail, but are mentioned
for completeness: the United Nation’s
Convention on the Law of the Sea
(UNCLOS)1, the Code of Conduct for
Responsible Fisheries of the Food and
Agriculture Organisation of the United
Nations (FAO)2 and the Convention on
Biological Diversity of the United Nations
(CBD)3.
UNCLOS represents the constitution of
the oceans and sets out the global legal
framework for human activities at sea.
UNCLOS requests Par ties, inter alia, to
cooperate in the conservation of marine
mammals in the EEZ and the high seas,
and in the case of cetaceans to work
through the appropriate international
organizations for their conservation,
management and study.
The FAO Code of Conduct for
Responsible Fisheries provides a
non-legally binding framework for
the international and national efforts
towards a sustainable use of living
aquatic resources, in harmony with the
environment. It sets an obligation to sh
in a responsible way, in order to ensure
the conser vation of target species and
species belonging to, or associated
with the same ecosystem. States
should minimise bycatch of non-target
species, and undertake research into
the selectivity of shing gear and their
environmental impact.
By signing the CBD, world leaders
committed, among other things, to the
conservation of biological diversity and
the sustainable use of its components.
For Belgium and The Netherlands,
OSPAR (see below) and the European
Directives can be considered as the most
important instruments for implementing
the marine aspects of the CBD.
The international conventions and
agreements detailed hereunder have
been signed and ratied, amongst
other nations, by Belgium and The
Netherlands. Specic sheries measures
taken at a national and an international
level, with a direct or indirect effect on
the harbour porpoise in the southern
North Sea, are being dealt with in
chapter 5.
The IWC
The International Whaling Commission
(IWC)4 was set up in 1946 by the
International Convention for the
Regulation of Whaling (ICRW) to work
on conservation and management of
whale stocks. Although the application
of the ICRW to small cetaceans is still
questioned by some Par ties, they have
been a focus of study and management
advice within the Sub-Committee
Small Cetaceans of the IWC Scientic
Committee. Several IWC resolutions5
have been adopted concerning directed
and incidental catches of small cetacean
species, and a reduction of bycatch
levels of porpoises in the North-Atlantic
has been recommended.
CMS and ASCOBANS
The Convention on the Conservation of
Migratory Species of wild animals6 (also
known as CMS or the Bonn Convention)
is a global convention concluded in 1979
under the United Nations Environment
Programme (UNEP). It aims to conserve
migratory species throughout their
range. Appendix I to the Convention lists
migratory species that are threatened
with extinction. Species that need,
or would benet from international
co-operation are listed in Appendix
II. For these species (a.o. the harbour
porpoise), CMS encourages states to
conclude regional Agreements. CMS
has repeatedly adopted resolutions
dealing with the assessment of human
impacts on cetaceans and the limitation
of incidental catches7. Such resolutions
however, do not specically refer to
harbour porpoises; they are very general,
and only impose a moral obligation upon
Parties.
THE HARBOUR PORPOISE
in the southern North Sea
12
Under CMS, the regional Agreement
ASCOBANS was concluded. This
Agreement, originally named the
Agreement on the Conservation of Small
Cetaceans of the Baltic and North Seas8,
aims to conserve the small cetaceans
(all toothed whales except for the
sperm whale) in the Baltic and North
Seas9. The current ASCOBANS area
(Fig. 3) also covers waters under the
jurisdiction of Ireland, Spain, Portugal,
Norway, Estonia, Latvia and the Russian
Federation, although these range states
have chosen not to become a party.
The principle aim of ASCOBANS is
to achieve an efcient protection of
small cetaceans through international
co-operation. This should be achieved
through the implementation of a
Conser vation and Management Plan
(annexed to the Agreement text) with
the following priority actions:
1. Habitat conservation and
management, including the
reduction of pollution and bycatch,
the regulation of activities with an
indirect impact on small cetaceans,
the prevention of disturbance and
the establishment of protected areas;
2. Surveys and research, including
research on stranded animals;
3. Effective national legal protection;
4. Information and education of the
public on the species and issues.
ASCOBANS has been successful in
bringing together administrators,
scientists and delegates from the
Parties, range states, and relevant inter-
governmental and non-governmental
organisations. One of its major
achievements is the development of the
generally adopted principle that annual
bycatch levels of small cetaceans should
be less than 1.7 % of the best population
estimates, and that a take of porpoises
of less than 1 % of the population size
should be an intermediate precautionary
objective10. An annual bycatch above 1.7
% of the population is unsustainable, and
therefore unacceptable.
ASCOBANS has recently been criticised
for its lack of progress. Indeed: it so far
failed to yield legally binding protection
measures. This is mainly due to a lack of
(legal) competence, especially in the eld
of sheries. On the other hand, many of
the subjects dealt with by ASCOBANS
have been taken up in other fora, and
part of the achievements of ASCOBANS
can be traced back in management
actions taken at other levels, both
internationally and nationally. Up to
now, most of the efforts in ASCOBANS
have been dedicated to the harbour
porpoise, although it is also competent
for the protection of the other small
cetaceans in the Agreement area.
North Sea Conferences
The International Conferences on the
protection of the North Sea (NSC)11
are political events where ministers
responsible for the protection of the
marine environment assess which
additional measures should be taken.
Although the commitments made
at the NSC are not legally binding,
they have high political force and
reect the solutions nations around
the North Sea are considering for
Figure 3: Map of the new ASCOBANS Area
indicating the original and extended Agreement area
and the r ange states. Current Parties are France,
the United Kingdom, Belgium, The Netherlands,
Germany, Denmark , Poland, Lithuania, Finland and
Sweden.
13
resolving environmental problems. In
many cases, these commitments are
endorsed afterwards in legally binding
fora such as the OSPAR Convention or
the European Community legislation.
At the 5th NSC, held in March 2002
in Bergen, Norway, it was agreed that
the porpoise bycatch level should be
reduced. As an interim objective, the
Ministers of Nor th Sea riparian states,
along the lines of ASCOBANS, agreed
to reduce annual bycatches to below
1.7 % of the best population estimate. In
2006 they agreed that special attention
should also be given to the development
of shing gear and shing methods that
will help to reduce by-catches of marine
mammals to less than 1 % of the best
available population estimates. They
furthermore agreed that a Recovery Plan
for the harbour porpoise in the North
Sea should be developed (Anonymous,
2002). The development of a Recovery
Plan was taken up by ASCOBANS,
building on the experience with a
recovery plan for the porpoise in the
Baltic (the Jastarna Plan).
EU Habitats Directive
The European Community Directive on the
Conser vation of Natural Habitats and of
Wild Fauna and Flora12 (commonly know
as the Habitats Directive) was adopted
in 1992. It contains a list of species of
Community importance that have to
be strictly protected (Annex IV). Next
to this, a number of species have been
listed for which sites need to be selected
by Member States to contribute to
conserving their habitats (Annex II).
These Special Areas of Conservation
(SACs) should constitute, together with
the areas listed under the European
Birds Directive (Special Protection
Areas or SPAs), a network of sites which
will act as the backbone of the European
policy concerning habitat protection: the
NATURA 2000 network. The harbour
porpoise is included in Annex II and
Annex IV of the Habitats Directive.
While through this Directive the
harbour porpoise is protected in all EU
Member States, it has been difcult to
establish protected areas specically
for this species throughout Europe.
