status of marine mammals in the north sea

427

Netherlands Journal of Sea Research 26 (2-4): 427-435 (1990)

STATUS OF MARINE MAMMALS IN THE NORTH SEA

P.J.H. REIJNDERS and K. LANKESTER

Research Institute for Nature Management, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands

ABSTRACT

Information on the population status of marine mammals in the North Sea is rather scarce. For Grey Seals and Common Seals, which regularly come ashore, fairly accurate population estimates exist. However for whale species, even the more commonly observed dolphins and Harbour porpoise, no reliable data on stock areas or stock sizes can be provided.

Nevertheless, it is assumed that most whale populations have decreased in numbers. And only after cessation of hunting, seal stocks have been increasing in most areas.

Major recent and potential threats to marine mammals are interactions with fisheries and pollution. Several aspects of interactions considered are: drowning in nets, damage to nets or catch, 'competition" for fish and marine mammals as hosts for parasites. Most of these issues can only be answered by more intense population studies combined with multispecies fisheries assess- ments. Observer networks and stranding data can provide useful indices for qualitative occurrence of marine mammals, but are of limited use for proper population analyses. Adequate management of an ecosystem requires understanding of interspecies relationships and the vulnerability of its com- ponents to changes in environmental conditions. Data on the status of marine mammals are urgently required to evaluate their role in the marine ecosystem.

1. INTRODUCTION

Marine mammals are amongst the more prominent members of the top predators in the marine ecosystem. This centuries old privilege is of a dualistic nature and is either based on curiosity about their adaption to the aquatic environment or on conflicts with human economical interests i.e. fisheries. In recent years, many concerns have been expressed about the status of marine mammals, not in the least because of solicitude

about the deterioration of the North Sea environment. To understand the functioning of marine mammals in the ecosystem of the North Sea it is important to assess interspecies relationships and sensitivity of its com- ponents to environmental changes. Major research topics in this area are marine mammals-fisheries interactions and consequences of pollution. Data on the status of marine mammals are essential to interprete their functioning and in particular population studies concen- trating on occurrence, abundance and energy demands in relation to condition and contaminants burden are urgently required. In this paper population estimates and threats to marine mammals will be discussed with em- phasis on lacunae to be covered by future research.

We acknowledge the critical reading of the manuscript by Bjarne Clausen and Carl Kinze.

2. POPULATION ESTIMATES AND STATUS

2.1. GREY SEAL

The main area of occurrence of grey seals in the North Sea is around the United Kingdom, Iceland and the northwest coast of Norway (Fig. 1). The total number in Norway is estimated at 3100 animals, however no indication on reliability can be provided (WIGG, 1987). Around Iceland, aerial surveys of grey seal pups carried out in 1982 and 1986 provided the basis to estimate the present grey seal population at about 10,000 individuals (HAUKSSON, 1987). Only for the United Kingdom do long time series on some grey seal stocks exist (Fig. 2). The methodology used is aerial photography of pups born, complemented by ground-thruthing (HARWOOD, 1987). The statistical procedures for estimating stock sizes from pup production censuses are described in HIBY ~t HARWOOD (1979). The total British grey seal population in 1987 is estimated to be close to 100,000 animals. Despite the uncertainty in the status for some relatively small stocks, the conclusion is that the grey seal population in Great Britain exhibits an exponential growth and population trajectories do not indicate a levelling off. The population increase documented since 1965 onwards had presumably already started decades

428 P.J.H. REIJNDERS 8 K. LANKESTER

#

Fig. 1. Distribution of Grey seals in the North Sea. 1 = Dutch Wadden Sea; 2 = German Wadden Sea.

before. Protection during the breeding season since 1914, and a decline in the number of subsistence fishermen -- crofters who killed seals for skin and oil

number ~oooo0 -

BOO00-

60000-

4000(

20000-

O-

J

t965 1970 t975 1980 1985 f.990

year

Fig. 2. Number of Grey seals in Great Britain (Source: Sea Mammal Research Unit, Cambridge, UK).

- - have substantially contributed to that increase (BON- NER, 1972). This early increase is supported by the observation that since the last half century the grey seal has been able to extend its breeding range especially to small islands where human depopulation has taken place.

In the rest of Europe only a few animals occur regularly in Brittany (France), a small colony in the German Wadden Sea and since 1980 a colony exists in the Dutch Wadden Sea (REIJNDERS, 1987). This latter colony seems to be permanent and to increase gradually (Fig. 3).

