areas. In the Ythan estuary, eider consume 39 % of the annual mussel production which is. 20 % of all zoobenthos production (Milne & Dunnet, 1972). In the St.
HELGOI.~NDER MEERESUNTERSUCHUNGEN Helgol~nder Meeresunters. 43, 385-393 (1989)
Occurrence and food consumption of the c o m m o n eider, Somateria mollissima, in the Wadden Sea of Schleswig-Holstein* Georg Nehls Staatliche Vogelschutzwarte, Biologiezentrum; Olshausenstrafle 40-60, D-2300 Kiel, Federal Republic of Germany
ABSTRACT: The number of eider in the Wadden Sea of Schleswig-Holstein was counted by aerial surveys during 1986 and 1987. The highest number occurred during migration in October 1987 with 151 000 ducks, the lowest number during the breeding time in May 1987 with 6000 ducks. About 100 000-120 000 eiders moult in July/August in the Wadden Sea of Schleswig-Holstein, but only 30 000-40 000 stay over winter. The average number was 62 000 ducks. Eider have increased in number since the seventies, when the average population size was estimated to be only 23 000. The increase referred mainly to moulting and migrating eider, whereas numbers in winter remained constant. There are substantial changes in the spatial distribution over the year. In most areas sites used during moult, migration and winter can be clearly separated, although so far no obvious differences in the morphology of these areas could be found. The annual food consumption was calculated to be 3.1 x 106kg AFDW or 1.3 g AFDW x m -2 x year -1, which is about 5 % of the average biomass of macrozoobenthos. The increase in the number of eider has led to a significant increase in total food consumption of carnivorous birds, which was estimated at 7.1 x 106 kg AFDW x year -1 in the seventies and now reaches 9.0 x 106 kg AFDW x year -1, of which the eider takes 34 %. The reasons for and consequences of the increase of the eider are discussed in context with the eutrophication of the North Sea and possible competition with shellfishery.
INTRODUCTION T h e e i d e r is t h e o n l y s e a d u c k o c c u r r i n g i n s u b s t a n t i a l n u m b e r s in t h e i n n e r p a r t s of t h e W a d d e n Sea. Birds o r i g i n a t i n g f r o m t h e Baltic S e a u s e this a r e a for m o u l t i n g , w i n t e r i n g a n d a f e w also for b r e e d i n g ( S w e n n e n , 1983). A l o n g l a s t i n g i n c r e a s e in t h e b r e e d i n g p o p u l a t i o n of t h e Baltic w a s f o l l o w e d b y g r o w i n g n u m b e r s in t h e W a d d e n S e a s i n c e t h e 1970s ( N e h l s et al., 1988). In 1986 a r e s e a r c h p r o g r a m w a s s t a r t e d i n t h e N a t i o n a l P a r k s of S c h l e s w i g - H o l s t e i n a n d N i e d e r s a c h s e n to s t u d y t h e n u m b e r of e i d e r , t h e i r s p a t i a l d i s t r i b u t i o n a n d c h a n g e s o v e r t h e y e a r , as w e l l as t h e i r f o o d a n d f e e d i n g h a b i t s . This p a p e r r e p o r t s o n t h e r e s u l t s f r o m S c h l e s w i g - H o l s t e i n .
* Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1-4 November 1988) 9 Biologische Anstalt Helgoland, Hamburg
386
Georg Nehls MATERIAL AND METHODS
From S e p t e m b e r 26th 1986 to October 18th 1987, 12 aerial surveys, covering the W a d d e n Sea of Schleswig-Holstein, were conducted to map the n u m b e r s a n d distribution of eider in the area. Flights were carried out during low tide, starting at the m o u t h of the river Elbe a n d e n d i n g at the island of Sylt. The counts were performed with 3 observers using a twin e n g i n e Cessna 337. The results of the counts were improved a n d checked by slides m a d e during the flight. For details a n d flight route see Nehls et al. (1988). Aerial surveys have b e e n used for m a n y years to count eider in the W a d d e n Sea (Drenckhahn et at., 1971; Joensen, 19731 1974; S w e n n e n , 1976), so that statements concerning c h a n g e s in n u m b e r s a n d distribution refer mainly to results that were attained by u s i n g the same method. RESULTS Numbers and distribution The results of 12 aerial surveys, carried out in 1986 a n d 1987, are s h o w n in Figure 1. It appears that very high n u m b e r s of eider occur during moult in July a n d August. After moulting, the n u m b e r s decline, but in October the a u t u m n migration leads to a m a r k e d increase again. The n u m b e r of w i n t e r i n g eider is much lower, a n d d u r i n g early s u m m e r only a small fraction stays i n the W a d d e n Sea. The function of the W a d d e n Sea of