Article 4 of the Habitats Directive
states that for aquatic species which
range over wide areas, [SAC’s] will be
proposed only where there is a clearly
identiable area representing the physical
and biological factors essential to their
life and reproduction. Given the lack of
detailed data on the distribution and
reproduction of the harbour porpoise,
the migratory nature of the animal and
its fairly irregular occurrence in certain
areas, it has been difcult to assess the
location of the most suitable sites. It is
now more and more acknow ledged that
it might not even be relevant to identify
SAC’s for this wide-ranging species. This
is not only the case for the harbour
porpoise. Given the lack of information
on the distribution of many marine
species and habitats, the establishment
of the NATURA 2000 network at sea is
still far from complete.
While the European nature conservation
legislation, together with international
treaties, imposes a strict protection
of the porpoise, the main threat for
porpoises are certain shing activities.
Fishing activities are mainly managed
by the European Commission in its
Common Fisheries Policy. The Habitats
Directive only touches sheries and
the bycatch problem marginally and in
general terms (in Article 12(4), Article
15, and Annex VI). According to Article
12(4), Member States must introduce a
system to monitor the incidental capture
and killing of Annex IV species, and are
required to under take further research
or conservation measures to ensure that
the incidental capture and killing does
not have a signicant impact on the
species concerned. Article 15 states that
Member States should prohibit the use
of all indiscriminate means capable of
causing local disappearance, or serious
disturbance to, populations of species
of Annex IVa for which a derogation is
applied [according to Article 16]. In
Annex VI prohibited means of capture
and killing are listed, among which nets
which are non-selective according to their
principle or their conditions of use.
While some sheries provisions seem
to have been taken in the Habitats
Directive, it is clear that in practice
they remained idle in most Member
States. This is due to the fact that in
most cases different national authorities
are responsible for sheries and for
environmental protection, and too little
communication exists between them.
The relevant sheries measures, both
on a European and national (Dutch
and Belgian) level, are being dealt with
further on in this document.
For the member states of the European
Community, the obligations under the
1979 Convention on the Conser vation of
Wildlife and Natural Habitats in Europe13,
also known as the Bern Convention,
have been largely taken up into the
European Habitats and Birds Directives.
Next to the Habitats Directive, also the
European Marine Strategy Directive,
adopted in 2008, is of relevance
14
.
Maintaining biological diversity is one of
the quality descriptors to achieve Good
Environmental Status. This means that
biological diversity is maintained, and that
the quality and occurrence of habitats, and
the distribution and abundance of species,
are in line with prevailing physiographic,
geographic and climatic conditions.
CITES
The 1973 Convention on the International
Trade in Endangered Species of wild fauna
and ora, also known as CITES or the
Convention of Washington
15
aims to
ensure that the trade in species does
not threaten their survival in the wild. In
several appendices, CITES sets varying
regulations for the international trade in
species: in live plants and animals, as well as
THE HARBOUR PORPOISE
in the southern North Sea
14
in parts of them. The harbour porpoise is
included in Appendix II, which lists species
that are not necessarily threatened with
extinction, but may become so unless
trade is closely controlled. International
trade in specimens of Appendix II species
may be authorized by the granting of an
export permit or a re-export certicate.
No import permit is necessary for these
species under CITES.
Within the European Union CITES
has been implemented since 1 January
1984 through regulations known as
the Wildlife Trade Regulations16. The
provisions in the EU Wildlife Trade
Regulations are stricter at some points
than CITES. All cetaceans are listed in
Annex A of Council Regulation 338/97,
which effectively treats them as if
they were CITES Appendix I species.
Commercial trade of these species
within the European Community is
therefore not allowed.
OSPAR Convention
The 1992 Convention for the protection of
the marine environment in the north-east
Atlantic (OSPAR)17 is managed by the
OSPAR Commission, which consists of
the representatives of the 15 OSPAR
Parties18 and the European Commission.
In its Annex V and Appendix 3,
OSPAR deals with the protection and
conservation of the ecosystems and
biological diversity of the maritime area.
In order to guide the setting of priorities for
the implementation of Annex V, OSPAR
has compiled an initial list of species and
habitats to be protected because they are
threatened and/or declining. The harbour
porpoise is one of the species on the
initial OSPAR list (OSPAR, 2004; 2006a).
Given this listing, OSPAR is looking into
the relevance to take measures in an
OSPAR framework. Undoubtedly the
OSPAR programmes and measures in the
framework of the reduction of pollution
are benecial to the harbour porpoise,
but they are generic, and not aimed
specically at marine mammals. OSPAR
is not competent to adopt measures in
the eld of sheries management, but it
can draw the attention of the relevant
authorities, including the European
Commission, to issues where it considers
that action is desirable.
To help full its commitments in
applying an ecosystem approach to the
management of human activities that may
affect the marine environment, OSPAR
is developing Ecological Quality Objectives
(EcoQO’s) for the North Sea as a test
case. These EcoQO’s can be considered
as objectives for a number of indicators,
which are related to environmental
problems. One of the objectives that
OSPAR has put forward is that annual
bycatch levels should be reduced to below
1.7 % of the best population estimate,
along the lines of ASCOBANS and NSC
recommendations (OSPAR, 2006b).
3.2 National legislation
in Belgium and The
Netherlands
The harbour porpoise is legally
protected through national legislation in
Belgium and The Netherlands.
In Belgium the harbour porpoise is a
protected species according to a Royal
Decree issued in 198019 and a Ministerial
Decree issued in 199720 (Flemish regional
legislation). In the marine environment,
the species is legally protected through
a framework law on the protection
of the marine environment issued in
199921. The general measures set out
in this law were further implemented
through a Royal Decree in 2001,
which was signed both by the minister
responsible for the environment of the
Belgian part of the Nor th Sea, as by the
minister responsible for marine sheries.
It contains protection measures as
well as measures to facilitate scientic
research22 .
In The Netherlands the harbour
porpoise is protected through the Law
on the Flora and Fauna23. According to
this legislation it is illegal to kill, wound,
catch, and obtain protected species, to
track them with the abovementioned
intentions, or to disturb them on
purpose.
15
4. The harbour porpoise in
Belgian and Dutch waters
4.1 Before the 20th century
There is substantial, but fairly anecdotal
information on the harbour porpoise in
Belgium and The Netherlands before the
20th century. Archaeolo gical excavations
of pre-historic coastal settlements have
revealed the use of harbour porpoises by
early Neolithic (human) hunter-gatherer
populations roaming the coasts of the
southern North Sea in what is now
called Belgium and The Netherlands
(Louwe Kooijmans et al., 2005). The
animal is frequently mentioned in
historical accounts that date back to
the Middle Ages. For instance, it can be
traced back in the accounts of the bailiffs
of the county of Flanders, because of the
claim that was laid on stranded animals
by the count (De Groote, 1999). The
animal is often mentioned in early natural
history publications, but in many cases it
was not discriminated from other small
cetaceans.
Early descriptions of porpoises which
include some life-history information,
are those given by Adriaen Coenen
(1585) and Mattheus Smallegange
(1696). Historic records indicate that
the porpoise was part of the diet of
coastal inhabitants, and that it was
actively hunted (Fig. 4). The rst
cooking book in Dutch even describes
a recipe to prepare porpoise with
pepper. The historic information on
strandings, sightings and catches of
harbour porpoises in Belgium and The
Netherlands is extensively described by
Camphuysen & Peet (2006).