2.2. COMMON SEALS

The common seal occurs in all countries around the North Sea (Fig. 4). Major populations are found around Iceland, in the Wash (U.K.), around the Orkney Island, in the Wadden Sea and in the Kattegat-Skagerrak- southern Norway area. Population estimates of common seals are difficult to obtain. Contrary to grey seals, whose pups stay on land for 2 - 3 weeks, pups of corn-

STATUS OF MARINE M A M M A L S IN THE NORTH SEA 429

number

0 / "

year ~0

Fig. 3. Number of Grey seals in a colony in the Dutch Wadden Sea.

mon seals enter the water just a couple of hours after being born. Therefore breeding success in common seals cannot be obtained directly, as the number of pups present at any given moment is a function of the numbers born and the numbers that died already. A recruitment model that provides the total number of animals born on basis of frequent counts of living pups during the whelping period was designed by FRANSZ E~ REIJNDERS (1978) and subsequently applied (REIJN- DERS, 1978). In most areas however, no frequent pup counts are carried out. Moreover information on the population structure, in order to estimate the Gross An- nual Reproduction Rate, is mostly lacking. Therefore

most estimates are based on counts from boats, aircraft or land of the numbers of seals hauled out in particular areas. Consequently these figures only provide minimum estimates of the total population.

To estimate breeding success is a general problem for virtually all seal species. Its solution is addressed by WARD et al. (1987) and BOWEN et al. (1987).

As the behaviour of common seals is not consistent in all areas and estimates are sometimes based on a single census, the accuracy of the different population figures can vary considerably. Nevertheless, survey techniques are comparable for specific areas and population trends can be provided for the Wadden Sea and Kattegat-Skagerrak-southwestern Norway (Fig. 5). In both areas there is a clear decline noticeable until the mid-seventies, followed by a rapid recovery (REIJN- DERS et al., 1981). Excessive hunting pressure was the major cause of the decline in all areas (REIJNDERS, 1981). After cessation of hunting some populations increased at an exponential rate of 0.12 (HEIDE- JORGENSON ~" HARK(3NEN, 1988), although the recovery did not occur at the same rate in all areas. Especially in the western part of the Wadden Sea pup production is suppressed as a result of pollution (REIJNDERS, 1981, 1986). Therefore the recovery potentials differ in the various areas of the Wadden Sea as is demonstrated by their respective population trajectories (Fig. 6).

The present minimum size of the British common seal population is estimated to be 24,700 (HARWOOD, 1987). Differences in survey techniques used for former

. ° . . , - - . - ~ . ; . .

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Fig. 4. Distribution of Common seals in the North Sea.

430 P.J.H. REIJNDERS 8- K. LANKESTER

n u m b e r

16°°°1 A t4°°° 1 / / /

"°°° L / 'oooo I J ...

"°°°I z ,/:i"..

...... i .................. t965 1970 t975 i980 t985 t990

--total - - - K a / S k / S . N w

• .. Wadden Sea

y e a r

Fig. 5. Number of Common seals in different areas in the North Sea, excluding the UK.

estimates do not allow a conclusion about the status of the populations. The common seal population in Iceland was estimated for the first time in 1980, based on aerial counts. From the figure provided by HAUKSSON (1987) it can be deduced that the Icelandic common seal population amounts to about 21,000 animals and appears stable.

The positive trend in population developments of

common seals, especially observed in the Kattegat- Skagerrak and Wadden Sea areas, has been abruptly interrupted. Since mid-April 1988, more than 18,000 common seals have been found dead at various sites around Europe. In the formerly mentioned areas, the in 1989 actually counted numbers of animals amounted to approximately 40% of the predicted population estimates without the epidemic. It is generally accepted

n u m b e r

8000

6000

4000

2000

~ . . " ' " • "~,.~, i.,.~.-~.~'-,~~.. .

0 ,'=7-=--,'-r=r-.--,--g=~-r-~--,--c r-;-. , . • . , , , , , , . i , ,

t960 1965 1970 1975 ig80 1985 1990 y e a r

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--- SH

• - NS

• .. DK

-- NL

- - H e l g .

Fig. 6. Number of Common seals in different parts of the Wadden Sea.

STATUS OF MARINE MAMMALS IN THE NORTH SEA 431

that the primary cause of death for most of these animals was infection with a previously undescribed morbillivirus, tentatively named as Phocine Distemper Virus. It will not be possible to assess the full impact of the epizootic on most populations, until aerial surveys are conducted during the moult period in August 1989 and further investigations on the possibble repetitive occurrence of the virus have been carried out. The Grey seal seems to be less sensitive to this virus, as out of a sample of 185 dead Grey seals, only very few showed symptoms of the disease. However, blood samples of 70 breeding females and 3 pups taken in 1988 at different Scottish colonies, showed a high (97%) prevalence of antibodies to Distemper Virus (HARWOOD et al., 1989).

