Biological Conservation 19 (198(~81) 213 221

M A N A G E M E N T A N D C O N S E R V A T I O N OF THE H A R B O U R SEAL, PHOCA VITULINA, P O P U L A T I O N IN THE

I N T E R N A T I O N A L W A D D E N SEA AREA

P.J.H. REIJNDERS

Research hTstituleo/or Nature Management, PO Box59, 1790 A B l )e . Burg Texel, 7he Netherlaml~.

A BS TRA CT

A populat ion assessment of ' the harbour seal in the international Wadden Sea area was carried out. The results show that the populat ions in the d([]'erent areas must he considered as one entity. Therefore management and conservation measurements should be placed in an international context. Because oJthe low pup product ion in the Dutch area the survival o f a harbour seal populat ion in the Dutch Wadden Sea depends on migration f r o m the adjacent areas as long as adverse environmental .['actors, i.e. pollut ion and disturbance, are not under control.

INTRODUCTION

The Wadden Sea is located along the coasts of The Netherlands, Germany (Niedersachsen and Schleswig Holstein) and Denmark (Fig. 1). It is part of the North Sea separated by a row of barrier islands and characterised by tidal flats. The harbour seal Phoca vitulina in this area has decreased drastically during the last decades, especially in The Netherlands, observed and reported by Van Haaften (1974) and Reijnders (1976). As seals do not acknowledge state boundaries the stock of seals should obviously be considered as one population with respect to conservation and management strategies. This has been done by population assessment and evaluating potential threats to it.

POPULATION ASSESSMENT

Size The number of seals counted in the Wadden Sea fluctuates throughout the year

(Wipper, 1974: Reijnders, 1976, 1978). At the end of July the highest numbers are

213 Biol. Conserv. 0006-3207/81/0019-0213/$02.50 ~ Applied Science Publishers Ltd, England, 1981 Printed in Great Britain

214 P.J.H. REIJNDERS

DENMARK

Fane

Reme

Sylt

A m r u m ~ C Hooge < •e•r ngeness

6de Nordstrand

NORTH SEA

Juist Borkum,~

• . o 1 1 ' . ~ SchlermonnlkooQ ~ 1 ~ ..

A m e l a n d ~

T e x e ~ T H E R L A N D S

Helgoland Trischen v

Scharh~rn n Neuwerk - O f -

Spiekeroog ~Wangerooge/

( Niedersachsen \ GERMANY

Schleswig Holstein

Fig. 1. Map of the international Wadden Sea area and the adjacent countries.

counted on the tidal flats. The number of pups born per mature female fluctuates each year and therefore the maximum number of subadults and adults should be regarded as an important character when comparing population sizes in subsequent years.

Censuses of the different Wadden Sea (sub)-populations have been carried out in the past mostly by boat and since about 1960 by plane in each of the four Wadden Sea countries. As boat censuses nearly always provide underestimates and take more time to cover the whole area only aerial surve2¢s are considered here. Unfortunately seals were counted in The Netherlands until 1974 around mid- September when numbers on sandbanks were already beginning to decrease (Wipper, 1974; Reijnders, 1978; Drescher, 1979). Revision of these numbers was therefore necessary but because the population decrease does not occur each year at

MANAGEMENT AND CONSERVATION OF THE HARBOUR SEAl 215

the same time and at the same rate, a mean conversion factor was calculated based on the data f rom 1974 to 1978 (Reijnders, 1978). These corrected figures have been added to the data available for Niedersachsen (Wipper, 1974; E. Manninga, pets. comm.), Schleswig Holstein (Behnke, 1974; Drescher, 1979a), for Denmark (S~b ndergaard et al., 1976) and Helgoland (Vauk, 1973, pers. comm.) and presented in Fig. 2. It is clear from this figure that the total population has decreased markedly, mainly in the western part of the Wadden Sea.

Pup production The max imum numbers of living juveniles counted were reported for

Niedersachsen by Wipper (1974) and E. Manninga (pers. comm.), for Schles- wig Holstein by Drescher (1978), for Denmark by SCndergaard et al. (1976) and for The Netherlands by Reijnders (1976, 1978). These numbers of pups fluctuated each year but the total numbers of subadult and adult seals have remained fairly stable during the last five years. Therefore, in each country a mean number of living pups counted during the last five years was calculated. On the basis of these data the actual numbers of pups born were calculated according to the method devised by Fransz & Reijnders(1978). The number of pups born in relation to the total number of seals is shown in Fig. 3. The data for Denmark published earlier are apparently incorrect, according to Drescher (1979a), who estimated the total population to be 250 specimens instead of 550 (Joensen et al., 1976).

It is clear that the lowest ratio occurred in the western part of the Wadden Sea (The Netherlands) and an increasing ratio in the northeast.