Figure 4: Information on the occurrence of the
porpoise in the (distant) past is largely anecdotal.
This painting by Frans Snyders (Antwerp, 1579-
1657) describes in a baroque way the sh market
in Flanders in the 17th century. Nex t to some
sh species that are rare now, we nd a har bour
porpoise at the lower right hand side of the painting
(© KMSK Antwerp).
THE HARBOUR PORPOISE
in the southern North Sea
16
4.2 Research on the harbour
porpoise in Belgium and
The Netherlands during
the 20th century
The harbour porpoise appears not to
have been an attractive study subject
in the southern North Sea during the
rst half of the 20th century, due in
part to its high abundance, but also to
its inconspicuous nature and small size.
Studies and publications of rarer and
more spectacular cetaceans that washed
ashore were seemingly more rewarding.
Also sightings were rarely reported,
probably because the animal was so
common. Therefore, information about
the harbour porpoise in the early 20th
century in The Netherlands and Belgium
is scarce.
Research in The Netherlands
Observations of porpoises were rarely
recorded in the past, except when they
occurred in unusual areas, such as far
upstream rivers or in cities. When the
Zuiderzee was closed by the Afsluitdijk
(a barrier dam connecting Friesland
with mainland Noord-Holland) in 1931,
and became the lake IJsselmeer, the
fate of the enclosed harbour porpoises
was reported in an anecdotal way only
(Stoppelaar et al., 1935). Given the
expected radical changes of this sea area
that was to change into a freshwater
lake, most organisms were carefully
monitored. However, our knowledge
of the seemingly resident porpoises
in the area prior to, and following the
enclosure is very incomplete (Heinsius,
1914; Redeke, 1922; Weber, 1922).
One of the rst to study the porpoise
in the wild was Jan Verwey, former
director of the Royal Netherlands Institute
for Sea Research (NIOZ). He described
the behaviour and seasonal trends of
harbour porpoises in Dutch nearshore
waters before and after World War II
(Verwey 1975a; b). His descriptions of
porpoises in the Marsdiep area are some
of the very few rst-hand accounts of
porpoise behaviour and uctuations in
seasonal abundance in Dutch nearshore
waters.
Antonius Boudewijn van Deinse (1885-
1965), who was fully devoted throughout
his life to the proper documentation of
cetaceans stranded in The Netherlands,
considered harbour porpoises too
abundant to be of interest. Only
‘abnormal’ cases were reported and
the annual number washed ashore was
often indicated by the mathematical
symbol (innite; Van Deinse,
1925; 1931; 1946 and annual repor ts
1944-1966). Growth rates of porpoises
and reproductive characteristics were
investigated by van Utrecht (1978).
After the death of Van Deinse in
1965, a systematic recording of
(reported) strandings of cetaceans in
The Netherlands was taken up again
only in 1970. From 1970 onwards,
details on porpoises were included in
the stranding reports (Husson & van
Bree, 1972; 1976; van Bree & Husson,
1974; van Bree & Smeenk, 1978; 1982;
Smeenk, 1986; 1989; 1992; 1995; 2003).
After the retirement of Smeenk in 2005,
the strandings data were digitised by
C.J. Camphuysen and forwarded to
Naturalis, Leiden (the Dutch national
Natural History Museum). Today,
strandings data are made available to
the general public through a website24,
and the bones of rarer cetaceans, and
a selection of stranded porpoises, are
added to the extensive collections of
this museum.
Dutch seawatchers, systematically
recording the passage of migratory
seabirds, waders and waterfowl since
1972, were the rst to report some
sightings of porpoises from the Dutch
coast (Camphuysen, 1982; 1985;
Camphuysen & Van Dijk, 1983). It was
from these effor t-corrected data that
the return of the harbour porpoise
in coastal waters of the southern
North Sea started to be properly
documented (Camphuysen, 1994;
2004a; Camphuysen & Leopold, 1993).
Research in Belgium
In the 19th century Pierre-Joseph Van
Beneden (1809-1894), professor at
the University of Leuven, had studied
cetaceans, but mostly the larger whales
and fossil cetaceans found in abundance in
deposits around Antwerp. A systematic
investigation of stranded animals did
not exist during the 19th century
and the rst half of the 20th century.
Data on small cetaceans in Belgium
remain therefore largely anecdotal and
scattered. It seems that only porpoises
caught tens of kilometres upstream the
river Scheldt found their way into journals
and local papers – nobody seems to have
expressed an interest in strandings of
porpoises on beaches, or what must have
been regular sightings at sea. At least
in the 19th century the porpoise was
considered common in the Scheldt up to
Antwerp, and at the Belgian coast, where
strandings frequently occurred (De Selys-
Longchamps, 1842; Lameere,1895).
Early Belgian strandings- and sightings
data were summarised by Wim De Smet
(°1932). He published two extensive
repor ts on the historic occurrence
of cetaceans along the Flemish coasts,
which stretch from Calais, France, to
the mouth of the river Scheldt, The
Netherlands (De Smet, 1974; 1981).
In the 1960s, Wim De Smet initiated,
together with the University of Antwerp
and later with the Fisheries Research
Station, a more systematic research of
stranded cetaceans (De Smet, 1979).
The increased interest in cetaceans
in those years followed an increasing
environmental awareness caused by
marine pollution and its consequences.
From the 1970s onwards, data on strandings
and sightings in Belgium were collected by
the veterinary surgeon John Van Gompel,
in co-operation with the Royal Belgian
17
Institute of Natural Sciences (RBINS) and
later with the University of Liège. From this
period onwards, a clearer picture exists
about the occurrence of porpoises in
Belgium (Van Gompel, 1991; 1996).
In order to contribute to specic
obligations of the Belgian government
in the framework of a number of
international commitments, a dedicated
and government supported research
network was established in 1992.
This multidisciplinary network is co-
ordinated by the Management Unit of
the North Sea Mathematical Models
(MUMM), a Department of the RBINS.
MUMM maintains a database on marine
mammal strandings and sightings, par t of
which can be consulted on its website2 5.
Selected remains of stranded cetaceans
are taken up in the extensive natural
histor y collection of the RBINS, and
some are on display in its museum.
4.3 The occurrence of
the harbour porpoise
in Belgium and The
Netherlands in the 20th
and early 21st century
Despite our data being very incomplete,
it is possible to at least reconstruct
shifts in abundance of porpoises
through most of the 20th and early
21st centuries (see also Camphuysen
& Peet, 2006; Reijnders, 1992; Smeenk
1987). From 1900 to the early 1950s,
harbour porpoises were considered
“abundant and widespread“ in coastal
waters throughout the southern
North Sea (Van Deinse, 1925). There
are no reasons to doubt that prior to
that period the shallow waters of the
southern North Sea, with its estuaries
and river mouths, and even the Wadden
Sea and the former Zuiderzee, had
been prime habitats for porpoises for
centuries (Camphuysen & Peet, 2006).
Occurrence in The Netherlands
According to Weber (1922) and Van
Deinse (1925), porpoises were common
not only along the Dutch coast, but
also in the Zuiderzee. In the latter
area, porpoises were observed to hunt
anchovy (Engraulis encrasicolus) and
garsh (Belone belone) during summer
months (Heinsius, 1914), but since this
is the only original published account of
some sightings, we have no idea of how
representative the report actually is for
these porpoises. Shor tly after the closing
of the Zuiderzee, its entire surface froze
during a severe winter, and all enclosed
porpoises died (Stoppelaar et al., 1935).