2.3. WHALES AND DOLPHINS

Many species of whales and dolphins have been observed in the North Sea but most of them are infrequent visitors. In total 18 species of which 7 commonly occurring. Only the harbour porpoise (Phocoena phocoena), a truly indigenous cetacean in the North Sea, and a few other toothed whale species can be considered to be more or less resident. These are the whitebeaked and the whitesided dolphin (Lagenorhynchus albirostris) and (L. acutus), the bottlenose dolphin (Tursiops truncatus), the pilot whale (Globicephala rnelaena), the minke whale (Balaenoptera acutorostrata), and the killer whale (Orcinus orca).

Data on the status of harbour porpoise in the North Sea suggest that the species has declined in numbers in coastal waters of the Netherlands and southern parts of the United Kingdom (NORTHRIDGE 8" LANKESTER, 1988). The foundations of those sugges- tions are narrow and largely derived from information on local observation of cetaceans, from data on strandings and from a few restricted sighting schemes.

VERWEY (1975) collected sightings data in 1932-1947 on the number of harbour porpoises and bottlenose dolphins in the Marsdiep, the tidal inlet south of Texel in the Netherlands. The author suggested that there had been a decline in both species, which would primarily have been caused by the closure of the Zuiderzee and the subsequent decline of spawning herring stocks in the Wadden Sea. However, such a conclusion is not warranted by analysis of his data, as actually remarked by the author himself.

Data on strandings have been collected for decades in the United Kingdom and the Netherlands and have led to discussions about the decline of the harbour porpoise in the southern North Sea (KLINOWSKA, 1987). However, in both instances reporting efficiency was not accounted for, and a lack of strandings does not necessarily imply a low population level. The caveats

for the interpretation of this type of data as discussed by KAYES (1985), apply to both time-series involved.

EVANS et al. (1986) reported on an observer scheme around the British coasts, which appears to indicate a decline in the number of harbour porpoises in the North Sea. It is not possible to deduce trends in local abundance from these data, since in several instances effort has not been quantified. Moreover, all but one of the observation sites were on the coast. An extra complica- tion involves the patchiness of distribution patterns, which may mean repetitive sightings of a few individuals which can in turn lead to overestimation of local abundance. Preliminary aerial surveys were conducted by BAPTIST (1987) along transect lines on the Dutch con- tinental shelf, mainly covering the area between 52 ° and 54 ° North. During 13 surveys, 22 primary sightings of harbour porpoise and 22 sightings of Lagenorhynchus species were recorded. A systematic shipborn survey has been conducted by the seabird group of the Danish Ornithological Society in the Danish part of the North Sea (KINZE, pers. comm.). Although no basis for population analyses could be derived, these types of systematic surveys are promising when carried out on a sufficiently large scale with well designed tracklines. Some technical aspects involved in the interpretation of sightings-data are discussed e.g. by COOKE (1988).

Anecdotal accounts of coastal residents and fishermen often refer to a decrease in the number of harbour porpoises and bottlenose dolphins seen in recent decades, but such circumstancial evidence can only be used as an indication and should not be interpreted as an actual decline.

On the other species, information is even more limited. Some dolphin species can occasionally be observed in fairly large numbers which does not necessarily imply a high abundance. Since these species are usually distributed further off-shore, the number of reported sightings is smaller and less regular. In conclusion, interpretation of this type of data is highly com- plicated and will not lead to conclusive support on population trends. However, a combination of these data with aerial and shipborn surveys might be useful in order to assess their applicability. In this review the status of harbour porpoise in the North Sea has been discussed. For population analyses as well as management objectives the possible existence of small func- tional population units within that area would be of in- fluence. GASKIN (1984) distinguished regional populations, however the basis for this was somewhat arbitrari- ly. To elucidate this question a project on stock identification by means of iso-enzyme characterization has been started by KINZE (Zoological Museum of Copenhagen).

Recently there have been significant advances in the use of molecular genetic techniques to analyse the structure of animal populations. DNA-fingerprinting has

432 P.J.H. REIJNDERS ~t K. LANKESTER

been applied to natural populations of several marine mammal species (HOELZEL 8- AMOS, 1988). Even in small skin samples collected with biopsy darts it is possible to extract DNA and obtain clear DNA-fingerprints. This technique is very valuable for harbour porpoise stock identification as it allows to gain information about their genetics without the need to kill animals.

