Haelters, J.

Jauniaux, T.

Kerckhof, F.

Ozer, J.

Scory, S.

Open Marine Archive

 1
CM 2006/L:03  
 
 
Using models to investigate a harbour porpoise bycatch problem in 
the southern North Sea – eastern Channel in spring 2005  
 
 
Jan HAELTERS1, Thierry JAUNIAUX2, Francis KERCKHOF1, 
José OZER3 & Serge SCORY3 
 
1: Management Unit of the North Sea Mathematical Models (MUMM), Royal Belgian Institute of Natural Sciences 
(RBINS), 3e en 23e Linieregimentsplein B-8400 Oostende, Belgium; email: J.Haelters@mumm.ac.be 
2: University of Liège, Veterinary Medicine, Pathology Department, Sart Tilman B43, B-4000 Liège, Belgium 
3: Management Unit of the North Sea Mathematical Models (MUMM), Royal Belgian Institute of Natural Sciences 
(RBINS), Gulledelle 100, B-1200 Brussels, Belgium 
 
 
Key words: Phocoena, harbour porpoise, bycatch, southern North Sea, eastern Channel, 
multidisciplinary research, modelling 
 
 
Abstract 
 
At the Belgian coast a remarkably high number of dead harbour porpoises Phocoena phocoena washed 
ashore in the first half of May 2005.  In total 16 were counted between 3 and 16 May.  This is a number 
without known precedent in this area and in such a short period, and it arises concern given the species’ 
conservation status.  The state of decomposition of all porpoises was very similar.  They were very 
decomposed, and it was estimated that they had died at least 2 to 3 weeks prior to their stranding.  All 
carcasses were subjected to a necropsy.  Two had been cut open ventrally, clearly indicating that they 
were bycaught in fishing gear.  Bycatch was also the most probable cause of death of the other 
porpoises.  Prior to the stranding in Belgium (and to a lesser extent The Netherlands), high numbers of 
dead stranded porpoises were reported from North Sea shores of Denmark and the United Kingdom.  
Given the mortality observed at Danish and British coasts, and the distribution and abundance of 
porpoises in the North Sea, we had expected that the animals that washed ashore at the Belgian coast in 
May 2005 would have originated from waters north of the area under Belgian jurisdiction.  However, 
the models developed at MUMM to simulate the drift at sea of objects or oil, indicated that the animals 
most probably had died between the western part of Belgian territorial waters, and an area offshore 
Boulogne – Le Touquet off northern France (eastern Channel).  The conclusions of this multi-
disciplinary research are that (1) necropsies on severely decayed porpoises can in many cases reveal the 
cause of death of the animals, which in this case was certainly or most probably bycatch; (2) 
mathematical modelling of the drift of cetacean carcasses can play a useful role in revealing where the 
animals died and (3) in spring 2005 a porpoise bycatch problem existed in the southernmost part of the 
North Sea - eastern Channel. 
 
 
 2
Introduction 
 
At the Belgian coast a remarkably high number of dead harbour porpoises Phocoena phocoena washed 
ashore in the first half of May 2005.  Between 6 and 10 May not less than 13 were counted, and in total 
16 were counted between 3 and 16 May.  This number of strandings in such a short period of time is 
highly unusual in this area, where only 3 to 5 porpoises washed ashore annually a decade ago.  Since 
1995 strandings and sightings of porpoises have been steadily increasing.  The increase in the number 
of porpoises in this area is most probably due to a shift of part of the North Sea population towards the 
south (CAMPHUYSEN, 2004).   
 
The state of decomposition of all animals was very similar: they were all coded as ‘fours’.  This means 
very decomposed, with parts of the skin and fins missing, and in many cases intestines hanging free 
from the body and skeletal parts protruding through the skin (KUIKEN & GARCÍA HARTMANN, 1991).  It 
is estimated that the animals had died at least 2 to 3 weeks prior to their stranding.  The porpoises 
concerned immatures.  The length of the 12 animals that were measured, ranged from 1.00m to 1.30m 
(average: 1.16m, S.D. 0.09m).  Next to the porpoises, also two seals - in a similar state of 
decomposition - washed ashore. 
 