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Schleswig-Holstein in the a n n u a l cycle of the eider is therefore quite different from the Danish a n d the Dutch part, where highest n u m b e r s occur during winter (Laursen, 1987; S w e n n e n , 1976). The n u m b e r of eider moulting in the W a d d e n Sea of Schleswig-Holstein has increased considerably since the seventies. Although n u m b e r s of moulters have tripled in this period, wintering n u m b e r s r e m a i n e d constant (Nehls et al., 1988). The c h a n g e in n u m b e r s refers m a i n l y to the long lasting increase of the b r e e d i n g population in the Baltic Sea, which is caused by a n increase in salinity and the eutrophication (Stjernberg, 1982; Nehls et al., 1988) but to a certain extent also to a change in the m o u l t i n g areas of the Danish b r e e d i n g population (Franzmann, 1983), which is however too small to explain all of it. The eider uses all areas of the W a d d e n Sea of Schleswig-Holstein, but in the course of the year substantial changes in the spatial distribution occur. These allow a distinction b e t w e e n moulting a n d wintering areas (Fig. 2). The eiders are constantly on the move through the W a d d e n Sea and some parts may be visited for only a few weeks of the year. The s u m m e r situation is seriously affected by recreational activities like boating, and high moulting concentrations are not to be found n e a r inhabited islands or busy shiproutes (Nehls & Thiel, 1988). So far, no obvious differences in the morphological or hydrological structure of these areas could be found to explain these movements. The increase of the eider showed regional differences. Strong increases were reported from the island of Trischen or the area of the Nordfriesische Aul3ensfinde, whereas the n u m b e r s north of the H i n d e n b u r g d a m m have r e m a i n e d constant for at least 20 years (Nehls et al., 1988). In Niedersachsen n e w sites have b e e n colonized during the last 15 years (Nehls et al., 1988). In the Dutch a n d Danish parts of the W a d d e n Sea no changes in n u m b e r s could be found in that time ( S w e n n e n et al., 1989).
Food and consumption Eider feed on marine invertebrates, mainly molluscs, throughout the year. In the W a d d e n Sea the mussel, Mytilus eduBs, a n d the cockle, Cerastoderma edule, are the most important food items. Shorecrabs, Carcinus maenas, Baltic tellins, Macoma baIthica, clams, Mya arenaria, periwinkles, Littorina littorea, a n d seastars, Asterias rubens, are only of minor importance. In the Dutch W a d d e n Sea mussels a n d cockles each take a share of 40 % in the elder's food (Swennen, 1976). In Schleswig-Holstein preliminary investigations, analysing about 7000 faeces in 1987 a n d 1988 (Nehls, u n p u b l , data), showed that cockles contribute about 75 % to the food of the eider during summer, w h e n n u m b e r s are highest. For the wintertime no data are available so far. Eider use various feeding techniques which allow feeding on tidal a n d subtidal areas. In the W a d d e n Sea feeding takes place mainly during rising or e b b i n g tide. At low tide, eider rest on exposed sandflats or on the water. Cockles are mainly t a k e n from tidal flats by trampling, which leaves small craters of about 50 cm in diameter a n d 20 cm deep. These are easily visible from the plane. The eider can however take cockles b y diving as they do regularly in the Baltic Sea (Madsen, 1954; Kirchhoff, 1979). Mussels are t a k e n by diving at high tide, w h e n eider aggregate in large flocks over the mussel beds, both on subtidal a n d intertidal areas (Swennen, pers. comm.; Ketzenberg, pers. comm.). Total consumption by eider in the W a d d e n Sea of Schleswig-Holstein was calculated
Georg Nehls