3. INTERACTIONS BETWEEN MARINE MAMMALS AND FISHERIES

In recent decades a conflict has been recognized between commercial fisheries and marine mammals. Fisheries and marine mammals compete to some extent for the same fish species, traditionally this has caused fishermen to consider marine mammals as a nuisance. A case which attracted particular attention in the United Kingdom is the effect of grey seals on salmon yields in British fisheries. Concern about the effect of grey seals on Scottish salmon lead to large scale pup hunting in the mid-1960's. Concern about general competition between seals and fishermen for all sorts of fish prompt- ed a culling scheme in the Hebrides and the Orkneys in 1977, which was never completed due to increasing public protest (HARWOOD 8- GREENWOOD, 1986). The issue of interaction has become subject to a lot of both political and scientific dispute. The approach by fisheries industries has been shown to be too simplified (BEVER- TON, 1985). For years it has been argued that fish con- sumption by seals was relatively high. Recent construc- tive work by INNES et al. (1986) supports that metabolic rates are comparable with other mammals. It is now realised that predator-prey relationships are quite com- plex, and that culling a number of marine mammals does not necessarily result in an increase in fish yield which is directly proportional. There may even be a countereffect depending on the position of a particular commercially interesting species in the marine ecosystem. This would be the case when seals prey on large, cannibalistic individuals, or on species of low commercial value which compete for food resources with commercially valuable fish.

An additional argument for seals being a potential hazard to fish yields is that seals are, besides many in- vertebrates, intermediate hosts for parasites. However, the response of levels of infestation in fish to changes in seal numbers is much less than its response to changes in fish numbers. Furthermore BJeRGE (1979) concludes that changes in food composition -- pelagic versus benthic food items -- are reflected in the species distribution of parasitic nematodes found in seals. So, changes in abundance of certain fish species can have an effect on the infestation of a specific parasite. In conclusion, the number of seals is not related to the level of parasitic infestation of cod and herring. Fisheries management will probably have the largest effect on

the efficiency of specific parasites. Therefore reducing numbers of seals will not solve the problem of parasitic infestation of fish species.

Another important aspect is the incidental catch of marine mammals in fishing gear. In the North Sea several species are known to be involved. Harbour porpoises and common dolphins are regularly caught in a variety of gear, while occasionally, Risso's and bottlenose dolphins as well as pilot whales are taken incidentally in trawl and gill nets. The extent of the bycatches is largely unknown and only on a sparse scale data have been collected. ANDERSEN 8- CLAUSEN, (1983) reported 91 harbour porpoises caught in Danish gill net fisheries in the area around 56 ° N in the North Sea, in a period of four months. KINZE (pers. comm.) has recently started to collect harbour porpoises from Danish ports. NORTHRIDGE (1988) has tried to estimate the extent of incidentally caught marine mammals in British fisheries. It appears that the type of gear greatly determines the incidence of bycatches, and that gill net- ting in particular is responsible for most of the victims.

What the direct consequences of the depletion of certain fish stocks like North Sea herring and mackerel are, is hard to determine. It may not have been deleterious, on the other hand it may have caused cetaceans to search for food sources in other areas.

4. CONTAMINANTS AND MARINE MAMMALS

To evaluate the importance of contaminants in marine mammals two aspects can be distinguished: 1) the possible effects of pollutants on physiological processes in marine mammals and 2) the potential of marine mammals as monitors of marine pollution. On (1), in the con- text of this paper, no detailed overview of studies on pollution in marine mammals will be presented. For that purpose see REIJNDERS (1988C). Here, only those compounds are discussed which are known to affect other vertebrates and are implicated as the cause of certain anomalies in marine mammals. Practically every group of contaminants can be found in any marine mammal. In general, the concentrations are highest in coastal species due to the discharge via rivers or directly from land. Marine mammals are, furthermore, usually top predators and therefore will accumulate certain types of contaminants. As most marine mammals have large fat reserves, they are particularly vulnerable to effects of lipophilic contaminants. Although considerable levels of many environmental contaminants - such as heavy metals, polynuclear aromatic hydrocarbons and organochlorines - have been recorded in marine mammal tissues, only the latter group appears to have caused significant problems. The PCB's, or some of their breakdown products or impurities, and probably the DDT family are found to cause reproductive failure, disrupt the metabolism of steroid hormones and affect

STATUS OF MARINE MAMMALS IN THE NORTH SEA 433

the immunesystem of seals (BERGMAN ~1- OLSSON, 1986; REIJNDERS, 1986, 1988a). Circumstantial evidence in- dicates that they may have similar effects on beluga whales (MARTINEAU et al., 1987) and California sealions (GILMARTIN et al., 1976). The mode of action for some of these compounds has been developed but the precise biochemical mechanism has yet to be identified (REUN- DERS, 1986, 1988b). There are a number of other organic compounds that are structurally sufficient similar to the chlorinated biphenyls to be expected to have similar effects on marine mammals. Other important compounds to be considered are the diphenyl sulfones, chlorinated xanthenes, chlordanes, chlorinated naphtalenes and phenoles.