Some authors claimed that during the
early 20th century harbour porpoises
were most numerous in summer
months, and go as far as claiming that
harbour porpoises were as strongly
coupled with good (war m) summer
weather as swallows may be expected as
spring migrants (IJsseling & Scheygrond,
1943). Neither the strandings data, nor
the anecdotal sightings data currently
available, seem to support that claim
(Camphuysen & Peet, 2006; Verwey,
1975a; b).
More detailed information on the
occurrence of the harbour porpoise
in The Netherlands is available from
around World War II, especially about
porpoises in the western Wadden
Sea. Before World War II, Jan Verwey
and his colleagues of the zoological
station at Den Helder saw porpoises
almost daily, although number s varied
on a monthly basis (Ver wey, 1975a;
b). Small numbers were observed
from February to May, and these
increased in June and July. The highest
numbers were seen from December
to Februar y. For reasons not quite well
understood, the formerly abundant
species gradually disappeared after
World War II, somewhere during the
1950s and 1960s. At rst, the decline
was reported by some naturalists, but
ignored - or denied - by established
scientists such as Van Deinse (1952;
1956; 1957; 1958; 1960; 1961) and
Vader (1956). An incidental report
published by Dudok van Heel (1960)
on 40 to 50 harbour porpoises in
mid-January 1958 in the Texelstroom
area (western Wadden Sea) seemed
to conrm that harbour porpoises
were still numerous. This was the last
sighting of any signicance, however,
and virtually none were repor ted in
the 1960s and 1970s in the waters
around The Netherlands. In the
1970s, sightings of harbour porpoises
were so rare that the animal might
as well be considered locally ex tinct
(Camphuysen, 1982). Between 1970
and 1985, Dutch seawatchers recorded
only 20 porpoises during 40.000 hours
of observations, illustrating well how
rare this species was in coastal waters.
Occurrence in Belgium
From the publications by Wim De
Smet (1974; 1981) it appears that the
harbour porpoise was common during
the rst half of the 20th century in
Belgian waters, but the evidence for
this is very anecdotal. As was noticed
in The Netherlands, it appeared that
the numbers of harbour porpoises had
declined in Belgian waters after World
War II. Wim De Smet tried to organise
a scientic investigation of stranded
marine mammals from the 1960s
onwards, but he succeeded in collecting
only very few porpoises. Also during
the period when John Van Gompel
recorded strandings, and performed
scientic investigations of stranded
animals, their numbers remained very
low (Van Gompel, 1991; 1996), with on
average only 1 recorded stranding per
year between 1970 and 1989. Although
a number of strandings were probably
not recorded, this indicates the scarcity
of the porpoise in Belgian waters during
those decades.
THE HARBOUR PORPOISE
in the southern North Sea
18
4.4 Increase in the number of
porpoises in the southern
North Sea at the end of
the 20th century: evidence
from stranding records
Not only sightings of porpoises were
rare from the 1960s to the 1980s, also
strandings were scarce and far apart.
The total number of stranded animals
recorded per year in The Netherlands
and Belgium between 1970 and 2007 is
shown in gure 5. In the 1970s between
6 and 29 porpoises were found stranded
annually in The Netherlands, clearly
contrasting with the innite number
repor ted by Van Deinse in the rst half
of the 20th century. The total number
of recorded strandings in the 1970s and
1980s in Belgium was only 21. From the
late 1980s (The Netherlands) and the
mid-1990s (Belgium) the number of
stranded porpoises steadily increased.
As with the historic decline (Reijnders,
1992; Verwey, 1975a; b), also the increase
in occurrence as evidenced through
sightings, appeared to be concurrent
with trends in strandings.
In The Netherlands on average 30
porpoises washed ashore per year in
the 1980s. This increased to 45 per
year in the 1990s, and 242 per year in
the early 21st century (data from the
national strandings database managed
at Naturalis Leiden and at Royal
NIOZ, Texel; Addink & Smeenk, 1999).
Recorded stranding numbers in Belgium
increased from on average 6 per year
in the 1990s to 49 per year in the early
21st century (data from De Smet, 1974;
1981; Van Gompel, 1991; 1996; database
MUMM). The number of stranded
porpoises was considerably lower in
2007 than in 2006. Data collected in
the rst four months of 2008 show
that it has declined even further. The
coming years will show whether this
recent decline will continue, or if it was a
temporary phenomenon.
4.5 Increase of the numbers of
porpoises in the southern
North Sea at the end of
the 20th century: evidence
from sightings
Sources of information
The numbers and distribution of
porpoises at sea originate from different
sources. In the late 1970s, a standardised
protocol to record seabirds at sea
was developed (Tasker et al., 1984).
The standardisation of observation
techniques (line-transect sur vey methods)
permitted the construction of a joint,
international database on seabirds:
the European Seabirds at Sea database
(ESAS database; Reid & Camphuysen,
1998). During the seabird surveys
also marine mammal sightings were
recorded, together with temporal and
spatial patterns in observer effort.
These marine mammal observations
constituted the rst, and to date the
most comprehensive dataset on the
abundance and spatial distribution of
cetaceans in the North Sea.
Both in the summer of 1994 and 2005
an international survey was carried
out, dedicated to the assessment of
the porpoise population size within
the North Sea and adjacent areas.
These so-called SCANS I and SCANS II26
0
100
200
300
400
500
600
700
1970
1972
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
The Netherlands
Belgium
Figure 5: Annual number of recorded stranded
harbour porpoises in Belgium and The Netherlands
from 1970 to 2007 (data: NIOZ, Natur alis and
RBINS/MUMM). In both countries a similar trend
occurred: a vir tual absence of strandings during the
1970s and 1980s, a steady increase between the
1990s and 2006, and a slight decline in 2007.
19
surveys included Belgian and Dutch
waters. Apart from these two extensive
international research campaigns, there
have been a number of dedicated
national aerial and ship-based marine
mammal surveys over smaller areas,
usually in the framework of projects, or
in the implementation of the obligations
under the Habitats Directive.
Another source of information for the
occurrence of porpoises in nearshore
waters is the sea-watching dataset of
the Nederlandse Zeevogelgroep (NZG),
containing besides numbers and species
of birds migrating along the coastline,
also sightings of marine mammals. These
effor t-corrected sightings data are
available form 1972 onwards. In recent
years, most seawatching results, also
from the Belgian coast, are instantly
made available on the internet27, allowing
immediate analyses and comparisons of
patterns in abundance throughout the
year and over the surveyed (coastal)
area.
Useful information can also be
obtained from the non-effort-related
sightings repor ted by the public, or by
people active at sea on board aircraft,
helicopters, shing vessels, ferries or
gas rigs28. Although not effort related,
these anecdotal data do give valuable
information, and fur ther encourage
people to report their sightings, to
learn how to discriminate the different
species, and to understand conservation
effor ts. Obvious restrictions of the
incidental sightings are the following:
1. Misidentications are more likely
to occur, although there are very
few cetacean species in Belgian and
Dutch waters that can be confused
with harbour porpoises.