A list of the strandings, with date and location, is given below.  This list is completed with similarly 
decomposed harbour porpoises that washed ashore in the same period in the south-western part of the 
Netherlands (NL; personal communication Jaap van der Hiele).  Vlissingen and Cadzand are very near 
to the border with Belgium, the other Dutch locations are 30 – 50km further to the northeast. 
 
Species Day Place Length (m) 
Phocoena phocoena 3-May-05 Oostende 1.12 
Phocoena phocoena 4-May-05 Vlissingen (NL)  
Phocoena phocoena 4-May-05 Vlissingen (NL)  
Phocoena phocoena 5-May-05 Oostende 1.20 
Phocoena phocoena 6-May-05 De Haan 1.10 
Phocoena phocoena 6-May-05 Middelkerke 1.20 
Seal sp. 6-May-05 Nieuwpoort  
Phocoena phocoena 7-May-05 Knokke-Heist 1.25 
Phocoena phocoena 8-May-05 De Haan  
Phocoena phocoena 8-May-05 Bredene  
Phocoena phocoena 8-May-05 Wenduine 1.02 
Phocoena phocoena 9-May-05 Bredene 1.22 
Phocoena phocoena 9-May-05 Koksijde 1.30 
Phocoena phocoena 9-May-05 Koksijde 1.15 
Phocoena phocoena 9-May-05 Koksijde 1.25 
Phocoena phocoena 9-May-05 Blankenberge 1.00 
Phocoena phocoena 10-May-05 Nieuwpoort  
Phocoena phocoena 10-May-05 Nieuwpoort  
Phocoena phocoena 11-May-05 Cadzand (NL)  
Phocoena phocoena 14-May-05 Renesse (NL)  
Phocoena phocoena 15-May-05 Bredene 1.14 
Phocoena phocoena 15-May-05 Vrouwenpolder (NL)  
Seal sp. 15-May-05 Blankenberge  
Phocoena phocoena 16-May-05 Neeltje Jans (NL)  
 
 
What did they die of? 
 
All dead animals that had washed ashore in Belgium were collected by MUMM, and subjected to a 
necropsy at the University of Liège, using a standard protocol (JAUNIAUX et al., 2002).  Two of the 
animals had been cut open ventrally, clearly indicating that they had been bycaught in fishing gear.  
Some fishermen cut open bycaught animals, hoping that the carcass will sink and therefore not wash 
ashore.  The most probable cause of death of the other porpoises was also determined as bycatch.  This 
was indicated by a relatively thick blubber layer, the presence of stomach content, the absence of 
chronic lesions and the presence of some specific criteria (KUIKEN, 1996).  Animals having died of 
illness and/or malnutrition typically show a very thin blubber layer and an empty stomach.  Bycatch is 
 3
most likely to occur in bottom set gillnet fisheries.  This kind of passive fishery is - apart from the 
marine mammal bycatch problem - relatively environmentally friendly.  It is characterised by a low 
level of bycatch of benthic invertebrates or undersized fish, a minor bottom disturbance and low fuel 
consumption relative to the total weight of marketed fish.  For tackling the cetacean bycatch problem in 
this fishery, the European Commission issued Council Regulation 812/20041.  This regulation does not 
take far-reaching measures in the southern North Sea, where porpoises were considered rare in the 
recent past, and where bycatches therefore occurred very rarely. 
 
Prior to the stranding in Belgium (and to a lesser extent The Netherlands), high numbers of dead 
stranded porpoises had been reported from North Sea shores of Denmark and the United Kingdom.  In 
mid-April, 55 dead porpoises were found on a stretch of beach of 10km in Denmark in only 24hrs.  The 
most probable reason for this mortality was bycatch in the gillnet fishery for lumpfish Cyclopterus 
lumpus.  Also at the east coast of the UK, between Berwick and the Wash Estuary, an unusually high 
number of dead porpoises had washed ashore in April (communication British Coastguard).  Danish 
and British authorities have investigated these mortalities.  Although porpoises have become fairly 
common in Belgian waters during the last years, especially in spring, we had expected that the 
carcasses that had washed ashore in May 2005 would have originated from waters north of the area 
under Belgian jurisdiction, given their state of decomposition, the mortality observed at Danish and 
British coasts, and the distribution and abundance of porpoises in the North Sea according to REID et al. 
(2003). 
 