Apart from the reproductive failure in common seals from the Dutch Wadden Sea, no other effects of pollutants on marine mammals in other parts of the North Sea have been observed so far. Some information on organochlorine burdens in miscellaneous tissues of cetaceans is available. Despite the handicaps of differences in analytical techniques there is enough con- sistency to conclude that the total levels of PCB's and DDT in porpoises from that area analysed in the late sixties and early seventies (REIJNDERS, in prep.) are of the same order of magnitude as those in porpoises from a recent Danish study (CLAUSEN 8- ANDERSEN, 1989). In this latter study, fecundity appeared to be normal. So it cannot be concluded that reproduction in harbour porpoise has been influenced by PCBs and DDT in a way analogous to what has been observed in common seals.

Consequently, the often cited statement that pollution i.e. by chlorinated hydrocarbons, is one of the fac- tors responsible for the disappearance of the harbour porpoise from the southern part of the North Sea is premature and rather speculative.

No conclusions can be drawn on possible effects that might have occurred until the mid-sixties as no data are available for that period. An argument against the hypothesis about the effects of pollution on the harbour porpoise population, particularly in the southern North Sea, is the fact that porpoise are believed not to be stationary animals. They will migrate and the uptake of pollutants will differ between the several areas. Therefore they can be considered as integrators for pollution in the entire North Sea, rather in temporal than in spatial respect.

On (2), monitoring of marine pollution through ex- amination of marine mammals is not yet feasible. Knowledge about both the kinetics of pollutants, due to changes in physiological state and the feeding habits of marine mammals, is still scarce. Furthermore the problem needs to be solved which organ of tissue is a representative indicator for the concentration ratio between residual chemicals in the animal and their environment (e.g. water, sediment), As long as these data

are not available only animals which are in a fairly steady state with their environment will be suitable for this por- pose. To assess whether this requirement can be fulfilled by marine mammals in the North Sea is a problem in itself.

Although the idea of monitoring marine pollution seems to be unrealistic at present, there are other ex- cellent perspectives for studies of pollution in marine mammals. As top marine predators they exhibit high levels of pollutants. This implies that subtle changes in physiological processes might already occur before any major phenomena, like reproductive failure or increased mortality, can be observed. This has been demonstrated for some compounds in common seals where changes in chemical blood parameters were indicative for af- fected physiological processes (REIJNDERS, 1988b). The long life-span of marine mammals and their properties as sensitive indicators and integrators for pollution renders marine mammalogy a valuable discipline in the predictive assessment of pollution effects in the marine environment.

5. CONCLUSIONS

Knowledge on the population status of common and grey seals is less fragmentary than for cetaceans, and for most areas estimates on population sizes exist. After cessation of hunting, most populations have recovered and exhibit a steady increase. In the western part of the Wadden Sea pup production is still hampered by pollution, predominantly PCB's.

The limited experience with cetaceans in the North Sea has largely been based on accidental encounters. Information has either been gathered from coastal sightings, incidental catches in fishing gear, or it had to be obtained from animals which had been washed ashore. Strandings and sightings have not been able to supply the necessary information upon distribution and abundance of cetaceans in the North Sea. Therefore, we have not yet been able to improve our understanding of the population dynamics of cetaceans beyond a mere notion of the presence of resident species and occasional stragglers, and some qualitative ideas about death causes inflicted by humans. The available anecdotal information on presence of cetaceans points to a decline in numbers, especially in the southern part of the North Sea. Pollution and interactions between marine mammals and fisheries have been identified as major, recent and potential, threats. However, no conclusions can be drawn on their effects with respect to the supposed decline.

Future research on marine mammals should be directed towards the understanding of population trends, interspecies relationships and their vulnerability to changes in environmental conditions. This implies the following main research topics: for seals, improvement

434 EJ.H. REIJNDERS 8 K. LANKESTER

of populat ion dynamic research by telemetry, freeze branding and aerial surveys; for cetaceans, aerial surveys, bycatch assessement, stock identif ication and dispersal studies by means of acoustic telemetry, radio telemetry and photo identif ication.

For both groups, contaminants analyses f rom bycatches, toxico-kinet ic studies and collection of blood parameters of live animals are urgently required to assess the impact of pollution on marine mammals as well as their potential to moni tor marine pollution.

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status of marine mammals in the north sea

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