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Recolonization of the Dutch Wadden Sea by the Grey Seal, Halichoerus Grypus

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Recolonization of the Dutch Wadden Sea by the Grey Seal, Halichoerus Grypus .. Not to be cited without prior reference to the authors International Council for C.M. 1992/N:7 the Exploration of the Sea Marine Mammals Committee Recolonization of the Dutch Wadden Sea by the Grey Seal, Halichoerus grypus • by l Peter J.H. Reijnders , Jan van Dij1c2 & Dirk Kuiper 'DLO-Institute for Forestry and Nature Research, Dept. Aquatic Ecosystems. P.D. Box 167, 1790 AD Den Burg, The Nether/ands 2Ministry ofAgn"cu/ture, Nature Management &: Fisheries, Section Fauna Management, 1he Nether/ands. Abstract Based on subfossil remains obtained during excavations, it is concluded that the grey seal Halichoerus grypus used to be a common seal species in the Wadden Sea area until the early Middle Ages. It is postulated that due to the increased human settlement in the area and • consequently the intensified hunting and disturbance at the rookeries, the grey seal population in the Wadden Sea area gradually decreased and the species became virtually extinct at the end of the Middle Ages. Because of the growing grey seal population on the Farne Islands, more (young) animals migrated to the Dutch Wadden Sea. Since 1980 they founded a colony between the islands of Vlieland and Terschelling, at a tidal flat which is usually not flooded. From 1980 onwards, nearly all of the animals that were rehabilitated in nursery stations have been subsequently released dose to that colony. The increase in the number of immigrants and to a lesser extent the number of released animals, caused the colony to increase and in 1992, a maximum of 178 animals were observed. Additionally since 1985, pups have been bom in the colony; 21 were counted in 1992. The seasonal occurrence is similar to that which is found for the grey seals at the Farne Islands, albeit the annual timing at present is different, probably caused by a difference in the timing of the breeding season. Morphodynamic changes of the tidal flat used by the grey seal colony has already • 2 influenced, and will continue to influence the growth of the colony. This could be avoided by management measures promoting establishment of the grey seal colonies in other more suitable areas. However, this should be thoroughly considered and the consequences profoundly studied, since an increasing grey seal stock has the potential to come into conflict with other components of the Wadden Sea ecosystem. Resume Base sur des restes subfossiles trouves lors de fouilles, ont a put constater que le phoque gris, Halichoerus grypus, etait une espeee de phoques commune dans la Mer des Wadden jusqu'au debut du Moyen Age. Il est postule que, du a l'accroissement de l'etablissement des humains dans la region et par consequence l'intensification de la chasse et perturbation la population de phoques gris dans la Mer des Wadden a graduellement baisse. A la fin du Moyen Age l'espeee avait pratiquement disparu. L'accroissement de la population de phoques gris dans les Iles Farne a amene une migration de plus en plus grande de (jeunes) animaux vers la Mer des Wadden neerlandaise. • Depuis 1980 ils ont fonde une colonie sur un banc de sable, qui n'est pas inonde habituellement, entre les Iles de Vlieland et de Terschelling. Aussi depuis 1980 tous les animaux qui ont ete rehabilites dans des centres, ont ete reläches pres de la colonie. Ceci a fait que la population a augmente, et en 1992 un maximum de 178 animaux ont ete observes. Depuis 1985 des petits sont nes dans cette colonie dont 21 en 1992. La presence saisonniere est comparable a celle trouvee chez le phoque gris au Iles Farne quoique la chronologie annuelle est differente due aune difference dans la chronologie de la reproduction. Des changements morphometriques du banc de sable ont dejä influence et continuent a influencer la croissance de la colonie. 11 est possible d'y echaper par des mesures de gestion favorisant l'etablissement de colonies dans d'autres regions plus appropriees. Mais ceci devraient etre envisage avec beaucoup d'attention, et les consequences devait etre etudiees profondement comme une augmentation du stock de phoques gris pourraita entrer en conflit avec d'autres elements de l'ecosysteme de la Mer des Wadden. • Introduction Two species of seals tegularly occur in the Dutch Wadden Sea, the harbour seal Phoca vitulina and the grey seal Halichoerus grypus (Reijnders & Wolff, 1981; Reijnders et al., 1990). At present the harbour seal is the most numerous seal and the grey seal is relatively rare. However, this was different in historical times. The purpose of this paper is to review the historical and current status of the grey seal in the Dutch Wadden Sea and to outline the factors that contributed to its disappearance and the subsequent recolonization process. 