 
Where did they come from? 
 
MUMM develops mathematical models to simulate the drift at sea of objects or oilspills (SCORY, 
2005).  In these models data of wind force and currents are used.  The models were used to simulate the 
drift of the porpoise carcasses, and as such establish the most likely place where the animals had died. 
 
 
Description of the model simulations 
 
The area of possible origin of the dead porpoises was taken very wide: between 002°W and 010°E and 
between 48°N and 57°N.  In that area (see figure 1), one point of release, indicating a priori a possible 
source for dead porpoises, is taken every ¼° in both directions.  The model was run, after initialisation, 
for a period of one month (10 April to 10 May 2005).  The model is driven by the tide and realistic 
atmospheric forcing (UKMO2).  
                                                 
1 Council Regulation (EC) No 812/2004 of 26 April 2004 laying down measures concerning incidental catches of 
cetaceans in fisheries and amending Regulation (EC) No 88/98; Official Journal of the European Union of  30 
April 2004. 
2 UK Meteorological Office 
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Figure 1: Positions where particles were released every 3 hours.  The area concerned is the 
eastern Channel, the southern and part of the central North Sea; the area under Belgian 
jurisdiction is indicated. 
 
 
During the model run, one particle was released at each release point every three hours.  The drift of 
each particle was computed according to: 
WDuu
dt
sd
wp
p rrr
r
α+==  
where t denotes time, ps
r
 is the position of the particle at time t, pu
r
 is its velocity, wu
r
 is the current 
speed computed by the hydrodynamic model (depth averaged current), W
r
 is the wind speed at 10m 
above the sea surface, α  is the wind drift factor and D  is a matrix that allows to introduce a deviation 
angle ϑ  between the wind and the wind drift velocity (degrees).  The deviation angle is to the right to 
the wind and it depends on the wind speed according to: 
W
r
840 −=ϑ  
when the wind speed is less than 25ms-1.  ϑ  is set equal to 0 when the wind speed is higher than 25 
ms-1.  Two model runs were performed.  In the first one, the wind drift factor was set equal to 0.03 (as 
usually done for the computation of oil drift).  In the second run, the wind drift factor was set equal to 
0.015. 
 
 
Results of the models 
 
We firstly started by identifying those particles that had reached the Belgian coast (from 2.54°E to 
3.33°E) between 3 and 9 May 2005 (included).  For each of these particles, we knew where and when it 
had been released.  We then classified the particles according to the time taken to reach the coast (less 
than 7 days; between 7 and 14 days; between 14 and 21 days; more than 21 days).  For each class, we 
produced a map showing the position of release.  The size of each circle is roughly proportional to the 
number (in each class) of particles that have been released from that position and that have reached the 
coast in the specified time. 
 5
  
 
  
Figures 2 and 3: Particles that reached the coast in less than 7 days (above left and right), and between 7 and 14 days (below left and right).  The circles indicate the 
release positions of the particles.  The radius of each circle is roughly proportional to the number of particles released from that point that reach the coast.  The results 
obtained with a wind drift factor set equal to 0.015 are shown in the left panels.  Those obtained with a wind drift factor set equal to 0.03 are shown in the right panels. 
 6
  
  
  
Figures 4 and 5: Modelling as for figures 2 and 3,  but now the travelling time of the particles is between 14 and 21 days (above left and right) and greater than 21 days 
(below left and right). 
 7
Discussion 
 
The state of decomposition of the porpoises that washed ashore in the first half of May 2005 was very 
similar in all cases.  It indicated that death had occurred in a defined, relatively short period of time.  
An uncertainty in this report concerns the actual date of death of the stranded porpoises, due to a lack 
of data on the speed at which dead porpoises decay in waters of around 10°C.  According to the state of 
the carcasses, this period was estimated to be situated at at least two to three weeks prior to the 
stranding.  Also data on the floating characteristics of dead porpoises at different states of 
decomposition, necessary for modelling the influence of the wind in the drifting process, is lacking.  
For these characteristics the values for the model used here, are those currently applied for oil at the sea 
surface, and an alternative thereof. 
 