2. Sightings tend to be concentrated
around marinas, coastal vantage
points and frequently used shipping
lanes for yachts.
3. One could expect that sightings
predominantly occur during periods
with a higher level of marine
recreation (summer).
4. The number of sightings reported
has increased due to an increasing
attention of the public towards
environmental matters, and the
easy way in which sightings can be
repor ted through the internet.
However, in all previous compilations
of these data it was obvious that
trends in incidental sightings of harbour
porpoises closely followed both spatial
and temporal trends in sightings from
effor t-corrected data and strandings
information (e.g. Camphuysen, 2004a).
Yearly trends in the occurrence
The Sea Watch Foundation, the Joint
Nature Conser vation Committee (JNCC)
and the Sea Mammal Research Unit
(SMRU) systematically analysed effort
related sightings data collected up to
around 2000 in British and adjacent
waters, including ESAS and SCANS I
data. The distribution of porpoises in
the North Sea which is presented (Reid
et al., 2003), is from before the species
returned to the southern Nor th Sea.
Porpoises were common throughout the
North Sea, except in its southernmost
part, including Belgian and Dutch coastal
waters.
The return of the harbour porpoise
in Dutch coastal waters was rst
noticed during the monitoring of
migrating birds from the coastline by
seawatchers (Fig. 6). This increase in
sightings in the 1990s and the early 21st
century could only be described as a
spec tacular come-back (Camphuysen,
2004a). From the mid-1990s to the
early 21st centur y, an annual increase
of on average 41 % was found, which is
clearly more than the potential natural
population growth. At rst, only fully-
grown animals were seen, and the
occurrence was virtually restricted to
mid-winter. In later years, numbers
not only sharply increased in winter
THE HARBOUR PORPOISE
in the southern North Sea
20
and spring, but more animals, including
some mother-calf pairs, were seen also
in summer and autumn.
An increase in sightings of harbour
porpoises in the Dutch sector of the North
Sea was reported from systematic aerial
surveys between 1985 and 1997 by Witte
et al. (1998); this was conrmed by a later
analysis of aerial survey data from 1991
to 2003 by Osinga (2005). The increase
in sightings reported from Zeeland
occurred after 2000, 12 years after the
observation of an increase in the northern
part of Dutch waters (Camphuysen &
Heijboer, 2008). In 2007, the number of
sightings reported by the public, and the
sightings from the seawatchers along the
coastline, indicated a drastic decline in
the abundance of harbour porpoises in
nearshore waters (Fig. 6).
Based on anecdotal records, the number
of sightings in Belgian waters started
to increase a few years later than in
The Netherlands, possibly indicating a
continued extension of the range of the
species towards the south during the
beginning of the 21st century (Fig. 7).
Data gathered by the Research Institute
for Nature and Forest (INBO) during
seabird surveys had also demonstrated the
increase in Belgian waters (Courtens et al.,
2008). The trends in sightings in Belgian
waters and in the most southerly waters
of The Netherlands (Zeeland) are similar.
Seasonal pattern of occurrence
Next to the annual trend, the sightings
data also indicate seasonal movements
of the species. An analysis of the
seasonal pattern in sightings reported
to the NZG Marine Mammal database
is interesting in the sense that prior to
2000, when nearshore abundance was
still low, distinct peaks in the number of
reported porpoises can be observed in
April and December (Fig. 8). Neither one
can be explained easily by peaks in likely
sightings from the activities of shermen,
yachtsmen and research vessels at sea.
The peak in coastal sightings is modest
and restricted to March.
In recent years (2000-2007), following a
drastic increase in sightings frequency, a
distinct spring peak in repor ted sightings
was obvious, followed by a dip in June and
a slightly higher level from July onwards
(Fig. 8). The seasonal pattern can be
described as follows: low densities during
summer, a gradual increase in autumn
towards a peak in abundance in winter
0
200
400
600
800
1000
1200
1400
1970
1972
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
Number of animals
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Sighting per hour
Incidental sightings
Sightings per hour during seabird
censuses
0
20
40
60
80
100
120
140
160
180
200
1970
1972
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
Figure 6: Annual sightings of harbour porpoises in Dutch coastal waters (coastal observations only), including
effor t corrected sightings from seawatchers (animals hour-1, orange line, right y-axis) and incidental sightings
repor ted by others (number of animals repor ted, red, lef t y-axis). Note the spectacular decline in sightings in
2007.
Figure 7: Annual number of porpoises repor ted by the public from 1970 to 20 07 in Belgian water s (anecdotal
sightings only) (data MUMM).
21
and early spring (in particular in February
and March), followed by a sharp decline
during spring. The pattern on the basis
of anecdotal observations in Belgium
between 2000 and 2007 was slightly
different: the bulk of the porpoises
seemed to arrive in Belgian waters
slightly later in the year than in Dutch
waters, and peaked from February
to April; hardly any sightings were
repor ted after May (Fig. 9). It should be
mentioned that only a small shift in the
distribution of porpoises can have large
effects on the sighting rates, especially
for the anecdotal observations which are
concentrated near the coastline.
The seasonal pattern could be explained
by a process whereby part of the
porpoises occurring offshore in the
central and southern North Sea migrate
towards shallower nearshore Belgian
and Dutch waters during autumn. They
reach Dutch waters rst, but mostly
along the mainland coast rather than off
the Wadden Sea islands. During winter,
numbers of porpoises start to increase
along the Belgian and northern French
coast. Porpoises star t leaving coastal
waters in early spring, and most have left
by late spring. The peak period of bir ths
(between May and August) coincides with
a minimal presence of harbour porpoises
in coastal waters, and the seasonal
movements may be motivated by a return
to breeding grounds for the summer.
One important difference between the
recent sightings and a reconstruction
of porpoise abundance from historical
information, even if sightings in the past
were not ‘typically in the summer’ and ‘with
warm weather’ (e.g. IJsseling & Scheygrond,
1943), is their near-complete absence
in June and scarcity in July in nearshore
waters in recent years.
An exception to this is the presence of the
porpoise in Zeeland, the southernmost
part of The Netherlands. The seasonal
pattern of sightings in Zeeland, including
(the enclosed) lake Grevelingen, indicates
that porpoises currently occur in this area
throughout the year (Camphuysen &
Heijboer, 2008).
As is obvious with the strandings data,
also sightings data recorded in 2007
suggest that the overall picture of the
occurrence of the porpoise in the
southern North Sea might be changing
again (Camphuysen, 2008).
0
200
400
600
800
1000
1200 1970-1999 All
1970-1999 Coast
2000-2007 All
2000-2007 Coast
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0
20
40
60
80
100
120 1970-1999
2000-2007
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Figure 8: Anecdotal obser vations of harbour por poises per month repor ted to the Marine Mammal database
(The Netherlands), 1970-1999 and 2000-2007, discriminating between animals seen from coastal vantage points
and all animals reported (including offshore sightings; data: NZG Mar ine Mammal Database, C.J. Camphuysen).
Figure 9: Anecdotal observations of porpoises per month in Belgium from 1970 to 1999, and from 2000 to 2007
(data reported to MUMM, unpublished). The observations made from the surveillance aircraft operated by
MUMM, which are numerous but irregular due to an irregular effor t, were excluded.