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by u s i n g the data given by S w e n n e n (1976), who estimated the daily d e m a n d of a n eider to be 138 g AFDW (ashfree dry weight) equal to 2.5 kg molluscs (wet weight, including shells). The total average n u m b e r of eider i n the W a d d e n Sea was calculated at 62 000, which was multiplied by 365 days a n d the daily d e m a n d , giving a total a n n u a l d e m a n d of: 62000 x 365 x 138 g AFDW -- 3.1 x 1 0 6 kg AFDW or 62 000 x 365 x 2.5 kg -- 56 575 t molluscs Related to the area of the W a d d e n Sea of Schleswig-Holstein this leads to a consumption of 1.3 g AFDW x m -2 x year -1, which is about 5 % of the biomass of the macrozoobenthos as f o u n d by Reise (pers. comm.) in the Nordstrander Bay or by B e u k e m a (1983) on tidal flats of the Dutch W a d d e n Sea. For their m a i n food species, cockles a n d mussels, the proportion t a k e n by eider is higher, reaching 12.5 % of the total biomass. Since mussels reach a m u c h higher biomass in the subtidal areas (de Wilde & Beukema, 1984), the proportion t a k e n by eider there is lower. C o n s u m p t i o n by the eider in the W a d d e n Sea of Schleswig-Holstein has tripled since the seventies, w h e n D r e n c k h a h n (in Busche, 1980) calculated the a n n u a l d e m a n d to be 1.2 • 106 kg AFDW. The total consumption b y carnivorous birds was then estimated to be 7.2 • 1 0 6 kg AFDW, of which the eider took 17 %. With increased eider n u m b e r s the total c o n s u m p t i o n b y carnivorous birds now a m o u n t s to 9.1 x 1 0 6 kg AFDW or 3.82 g AFDW x m -2, of which the eider takes a share of 34 %. Since some other important bird species of the W a d d e n Sea like the shelduck or the gulls have increased as well (Riiger et al., 1986; V a u k & Priiter, 1987), the total c o n s u m p t i o n m a y reach 5 g AFDW x m -2 x year -1. C o n s u m p t i o n b y the eider in Schleswig-Holstein has only recently r e a c h e d the level reported from the Netherlands for the seventies (Swerlnen, 1976). To discover whether the c h a n g i n g spatial distribution leads to a different pattern in predation pressure, the W a d d e n Sea of Schleswig-Holstein was divided into 10 sections, where the c o n s u m p t i o n by eider was calculated separately. Figure 3 shows the phenol~ ogy of the eider a n d the average n u m b e r s per area for each section, r e p r e s e n t i n g the consumption. The a n n u a l consumption in the different sections varies from 0.37 to 1.95 g AFDW x m -2 x year -1, b u t the figures for sections that are i m p o r t a n t for eider are surprisingly similar, although they have clearly different functions in the a n n u a l cycle of the eider in the W a d d e n Sea. DISCUSSION The strong increase of the eider in the W a d d e n Sea of Schleswig-Holstein, the regular c h a n g e s in their spatial distribution, a n d the conspicuous similarities in the a n n u a l c o n s u m p t i o n in the different sections of the W a d d e n Sea of Schleswig-Holstein, as well as in the other parts of the W a d d e n Sea, leads to questions about t h e function of the eider in this ecosystem. Two questions are of special interest: (1) How was it possible that the most important avian predator colonized m a n y areas of the W a d d e n Sea just recently? (2) What is the limiting factor for the n u m b e r s of eider in the W a d d e n Sea? R e g a r d i n g Q u e s t i o n ( 1 ) : The reasons for the increase of the e i d e r are found in the hydrological changes a n d the eutrophication of the Baltic Sea. In t h e W a d d e n Sea no such drastic changes, like mussels or cockles spreading into n e w areas, were reported
Occurrence and food consumptionof Somateria mollissima
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that could explain the increase. So it seems likely that the eider, as S w e n n e n (1976) suggested, found unexploited food resources in the W a d d e n Sea. Although mussels a n d cockles are a preferred prey of m a n y other animals, this seems to be possible. O n the tidal flats about ten different bird species (Smit & Wolff, 1983) as well as crabs, shrimps, flatfish a n d e v e n polychaetes (Reise, 1985) prey on the various age classes of cockles, b u t only oystercatcher a n d eider feed on cockles larger than 2 cm. Since oystercatchers preferably feed on rather high mudflats close to their roosts (Zwarts, 1983), there seems to b e little interference with the eider, which can easily take a d v a n t a g e of this situation a n d move into n e w places without competing w i t h other predators. In the Kbnigshafen of Sylt where only few eiders occur, Reise (1985) found almost no predation and, therefore, r e d u c e d mortality in cockles larger than 3 cm. Regarding Question ( 2 ) : In several parts of the