The models indicate that the porpoises most probably died in an area between the western part of 
Belgian territorial waters, and an area offshore Boulogne – Le Touquet in northern France (eastern 
Channel).  During the last years an increasing number of porpoises has been observed in the southern 
North Sea and the eastern Channel in late winter and spring, after a virtual absence of some decades in 
this area.  The necropsies revealed for all porpoises bycatch as the certain or most probable cause of 
death.  In the area of origin indicated by the model a high number of professional gillnet fishermen are 
active.  In spring this fishery targets predominantly Dover sole Solea solea and other  flatfish, and it is 
carried out with small meshed (around 100mm) bottom set gillnets.  Bottom set gillnet fisheries are 
known to have a bycatch problem in certain areas and periods, and recently measures were taken in the 
framework of the European Common Fisheries Policy. 
 
From the results of this investigation some important conclusions can be drawn: 
 
1) Necropsies on severely decayed porpoises can in many cases reveal the cause of death of the 
animals, which in this case was certainly or most probably bycatch. 
 
2) Mathematical modelling of the drift of cetacean carcasses can play a useful role in revealing 
the origin of dead cetaceans. 
 
3) A porpoise bycatch problem exists (or existed at least in spring 2005) in the southern North 
Sea – eastern Channel.  It should be clear that only a proportion of bycaught porpoises wash 
ashore; this is very dependent on meteorological conditions.  The number of porpoises washed 
ashore and their cause of death however indicate that commitments taken in international fora 
such as ASCOBANS, the European Habitats Directive and the international North Sea 
Conferences regarding incidental catches of harbour porpoises might not met in the southern 
North Sea – eastern Channel. The porpoise bycatch problem in this area is relatively recent, 
and follows the regional increase in the numbers of porpoises, most probably due to a shift of 
part of the North Sea population towards the south. 
 
 
Acknowledgements 
 
This research is partly sponsored by the Belgian federal science policy, by the Royal Belgian Institute 
of Natural Sciences (RBINS) and by the FIOV Project VIS/02/B/OS/DIV (IDEV-ILVO-Visserij).  We 
would also like to thank all other authorities and volunteers involved in the work of the intervention 
network dealing with stranded marine mammals. 
 
 
Literature 
 
CAMPHUYSEN, K., 2004.  The return of the harbour porpoise (Phocoena phocoena) in Dutch coastal 
waters.  Lutra 47(2): 113-122  
JAUNIAUX, T., GARCIA HARTMANN, M., HAELTERS, J., TAVERNIER, J. & COIGNOUL, F., 2002.  Echouage 
de mammifères marins: guide d'intervention et procédures d'autopsie.  Annales de Médecine 
Vétérinaire 146: 261-276 
KUIKEN, T., 1996.  Review of the criteria for the diagnosis of by-catch in cetaceans.  In: Proceedings of 
the second ECS workshop on Cetacean pathology: diagnosis of by-catch in cetaceans. Montpellier, 
2 March 1994.  ECS newsletter 26 - Special Issue: 38-43 
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KUIKEN, T. & GARCÍA HARTMANN, M., (EDS.), 1991.  Standard protocol for the basic postmortem 
examination and tissue sampling of small cetaceans.  In: Proceedings of the first ECS workshop on 
Cetacean pathology: dissection techniques and tissue sampling.  Leiden, 13-14 September 1991.  
ECS newsletter 17 – Special Issue: 26-39 
REID, J.B., EVANS, P.G.H. & NORTHRIDGE, S.P. (EDS), 2003.  Atlas of cetacean distribution in north-
west European waters.  Joint Nature Conservation Committee, Peterborough, UK.  76p. 
SCORY S., 2005.  The use of mathematical models for estimating oil pollution damage at sea.  In: Maes, 
F. (ED.), 2005.  Marine resource damage assessment. pp. 211-251.  Springer, The Netherlands. 
 
 
 


Using models to investigate a harbour porpoise bycatch problem in the southern North Sea – eastern Channel in spring 2005

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