Mortality The method used for calculating the numbers of pups born also provided the

opportunity to estimate the juvenile mortality. Although for some areas not many data were available, juvenile mortality in each of the (sub)-populations calculated appeared to be almost equal at 60%. Direct data on mortality amongst subadults and adults are lacking. However, by assuming an age independent mortality (Sergeant & Fisher, 1960; Bigg, 1969; Boulva, 1973; Smith, 1973) for harbour seals in the year classes 1 and more, the mortality amongst subadults and adults was estimated, on the basis of a life table for a stable harbour seal population, at about 13% (Reijnders, 1978).

Migration The migration of the harbour seals in the Wadden Sea was studied by several

tagging programmes. Wild seals and young reared in captivity were tagged and those recovered showed an identical pattern in all studies (Van Haaften, 1972; Wipper, 1975; Drescher, 1979a). They showed a tendency to disperse in all directions from the point of tagging. According to Drescher (1979a) this is unexpected, as generally the harbour seal is thought to be rather stationary. However it is well known in

5600 / n u m b e r s

216 P.J.H. REIJNDERS

TOTAL

3200

28OO

24OO-

2000

8001 DENMARK ° t; o o

NETHERLANDS 400

HELGOLAND . . . . ~ , , • t ; , , : i " "

1960 1965 1970 1975 y e a r s

Fig. 2. Total numbers of harbour seals (including juveniles) in different areas of the Wadden Sea and for the whole area. The numbers are calculated on surveys carried out at the end of July each year. The numbers for Denmark are one rough estimate for the whole period as the population is believed to be

stable (Jo6nsen et al., 1976).

MANAGEMENT AND CONSERVATION OF THE HARBOUR SEAl. 217

D E N M A R K

24%

Fig. 3

30% ~ Schleswig NORTH SEA Holstein

0

Do

24%

THERLANDS \ GERMANY

Number of pups horn in relation to the total numbers of seals in the different areas in the Wadden Sea during 1974-1978. The figure for Denmark is a rough estimate.

many species that young animals tend to disperse more than adults. There is also evidence that migration among young marine mammals is quite common (Wada, 1969; Johnson, 1975). Older seals are assumed to be stationary because the same sand banks and even the same sites are chosen year after year as resting, whelping and nursing sites. Drescher (1979b) even states that certain age classes prefer specific sandbanks but agrees that so far it is unknown whether the same seals return to the same places after winter time.

M ohr (1952) and Van Haaften (1974) concluded that the harbour seal leaves the Wadden Sea during winter because fewer seals were counted on the tidal flats than during the summer. However, it may be possible that seals stay in the water during winter and so avoid being counted. Activities such as whelping, nursing, moulting

218 P.J.H. REIJNDERS

and possibly some social funct ions--al l of which take place on the beach--are confined to the summer months. Furthermore, seals are capable of sleeping and resting in the water for months (Wada, 1969; Wipper, 1975; Drescher, 1979a). For these reasons we assume that seals haul out onto the sandbanks during the winter only when weather conditions are favourable. If this is true, then seals are still present in the Wadden Sea but are seldom observed. This assumption is supported by monthly aerial surveys by the author from October to May, 1977-78. More than half of the maximum numbers present on the tidal flats during the previous summer were observed during these surveys. Two further independent data strengthen this assumption. The first is an observation by the author on two subsequent days in February when the same area in the Wadden Sea was visited. On the first day only 6 animals were observed and the water and air temperature were both 2°C, The next day 28 specimens were observed, the water temperature naturally was still at 2°C,but a bright sun warmed up the air temperature to nearly 8 ° C. The second is a statement by Vauk (1973) that seals staying on the rocks at the east side of Helgoland do not haul out for weeks during stormy weather and return immediately when the sea is calm again. These data indicate that most seals, although not observed by aerial surveys, during the winter still stay in the Wadden Sea but generally aggregate on sandbanks with ready access to the North Sea. Only when ice forms in the Wadden Sea do they disappear to the North Sea. This has been noted before but during the past winter the author observed that within a week after the melting or disappearance of ice-floes the number of seals increased to what it was before the ice had disappeared. The animals must have stayed in the North Sea but relatively close to the Wadden Sea. This aspect will be studied in more detail by means of telemetry.

CAUSE OF FLUCTUATIONS

Hunting Seals have been persecuted by man for centuries. In all of the countries adjacent

to the Wadden Sea hunting pressure has been heavy, as can be deduced by bounty statistics (Wipper, 1974; Drescher, 1975; Reijnders, 1976). Especially in the Dutch Wadden Sea the collapse in numbers of seals was initially caused by uncontrolled hunting of juveniles (Reijnders, 1976). Hunting was definitely prohibited in The Netherlands in 1962, in Niedersachsen in 1971, in Schleswig Holstein in 1973 and in Denmark in 1977. In Schleswig Holstein this prohibition did not seem to affect the population as this group of seals has remained fairly stable during last 25 years. Drescher (1979a) has suggested, without giving evidence, that this is not true because the actual decrease may only be visible in those regions where conditions are suboptimal. However, evidence for the impact of hunting on the total population in, for example, Schleswig Holstein can be deduced f rom data on