3 Early occurrence Subfossil grey seal remains from as early as the Neolithicum show that the grey seal has inhabited the entire Wadden Sea area from Denmark to The Netherlands (for area location see Fig.l). Excavations from more than fifty settlements in the Danish Wadden Sea between 6000 and 1000 RC. revealed the remains of grey seals exclusively (Joensen et al. , 1976). The grey seal was the most common species found in prehistoric discoveries in the German Wadden Sea area (Requate, 1957). Four out of five discoveries of seal remains from the early Middle Ages in the Dutch Wadden Sea area, are of grey seals (van Giffen, 1913, 1916; van Deinse, 1926; Clason, 1988). Flgure 1. Localities in the Wadden Sea of the grey seal colony between Vlieland and Terschelling and the colony close to Amrum. Discoveries show a gradual change in the ratio of grey seals to harbour seals, during the course ofthe early Middle Ages. Around 1000 A.D., the ratio was about equal (Requate, 1956, 1962). Reports on the occurrence of grey seals in the whole Wadden Sea area are scarce from the end of the Middle Ages onwards until the second half of the 1900s. In fact they were considered to have been a rare species during this time (Mohr, 1931, 1952; van Haaften, 1974; Joensen et al., 1976). Based on data on the early' occurrence of grey seals in the whole Wadden Sea area it is concluded that until the early Middle Ages the grey seal was a common inhabitant of the Dutch Wadden Sea. The numbers gradually decreased over the centuries and at the end of the Middle Ages the species occurred irregularly and only in very low numbers. A factor coincident in time with the mentioned decrease was the increased human inhabitation in that area. Even until the 9th century, relatively few people lived in the Wadden Sea area (mainland and the Frisian islands). They mainly lived on mounds in the satt marshes. Human settlements increased in numbers only around 1000 A.D., when dikes were constructed to prevent the sea from flooding the area. At the end of the' Middle Ages human settlements could be found on all the Frisian Islands in The Netherlands. A similar - - -- ----------~~~ ~- 1 4 development is found elsewhere in the Wadden Sea area (see e.g. Waterbolk, 1974). Seals have been hunted in northern Europe since the Stone Ages (Clark, 1946). At that time there was already a sealing station on Hessel0 (M0hl, 1970). It is obvious that in the early centuries grey seals were a worthwhile prey to hunt: for meat and fat as food, fur skin for clothing and leather, and fat again as fuel in lamps. They are a fairly easy prey to obtain, especially during the breeding season when the animals and their off-spring remain on land. It is conceivable that the increased human presence led to an intensified hunting and disturbance at the rookeries, which initially caused a decline in numbers and finally the virtual extinction of the grey seal in the Wadden Sea area. Recent development annual occurrence In the first half of the 20th century, observations of grey seals were so rare that in a review of mammals of The Netherlands, the species was not considered to belong to the Dutch fauna. Observations of grey seals in the Wadden Sea have become more regular since the 1950s. Although they were observed throughout the whole year, the small groups or individuals were never observed for more than a few months at the same sites (van Haaften, 1974). The biotope was apparently still not suitable for a spontaneous renewed settlement of the species. NUUBERS Figure 2. Annual maximum numbers ofgrey seal counts in the colony between Vlieland and Terschelling. The only place in the Wadden Sea where a small colony could be observed from 1967 onwards, was a tidal flat west of the island Amrum in Germany (Drescher, 1979; Scheibel & WeideI, 1988). However, the situation in the Dutch Wadden Sea changed from 1980 onwards. On a relatively high tidal flat between the islands of V1ieland and Terschelling 5 (Fig.I), a small group of grey seals were observed more permanently. That tidal flat was only flooded during spring tides in combination with strong winds, and obviously fulfilled the habitat requirements of the grey seals. The development of the colony there is shown in Fig.2. It shows a steady increase in the early 1980s, a short levelling off between 1985-88, followed by a continuing increase until now. A maximum of 178 grey seals were counted in April 1992. % 100 0 •• 0 7Ii 10 • 0 J F .. A .. JJ A • 0 ND MONTHS Figure 3. Means (+ 1 S.E.) relative monthly grey seal counts. Dates of maximum counts in those years: 1985= 0; 1986= .; 1990= 0; 1991= •. In Jan. to Dec. respectively n= 4,5, 10, 11,8,6,7,6,4, 3,4,6.
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