THE HARBOUR PORPOISE
in the southern North Sea
22
Summer distribution trend
observed over the North Sea
The comparison between the results
obtained from SCANS I and SCANS
II29 (Hammond et al., 1995; SCANS,
2008) clearly demonstrates the shift
in distribution from northern waters
towards the south between 1994 and
2005 (Fig. 10). Although the results of
SCANS II indicated a relatively high
density of porpoises in the southern
North Sea, the data were gathered
in July, outside the months with the
highest density of porpoises in Belgian
and Dutch waters. A clear seasonal
pattern exists, and therefore SCANS
results should be used to assess (as they
generally are) abundance and summer
distribution only (CEC, 2002). Actual
densities in coastal waters of Belgium
and The Netherlands during winter and
spring were undoubtedly higher than
the gures repor ted in SCANS II for July
(SCANS II, 2008; Haelters & Jacques,
2006; Depestele et al., 2007).
4.6 Possible reasons for the
irregular occurrence of
the harbour porpoise in
the southern North Sea
during the 20th century
The different aspects in the decline of the
harbour porpoise in Belgian and Dutch
coastal waters after World War II have
not been documented in detail. In the
absence of extensive scientic studies,
the cause-effect relations put forward
therefore can never exceed the level of
postulation. The explanations proposed
are summed up below; it is likely that the
real reason behind the disappearance is
a combination of several of them.
1. The construction of the Afsluitdijk,
enclosing part of the Wadden Sea, was
mentioned by some as the reason for
the decline. Although this might have
had important local effects on the prey
of porpoises, this barrier was already
completed in the 1930s; the rst signs
of a decline were observed more than
a decade later.
2. As top predators, porpoises are very
sensitive for pollutants such as heavy
metals, PCBs and pesticides. During
the decline of the porpoise population
also the numbers of harbour seals in
the Dutch Wadden Sea dropped
dramatically: PCBs have been put
forward as the reason for a decline
in their reproductive capacity in that
area (Reijnders, 1986). Likewise,
high pesticide levels were associated
with the mass die-off of seabirds in
the Wadden Sea area in the 1960s
(Koeman, 1971).
3. Directed catches might have occurred
up to the rst half of the 20th century,
but the percentage of the population
caught in coastal waters of the
southern North Sea was probably
very small.
4. Porpoises are vulnerable to incidental
death in certain shing gears; it is
not clear to which extent incidental
catches in gillnets or driftnets could
have caused the decline in numbers in
the southern North Sea.
5. It is unlikely that the increase in
disturbing human activities such as
shipping, offshore construction works,
seismic research and recreation was
a major cause for the near complete
disappearance of the porpoise from the
southern North Sea, given the current
level of these forms of disturbance, and
the current number of porpoises.
6. Overshing has lead to a severe decline
in the stocks of herring and sprat in the
Figure 10: A comparison between the results of
the two major cetacean abundance and distribution
surveys (left: SCANS I, July 1994, right: SCANS II,
July 2005) clear ly indicates the shift in distribution of
harbour porpoises in the North Sea. The colour bar
indicates porpoise density (no of animals per km²;
SCANS II, 2008).
23
North Sea from the 1960s onwards.
These clupeids were a prey of choice
for porpoises, which had to switch to
other and less suitable prey species.
Overshing in the southern North Sea,
and a better availability of suitable prey
in the central and northern North Sea,
might have caused a shift in the North
Sea porpoise population in the middle
of the 20th century.
7. Climate changes might have had a direct
or indirect impact on the number of
porpoises in the North Sea, on their
distribution and on migration patterns.
Unambiguous explanations for the
initial decline in the abundance of
the harbour porpoise have never
been given. It appears equally difcult
to fully understand its come-back,
which occurred over a similar period
of time. There is evidence, however,
that distributional shifts rather than
population uctuations underlie the
recent trends observed.
The redistribution of harbour porpoises
in the Nor th Sea may have been
triggered by local reductions or shifts
in principal prey availability, in particular
in the northern part of the Nor th Sea
(Camphuysen 2004a, SCANS II, 2008).
This is probably caused by changes in
environmental conditions. Similarly,
feeding conditions for certain seabirds
nesting in colonies in the northern North
Sea have severely worsened during the
last years, as seen in a generally very
poor breeding success. While porpoises
can under take migrations towards
better feeding grounds, it is not possible
for seabirds to change the location of
their breeding colonies on a short term
basis this is something that may take
several generation times.
However, bad local feeding conditions
can still cause serious problems for
porpoises. Being amongst the smallest
of marine mammals, they cannot cope
for long periods without food. Studying
animals in captivity, Kastelein et al. (1997)
demonstrated that porpoises need to
take in 4 to 9.5 % of their body weight
in food on a daily basis to stay healthy. In
the wild the level of food intake is likely
to be even higher, given the colder water
and the greater effort needed to obtain
prey. Taking in sufcient food is also
prey-related: some sh species (such
as herring) are better suited as prey
than others (such as gadoids) due to a
higher caloric value. When food intake
is irregular and insufcient, porpoises
depend on their fat reserves. Given that
the fat reserve also serves as thermo-
insulation, an animal can die because of
hypothermia before the total fat reserve
is used.
4.7 An analysis of strandings data
Introduction
In many cases, peaks in the numbers
of stranded animals reect peaks in
numbers at sea. However, a bias can
exist due to an increased seasonal
mortality due to bycatch, or a high
mortality of very young animals. There
is also a bias due to the location of
death: the chance of a dead porpoise
being washed ashore depends greatly
on its distance from the coastline and
on meteorological conditions during the
period the carcass oats at sea.
Stranded porpoises potentially
provide us with valuable information
on their ecology, on the structure
and health status of the par t of the
population found ashore, and on human
impacts. Depending on their state of
decomposition, stranded cetaceans
are generally thoroughly investigated
nowadays. Next to gathering the
standard data such as length, weight and
sex, additional research – depending on
the state of the carcass - is performed
in the elds of virology, bacteriology,
toxicology, reproduction and feeding
ecology. However, this type of research
has only been conducted widely since
the early 1990s. Furthermore, the
THE HARBOUR PORPOISE
in the southern North Sea
24
representativeness of the investigated
animals regarding the entire population
can be disputed.
It is possible to record trends in strandings
by using the basic data that were
routinely recorded in the past. These
basic data include stranding location and
date, and length and sex of the stranded
animals. For a few years after the death
of Van Deinse in 1965, virtually no data
on stranded animals were collected in
The Netherlands. In Belgium a systematic
recording of stranded animals only started
during the 1970s. For these reasons,
1970 is chosen as the starting point of
the analysis presented below. The older
the data sets, the more incomplete they
get. However, they still provide us with
an indication of the changes in strandings
that have occurred.
Monthly stranding rates
between 1970 and 2007
The monthly stranding rates between
1970 and 2007 show very similar trends
in Belgium and in The Netherlands (Fig.
11 and 12). They indicate an irregular
occurrence of strandings throughout
the year, with a considerably less distinct
peak than the seasonal observations in
nearshore waters (Fig. 8 and 9). During
the 21st century, strandings peaked from
March to May, and in August. The peak
at the Belgian coast in May is for a large
part a consequence of the stranding of
decomposed animals, which probably
had died already in April. The peak in
strandings of porpoises during March
and April is for an important par t
related to incidental catches during
these months (see further). Strandings
during the summer months, when only
few obser vations are reported, concern
in many cases newborn or very young
animals, or very decomposed animals
that may have drifted in from further
offshore.