MANAGEMENT AND CONSERVATION OF THE HARBOUR SEAl. 219

dispersal of juveniles (Wipper, 1974; Drescher, 1979a). Assuming that tagging does not influence the dispersal behaviour of juveniles and that the chances of tag recovery are the same in each country, tagging data show that a balance must have existed between the numbers of seals that moved eastwards and westwards before the severe decrease started. These authors gave data from which it is possible to calculate that both from the Schleswig Holstein and the Niedersachsen population about 15-20% of juveniles moved westwards and the same quantum northeastwards. J.L. van Haaften (pers. comm.) states that the same percentage from the Dutch population moved eastwards. This means that in the past about an equal number of juveniles moved from Schleswig Holstein to Niedersachsen and back again. The same holds for Niedersachsen and The Netherlands. Because pup production in The Netherlands decreased, more seals moved there from Niedersachsen. This together with the hunting pressure, caused a decrease in Niedersachsen, although at a slower rate than in The Netherlands.

Since hunting was stopped in Niedersachsen and Schleswig Holstein the population in the Dutch Wadden Sea decreased at a much slower rate and remained fairly stable (Reijnders, 1978). This ' improvement ' is only due, however, to an immigration from Niedersachsen and probably to a lesser extent from Schleswig Holstein, as shown by Reijnders (1978) by assessing age class composition in the Dutch population. So it is clear that hunting, even in Schleswig Holstein, inhibits the unrestricted dispersal of juvenile seals and indirectly the possible birth surplus from that area to replace the gap in pup production in The Netherlands.

Disturbance

During the whelping and nursing period seals are very dependent on sandbanks during low tide and the growing demand of tourists who use them for recreation in the Wadden Sea causes disturbance. Seals are often forced to leave the tidal flats when disturbed by boats. This results in less t ime for nursing and resting and possibly increased pup mortality. The pup mortality in all the Wadden Sea areas of about 60% does not mean that disturbance plays a minor role. The tourist pressure in the whole area is probably of the same magnitude, but it is not unlikely that in earlier times the pup mortali ty was considerably lower. Moreover, Drescher (1978) found that disturbance played a significant role in the occurrence of skin lesions on seals. When seals are chased into the water, mechanical injury to the nearly healed umbilicus may occur and this can lead to infection of the wound.

Pollut ion Koeman et al. (1975) and Drescher et al. (1977) recorded considerable amounts

of toxic chemicals in seals. Reijnders (1979) related pup production to the quantity of biocide residues present in seals of different Wadden Sea areas and found higher levels, especially of PCBs, present in seals from The Netherlands which exhibit a

220 P.J.H. REIJNDERS

low pup production. Comparing that result with the findings of Olsson e t al. (1975) and Helle et al. (1976a,b) in Baltic seals it is tentatively suggested that PCBs may be responsible for the recorded seal decrease in the Dutch Wadden Sea.

In relation to Drescher's work (1978) on skin lesions in seals, it is likely that PCBs indirectly contribute to infection. It is well known that PCBshave a strong immuno- suppressive capacity, and bearing in mind the rather high residues already present in young seals, it is assumed that after mechanical injury due to disturbance, the PCB-weakened pups are more vulnerable to infection which leads to the process described by Drescher.

MANAGEMENT AND CONSERVATION

Summarising the environmental and other factors which determine the several sub- populations it is clear that the seals in the Wadden Sea have to be considered as one population.

Consequently, management must be carried out in an international context based on current research data. The International Seal Working Group has already started work but unfortunately it is still doubtful whether its composition and resources will be guaranteed in the future.

Furthermore, conservation measures must be taken soon; for example, undisturbed rearing areas for pups have to be settled. In Niedersachsen some sanctuaries already exist and in The Netherlands several areas have been established or proposed as reserves for seals as well as for shore birds. In Schleswig Holstein, however, the promising plans for a National Park (Erz, 1972) are cancelled and a few areas nominated as reserves are still free for people to enter.

The voluntary co-operation of the several seal nursery stations along the Wadden Sea coast works successfully and will help to prepare for an eventual long-term management strategy when conditions are favourable. The influence of pollution which is the major environmental stress on the harbour seal population, cannot be stopped by the creation of sanctuaries. International co-operation is necessary between people concerned with environmental management and conservation, and those concerned with clean water, such as farmers, water companies and industries, together with politicians of all countries located along the Wadden Sea and along the large rivers, especially the Rhine. If this can happen soon the seals will do the rest.

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M A N A G E M E N T AND CONSERVATION OF THE H A R B O U R SEAl 221

DRESCHER, H.E., HUSCHENBETH, E. & HARMS. U. (1977). Organochlorines and heavy metals in the harbour seal (Phoea vitulina), from the German North Sea coast. Mat'. Biol., 41, 9%106.

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