study area the n u m b e r of eider did not increase, inspite of the fact that more and more eider from the Baltic a p p e a r e d in the W a d d e n Sea. The a n n u a l consumption in these areas is rather similar, so it could be that the population of eider has reached the m a x i m u m density that these areas can support. This seems to be reasonable, since other studies (Pehrsson, 1973, 1978, 1984) have shown the available food to be a main factor regulating the n u m b e r of eider. However, the consumption by eider in relation to the available food resources in the W a d d e n Sea (12.5 % of cockles a n d mussels) is m u c h lower than reported from other areas. In the Ythan estuary, eider c o n s u m e 39 % of the a n n u a l mussel production which is 20 % of all zoobenthos production (Milne & Dunnet, 1972). In the St. Lawrence estuary, Canada, eiders take during s u m m e r 10-30 % of their preferred prey, a Littorina species (Cantin et al., 1974). Furthermore, there has b e e n a major increase in the biomass of the zoobenthos in the W a d d e n Sea, at least in parts of the areas where eiders occur b u t did n o t increase (Beukema & Cad6e, 1986) even though there should be better feeding conditions today. The biggest problem for birds feeding on cockles or mussels in the W a d d e n Sea is the occurrence of m a r k e d fluctuations in these molluscs, due to severe losses in cold winters (Beukema, 1979; Dbrjes, 1980; Heiber, 1985). The cockle population of tidal areas m a y be totally exterminated w h e n ice covers the flats. After a cold winter it lasts until late August, w h e n eiders can feed on the n e w age class again, which m e a n s that both wintering and moulting eiders are affected by high cockle mortality in cold winters. As cold winters occur regularly, the n u m b e r of eider in the W a d d e n Sea is probably limited by the surviving a m o u n t of food, which also explains the low level of c o n s u m p t i o n in the relation to the average biomass of their prey. In the W a d d e n Sea, no c h a n g e s in n u m b e r s that could be related to fluctuations in the a b u n d a n c e of cockles or mussels have b e e n found so far, although eider counts have b e e n m a d e for more than 20 years. This all m e a n s that eiders have probably got little benefit from the eutrophication of the W a d d e n Sea and the following increase of macrozoobenthos biomass. It has m a i n l y improved the feeding conditions of good years a n d has not resulted in constantly a n d predictably larger food resources, b e c a u s e the winter mortality of their food is still the same. The relation b e t w e e n eider a n d shellfishery is not clear in this case. Nehls et al. (1988) showed that the n u m b e r of eider does not d e p e n d on mussel cultures. O n the other hand, eiders do not seem to be n e g a t i v e l y affected by mussel fishery although it reduces the standing stock of mussels considerably in a u t u m n . One explanation is the preference
392
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for s m a l l m u s s e l s of 2 - 3 c m o n t h e p a r t of t h e e i d e r (Kirchhoff, 1979; M i l n e & D u n n e t , 1972), w h e r e a s h a r v e s t e d m u s s e l s a r e u s u a l l y l a r g e r t h a n 5 cm. T h e s i t u a t i o n is q u i t e d i f f e r e n t for t h e c o c k l e , s i n c e h a r v e s t e d c o c k l e s of 3 c m in s i z e a r e w i t h i n t h e r a n g e p r e f e r r e d b y e i d e r . C o m p e t i t i o n b e t w e e n e i d e r a n d f i s h e r m e n will, at l e a s t i n t h e p r e s e n t s i t u a t i o n , o n l y o c c u r i n y e a r s w i t h l o w c o c k l e or m u s s e l p o p u l a t i o n s . But s i n c e b o t h t h e e i d e r p o p u l a t i o n a n d t h e s h e l l f i s h e r y a r e still e x p a n d i n g in t h e W a d d e n S e a , f u r t h e r r e s e a r c h is n e c e s s a r y to m o n i t o r t h e p o p u l a t i o n of t h e e i d e r a n d to s t u d y its f e e d i n g e c o l o g y a n d its r e l a t i o n to t h e f i s h e r y .
Acknowledgements. This study was initiated and financed by the regional office of t h e Wadden Sea National Park of Schleswig-Holstein. I wish to thank Stefan Br~iger, Jan MeiBner a n d Martin Thiel for spending many hours with me in the air counting eiders. Stefan Br:iger also h e l p e d to improve the manuscript.