The apparent mismatch between
sightings data and strandings data can
also be caused by the fact that porpoises
abandon just the coastal zone rather
than the entire Southern Bight. The
presence of porpoises at sea in summer,
at least in Dutch waters, is conrmed
from ship-based surveys (ESAS unpubl.
data), aerial surveys (Witte et al., 1998;
Osinga, 2005), anecdotal information
(Marine Mammal Database, C.J.
Camphuysen) and the SCANS II sur vey
(SMRU, 2008).
0
50
100
150
200
250 1970-1979
1980-1989
1990-1999
2000-2007
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0
10
20
30
40
50
60
70
80
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
1970-1979
1980-1989
1990-1999
2000-2007
Figure 11: Numbers of porpoises washed ashore in The Netherlands per month over decades between 1970
and 2007.
Figure 12: Numbers of porpoises washed ashore in Belgium per month over decades between 1970 and 2007.
25
Population structure of
stranded animals
Since 1970, many of the carcasses of
porpoises washed ashore in Belgium
and The Netherlands were measured
and sexed, even when they were not
collected for further scientic research.
Although length is not the best estimate
for age, it can be used for rough age
group assessment. Length at maturity is
still disputed, but undoubtedly a lot of
variation exists between individuals. As
no information about sexual maturity
was available for most of the animals
in our databases, we have classied
the animals for which length data were
available into different age categories.
Animals were classied as neonate
when they were smaller than 0.90 m,
and therefore in most cases probably
only a few weeks old. We classied
them as juvenile when at least 0.90 m
long and smaller than 1.35 m or 1.40 m
respectively in males and females. The
remainder was considered as having
reached adulthood. It was not possible
to indicate whether animals could be
considered as calves, which would
concern the whole lactation period.
In total 2.328 porpoises (1.906 from The
Netherlands, 422 from Belgium) that
stranded between 1970 and 2007 were
categorised into age categories. These
data indicate that the age composition of
stranded animals has changed. Between
1970 and 1999, 37 % of the animals was
considered adult, while this was only the
case for 20 % between 2000 and 2007
(Fig. 13). The difference in the percentage
of neonates between the periods before
and after 2000 was less pronounced:
7respectively 8 % of the stranded animals
was classied as neonate. The data indicate
that the increase in stranded animals
is predominantly due to an increase
in strandings of juveniles (Fig. 14). The
suggested increase in sightings of presumed
mother-calve pairs during the last years
is conrmed by the slight increase in the
number of stranded neonates.
For 63 % of the porpoises recorded from
Belgian and Dutch beaches between
1970 and 2007 (1.981 of 3.166), the sex
was recorded. Considered over the
whole period between 1970 and 2007,
54 % were males and 46 % were females.
However, between 1970 and 1999 more
females were recorded than males (44
% males), while between 2000 and 2007
more males were recorded (59 % males;
Fig. 15). The apparent change in sex ratio
coincides with the pronounced increase
in strandings since 1999.
0
50
100
150
200
250
300
350
400
450
500
1970
1972
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
Total number
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
% Adults
Total no. of stranded animals
% Adults
0
50
100
150
200
250
300
350
1970
1972
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
Neonate
Juvenile
Adult
Figure 13: Annual percentage of adults in stranded porpoises combined from Belgium and The Netherlands
(data: NIOZ, Naturalis and RBINS/MUMM).
Figure 14: Annual age category distribution in stranded animals combined from Belgium and The Netherlands:
neonates, juveniles and adults (data: NIOZ, Naturalis and RBINS/MUMM).
THE HARBOUR PORPOISE
in the southern North Sea
26
The data presented above indicate that
the increase in numbers of stranded
porpoises in Belgian and Dutch coastal
waters is mainly caused by juveniles,
with larger numbers of males than
females. Given that many of the
stranded animals were bycaught, this
gure might be biased by differences in
bycatch probability related to age and
sex. However, studies that included
samples of stranded Dutch porpoises
collected in the 1990s, suggest that
males may disperse more than females
(Walton, 1997).
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1970-1970 1975-1979 1980-1984 1985-1989 1990-1994 1995-1999 2000-2004 2005-2007
Figure 15: Sex r atio in stranded animals from 1970 to 2007 in Belgium and The Netherlands: percentage of
males: average per time period (blue) and yearly variation (extreme values, 25 and 75 percentile; exclusion of
sample sizes < 10) (data: NIOZ, Naturalis and RBINS/MUMM).
27
5. Bycatch of porpoises in
Belgium and The Netherlands
5.1 Introduction
At the end of the 20th century, many
of the stranded porpoises were a
testimony of one of the major problems
for this vulnerable species: bycatch in
shing gear. In some rare cases bycatch
has been repor ted by shermen, but the
external and internal signs on stranded
porpoises provide evidence for the fact
that bycatch is not a rare event.
5.2 Fishing gears leading to
bycatch
The most common commercial shing
practice in the southern North Sea is
bottom trawling (beam- and ottertrawling)
for demersal sh and shrimp. Next to
bottom trawling, a more limited shing
effort exists with pelagic trawls and static
gear. There is little evidence of porpoise
bycatch in trawls in the southern North
Sea, probably due to the avoidance
behaviour of porpoises towards motorised
vessels. While dead porpoises may end up
in the nets of the trawlers, such events are
not considered as bycatch.
The shing gear known as the major
cause of bycatch of porpoises in the
southern North Sea is static gear,
especially gill and tangle nets (Fig. 16).
These long nets are anchored on the
seaoor. In comparison to bottom
trawls, and especially beamtrawls,
their use is considered as relatively
environmentally friendly: they cause
little bottom disturbance, there is a
low bycatch of unwanted organisms or
undersized target sh species, and the
fuel consumption per kg marketed sh is
only a fraction of the fuel consumption
in towed gears. One of the major
environmental concerns in these
sheries is bycatch of marine mammals: it
is considered as the main anthropogenic
mortality factor for harbour porpoises
worldwide (Jefferson & Curry, 1994;
Lewison et al., 2004).
Catches of marine mammals in the
southern Nor th Sea are nowadays
always incidental. It is an impor tant
issue for conservation and animal
welfare. Bycatch of these charismatic
and popular animals is perceived as
negative in the eyes of the public. Also
most shermen themselves regret that
bycatches occur, but not in the least
for economical reasons. In many cases
bycaught animals cause gear damage,
slow down regular shing activities,
and may cause a reduction in sh
catches. Therefore, shermen are or
should be inclined to cooperate with
scientists and administrators to discuss,
develop and use bycatch mitigation
measures. The development of such
measures is challenging, given that they
should not lead to a nancial burden
to the shermen, nor signicantly
reduce catches or cause other negative
environmental impacts.
5.3 Gill and tangle net
fisheries in Belgium and
The Netherlands
Belgium and The Netherlands are not
considered as gillnetting nations. Of
the (currently around) 105 Belgian
commercial shing vessels, only 3 to 4
deploy gillnets and tangle nets. In The
Netherlands 60 to 70 small vessels
regularly deploy static gear (2006), but
this number is increasing (Anonymous,
2006a; de Graaff & Smit, 2007). The
total net length per shermen varies
from a few kms to up to 20 kms.
The main target species in static gear
shery in the southern North Sea are
sole (Solea solea) and other atsh,
which are shed in the coastal zone
Figure 16: Nearshore gillnet deployment off Wijk
aan Zee, The Netherlands, 2007.
THE HARBOUR PORPOISE
in the southern North Sea
28
between March and November.
Cod (Gadus morhua) is targeted
during winter months, especially near
shipwrecks. During summer months
some effort is dedicated to bass
(Dicentrarchus labrax), a valuable species
which is becoming more abundant, and
for which no European catch quota are
set. Gill and tangle net sheries are
fairly selective. For each target species
or group of target species a specic
gear type is being used.
In Belgium recreational sheries
with gillnets are limited by law to
the inter tidal zone it is illegal to use
them at sea. In The Netherlands, there
is no information on the number of
recreational vessels deploying gill and
tangle nets, the length of the nets set
and the areas most frequented.
The numbers of professional static gear
vessels in Belgium and The Netherlands
are low compared to those with home
ports in the eastern Channel. Probably
more than 150 professional static gear
shing vessels are active in the Channel
and/or the southern North Sea (ICES
Areas VIId and IVc) from the south-east
coast of England, and the same number
from the ports of northern France
(Guitton et al., 2003). Next to these,
also Danish static gear shing vessels
are active in the southern North Sea,
including Belgian and Dutch waters (Fig.
17).
It is expected however that in the near
future more European shermen will
switch from towed gear to static gear.
This is not only due to the increasing
knowledge of, and awareness about the
negative impacts of bottom trawling on
species and habitats, but especially for
economical reasons. Fuel prices affect
trawlers in particular, and soaring fuel
prices in recent years (up to the end
of 2008) make a switch from trawlnets
to static gears attractive. An important
part of the funding under the European
Fishery Fund 2007-201330 is being
dedicated towards initiatives reducing
fuel consumption, including eet
conversions.
The incidence and scale of porpoise bycatch
have been studied in many parts of the
north Atlantic, including the North Sea, the
Channel and the Irish Sea (e.g. Northridge
& Hammond, 1999; Northridge et al., 2003;
Siebert et al., 2001; Tregenza & Berrow,
1997; Vinther, 1999; Vinther & Larsen,
2002; 2004). Many variables affect bycatch
rate. There are clear indications that
some types of net, such as cod nets, have
relatively higher bycatch rates. However,
also local conditions can have an inuence,
Figure 17: Danish static gear shing vessels are
active in Belgian and Dutch waters (images: Dutch
coastal water s, 2007).
29
as can other less studied variables such as
porpoise foraging behaviour. The results
of bycatch research thus sometimes seem
contradictory. Variables that can have
profound effects on the potential of nets
to cause incidental bycatches of porpoises
are the following:
1. Season of the year (given the
porpoise is a migratory species);
2. Type of net (which depends mainly on
the target species): height of the net
and hanging ratio (the ratio between
the height of the stretched net, and
the effective height on the seaoor),
and type and thickness of twine
(having an effect on the visibility, the
acoustic reectiveness, the entangling
ability and the escape possibilities);
3. Position of the net (e.g. wreck or not);
4. Use and type of pinger;
5. Water depth;
6. Water current;
7. Porpoise feeding behaviour (e.g.
pelagic or demersal).
Next to the bycatch issue, also the
catch rate of target species has been
an important factor in bycatch studies.
Fishermen might be reluctant to accept
a net with a lower bycatch rate for
porpoises, if it also has a lower catch
rate for target sh species.
Some of the static gear types used
in Belgium and The Netherlands are
illustrated below (Fig. 18). During
interviews, shermen (all Belgian)
indicated their experiences with bycatch
of porpoises in each of these net types.
The results of the interviews seem
to conrm that bycatch rate highly
depends on the type of net, the material
it is made of, and the water depth in
which it is deployed. The shermen we
interviewed consider marine mammal
bycatch as a problem, and are very co-
operative in nding solutions.
During discussions with Dutch static
gear shermen, and a more informal
follow-up at a symposium in Noordwijk
aan Zee organised in the frame of The
year of the Dolphin in 2007, the problem
of bycatches of harbour porpoises in set-
nets was reluctantly recognised. Without
exactly indicating when and where, it
became obvious however that some
deployments at certain times of the year
and in cer tain areas posed greater risks
for porpoises than others. The potential
use of deterrents (such as pingers) was
discussed and the overall opinion was
that these should be deployed with
care, using the experience and initiative
of the shermen themselves, who would
known when and where to deploy them
with the greatest effect. A free access
to some deterrents-supply (provided
by the government for example) was
considered useful in this respect, so that
the extra cost for shermen would be
minimal. These discussions made two
points very clear: shermen did conrm
the suspicion that bycatches occurred in
their nets (although the exact scale of
the problem was not made clear) and,
most importantly, that any solutions
or other initiatives to help minimise
the problem were welcomed by the
shermen.
It is important to realise that the
reluctance of commercial shermen
to provide detailed information on
bycatches is driven by the simple fact that
they have little to gain in that process.
Fishermen don’t normally provide
information about their exact shing
locations in the rst place. They consider
that information condential because
it can only be misused, for example
by competitors (other shermen).
Broadcasting any information about
potential damage they might inict on
natural resources during routine shery
operations is clearly not benecial
for them, given the restrictions and
limitations that knowledge might lead
to. Like in other commercial activities,
any information or knowledge that
might harm the immediate commercial
interests of shermen is treated as
condential by them.
THE HARBOUR PORPOISE
in the southern North Sea
30
Examples of static gear types used
by Belgian and Dutch shermen
Nylon sole net
Nylon trammel nets are very visible under water. They
are mainly used to sh for sole. The mesh size is 90-110
mm, and the net height is around 1 m. The hanging ratio
is around 0.6. Although they do not catch as much sh
as monolament sole nets (see below), they are popular
because of their selectivity: these nets are very selec tive
for sole, and they catch less undersized sh. These nets also
take less garbage and invertebrates than the monolament
type (see further). On the other hand, they are less
suitable for catching certain other valuable species such
as turbot (Scophthalmus maximus) and plaice (Pleuronec tes
platessa), and they are three times more expensive than
the monolament type sole net. However, because the sh
are less entangled, they can be taken out of the net much
faster, which makes the shing operation more efcient.
Fishermen claim they never took a porpoise in these nets.
Monofilament sole net
This type of trammel net is, when new, invisible in the water.
It is stretched around 1 m high, but only stands around 50 cm
high above the seabed when deployed because of the low
lifting capacity of the oat line (hanging ratio: 0.5). It catches,
compared to nylon nets, more turbot, brill (Scophthalmus
rhombus) and plaice, and the same amount of sole. On the
other hand more undersized sh and non-commercial species
are being entangled as well, and more garbage gets stuck.
Fish are entangled more tightly, which may be problematic
for instance in areas with high densities of dogsh (Scyliorhinus
canicula). As a consequence, these nets take longer to be
emptied and reset. One of the shermen interviewed
indicates that he never caught a porpoise in these nets, set
in depths of 20 to 30 m. However, another sherman, setting
his net in shallower water nearer to the coastline, did report
bycatch (0 to 6 per year) in this type of net.
Monofilament cod/bass net
These types of gillnet are mainly used for catching bass or cod.
They have, according to the main target species, a mesh size
of 120 – 160 mm, and stand 3 to 5 m high on the seabed. In
these nets a fairly