LITERATURE CITED Beukema, J. J., 1979. Biomass and species richness of the macrobenthic animals h v i n g on a tidal fiat area in the Dutch Waddensea: Effects of a severe winter. - Neth. J. Sea Res. 13, 203-223. Beukema, J: J., 1983. Quantitative data on the benthos of the Wadden Sea proper. - Rep. Wadden Sea Work. Group 4, 134-142, Beukema, J. J. & Cad~e, G. C., 1986. Zoobenthos responses to eutrophication in the Dutch Wadden Sea. - Ophelia 26, 55-64. Busche, G., 1980. Vogelbest~inde des Wattenmeeres. Kilda-Verl., Greven, 155 pp. Cantin, M., Bedard, J. & Milne, H., 1974. The food and feeding of common eiders in t h e St. Lawrence estuary in summer. - Can. J. Zool. 52, 319-334. D6rjes, J., 1980. Auswirkungen des kalten Winters 1978/79 auf das marine Makrobenthos. - Natur Mus., Frankf. 110, 109-115. Drenckhahn, D., Heldt, R. & Heldt, R., 1971. Die Bedeutung der Nordseekfiste Schleswig-Holsteins ffir einige eurasische Watt- und Wasserv6gel mit besonderer Berficksichtigung des nordfriesischen Wattenmeeres. - Natur Landsch. 46, 338-346. Franzmann, N. E., 1983. The migration and survival of an eider, Somateria mollissima, population in the southern Baltic. - Ornis fenn. (Suppl.) 3, 73-74. Heiber, W., 1985. M6giichkeiten der Wiederbesiedlung yon Wattfl~ichen n a c h ,Umweltkatastrophen". - Seev6gel 6 (Sonderh.), 89-97. Joensen, A. H., 1973. Moult migration and wing-feather moult of seaducks in Denmark. - Dan. Rev. Game Biol. 8, 1--42. Joensen, A. H., 1974. Waterfowl populations in Denmark 1965-73. - Dan. Rev. G a m e Biol. 9, 1-209. Kirchhoff, K., 1979. Nahrungs6kologische Untersuchungen an b e n t h o s f r e s s e n d e n lenten in der Hohwachter Bucht. Dipl.Arb. Univ. Kiel, 105 pp. Laursen, K., 1987. Consumption of mussels by eiders in the Danish Wadden Sea. In: Proceedings of the 5th International Wadden Sea Symposium, Esbjerg. Ed. by S. Tougaard. The National Forest and Nature Agency and The Museum of Fisheries and Shipping, Esbjerg, 239-245. Madsen, F. J., 1954. On the food habits of the diving ducks in Denmark. - Dan. Rev. Game Biol. 2, 157-226. Milne, H. & Dunnet, G. M., 1972. Standing crops, productivity and trophic relations of the fauna of Ythan estuary, Aberdeenshire. In: The estuarine environment. Ed. by R. S. K. Barnes & J. Green. Appl. Science Publ., London, 86--106. Nehls, G. & Thiel, M., 1988. Wassersport im Nationalpark Schleswig-Holsteinisches Wattenmeer. Rep. WWF, Kiel, 24 pp. Nehls, G., Br~iger, S., MeiBner, J. & Thiel, M., 1988. Zum Vorkommen der Eiderente, Somatena mollissima, an der deutschen Nordseekfiste. - Corax 13, 41-58. Pehrsson, O., 1973. Chief prey as a factor regulating populations of eiders (Somateria mollissima) and long-tailed duck (Clangula hyemalis). - Zool. Rev. 35, 89-92.
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Pehrsson, O., 1978. A ten-year fluctuation pattern of the common eider (Somateria mollissima) on the Swedish west coast as a result of food availability. In: Proceedings of the Symposium on Sea Ducks, June 16-17, 1975, Stockholm, Sweden. Ed. by A. Andersson & S. Fredga. Shmbridge, 91-101. Pehrsson, O., 1984. Diving duck populations in relation to their food supplies. In: Coastal waders and wildfowl in winter. Ed. by P. R. Evans, J. D. Goss-Custard & W. G. Hall. Univ. Press, Cambridge, 101-116. Reise, K., 1985. Tidal flat ecology. Springer, Berlin, 191 pp. Rfiger, A., Prentice, C. & Owen, M., 1986. Results of the IWRB International Waterfowl Census 1967-1983.- IWRB spec. Publ. Slimbridge 6, 1-118. Smit, C. J. & Wolff, W. J., 1983. Birds of the Wadden Sea. Balkema, Rotterdam, 308 pp. Stjernberg, T., 1982. The size of the breeding eider population of the Baltic in the early 1980s. Ornis fenn. 59, 135-140. Swennen, C., 1976. Populatie-Struktur e n Voedsel van de Eidereend Somatefia molllssirna moll. in de Nederlandse Waddenzee. - Ardea 64, 311-371. Swennen, C., 1983. Eider. - Rep. Wadden Sea Work. Group 6, 78-84. Swennen, C., Nehls, G. & Laursen, K., 1989. Numbers and distribution of Eider Somatefia mollissima in the Wadden Sea. - Neth. J. Sea Res. 24 (in press}. Vauk, G. & Prilter, J., 1987. M5wen. Niederelbe-Verl. Otterndorf, 299 pp. Wilde, P. A. W. J. de & Beukema, J. J., 1984. The role of zoobenthos in the consumption of organic matter in the Dutch Wadden Sea. - Neth. Inst. Sea Res. Pubh Set. 10, 145-158. Zwarts, L., 1983. Habitat selection and competition in wading birds. - Rep. W a d d e n Sea Work. Group 6, 271-279.
