..

Not to be cited without prior reference to the authors

International Council forthe Exploration of the Sea

C.M. 1992/N:7Marine Mammals Committee

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Recolonization of the Dutch Wadden Sea by theGrey Seal, Halichoerus grypus

by

Peter J.H. Reijnders l , 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 greyseal Halichoerus grypus used to be a common seal species in the Wadden Sea area until theearly Middle Ages. It is postulated that due to the increased human settlement in the area andconsequently the intensified hunting and disturbance at the rookeries, the grey seal populationin the Wadden Sea area gradually decreased and the species became virtually extinct at theend 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 theislands of Vlieland and Terschelling, at a tidal flat which is usually not flooded. From 1980onwards, nearly all of the animals that were rehabilitated in nursery stations have beensubsequently released dose to that colony. The increase in the number of immigrants and toa 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 bomin the colony; 21 were counted in 1992.

The seasonal occurrence is similar to that which is found for the grey seals at theFarne Islands, albeit the annual timing at present is different, probably caused by a differencein the timing of the breeding season.

Morphodynamic changes of the tidal flat used by the grey seal colony has already

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influenced, and will continue to influence the growth of the colony. This could be avoidedby management measures promoting establishment of the grey seal colonies in other moresuitable areas. However, this should be thoroughly considered and the consequencesprofoundly studied, since an increasing grey seal stock has the potential to come into conflictwith other components of the Wadden Sea ecosystem.

Resume

Base sur des restes subfossiles trouves lors de fouilles, ont a put constater que lephoque gris, Halichoerus grypus, etait une espeee de phoques commune dans la Mer desWadden jusqu'au debut du Moyen Age. Il est postule que, du a l'accroissement del'etablissement des humains dans la region et par consequence l'intensification de la chasseet perturbation la population de phoques gris dans la Mer des Wadden a graduellementbaisse. 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 unemigration 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 inondehabituellement, entre les Iles de Vlieland et de Terschelling. Aussi depuis 1980 tous lesanimaux qui ont ete rehabilites dans des centres, ont ete reläches pres de la colonie. Ceci afait 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 IlesFarne quoique la chronologie annuelle est differente due aune difference dans la chronologiede la reproduction.

Des changements morphometriques du banc de sable ont dejä influence et continuenta influencer la croissance de la colonie. 11 est possible d'y echaper par des mesures degestion favorisant l'etablissement de colonies dans d'autres regions plus appropriees. Maisceci devraient etre envisage avec beaucoup d'attention, et les consequences devait etreetudiees profondement comme une augmentation du stock de phoques gris pourraita entreren 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 sealPhoca vitulina and the grey seal Halichoerus grypus (Reijnders & Wolff, 1981; Reijnderset al., 1990). At present the harbour seal is the most numerous seal and the grey seal isrelatively rare. However, this was different in historical times.

The purpose of this paper is to review the historical and current status of the grey sealin the Dutch Wadden Sea and to outline the factors that contributed to its disappearance andthe subsequent recolonization process.

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Early occurrence

Subfossil grey seal remains from as early as the Neolithicum show that the grey sealhas inhabited the entire Wadden Sea area from Denmark to The Netherlands (for arealocation see Fig.l). Excavations from more than fifty settlements in the Danish Wadden Seabetween 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 theGerman Wadden Sea area (Requate, 1957). Four out of five discoveries of seal remains fromthe 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 colonybetween Vlieland and Terschelling and the colony close to Amrum.

Discoveries show a gradual change in the ratio of grey seals to harbour seals, duringthe 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 arescarce from the end of the Middle Ages onwards until the second half of the 1900s. In factthey were considered to have been a rare species during this time (Mohr, 1931, 1952; vanHaaften, 1974; Joensen et al., 1976).

Based on data on the early' occurrence of grey seals in the whole Wadden Sea areait is concluded that until the early Middle Ages the grey seal was a common inhabitant of theDutch Wadden Sea. The numbers gradually decreased over the centuries and at the end ofthe Middle Ages the species occurred irregularly and only in very low numbers.

A factor coincident in time with the mentioned decrease was the increased humaninhabitation in that area. Even until the 9th century, relatively few people lived in theWadden Sea area (mainland and the Frisian islands). They mainly lived on mounds in the sattmarshes. Human settlements increased in numbers only around 1000 A.D., when dikes wereconstructed to prevent the sea from flooding the area. At the end of the' Middle Ages humansettlements could be found on all the Frisian Islands in The Netherlands. A similar

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development is found elsewhere in the Wadden Sea area (see e.g. Waterbolk, 1974). Sealshave been hunted in northern Europe since the Stone Ages (Clark, 1946). At that time therewas already a sealing station on Hessel0 (M0hl, 1970). It is obvious that in the earlycenturies grey seals were a worthwhile prey to hunt: for meat and fat as food, fur skin forclothing 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 anddisturbance at the rookeries, which initially caused a decline in numbers and finally thevirtual extinction of the grey seal in the Wadden Sea area.

Recent development

annual occurrenceIn 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 theDutch fauna. Observations of grey seals in the Wadden Sea have become more regular sincethe 1950s. Although they were observed throughout the whole year, the small groups orindividuals 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 ofthe species.

NUUBERS

Figure 2. Annual maximum numbers of grey seal counts in the colonybetween Vlieland and Terschelling.

The only place in the Wadden Sea where a small colony could be observed from 1967onwards, 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 1980onwards. On a relatively high tidal flat between the islands of V1ieland and Terschelling

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(Fig.I), a small group of grey seals were observed more permanently. That tidal flat wasonly flooded during spring tides in combination with strong winds, and obviously fulfilledthe habitat requirements of the grey seals. The development of the colony there is shown inFig.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 inApril 1992.

%

100 0 •• 0

7Ii

10

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0J F .. A .. J J A • 0 N D

MONTHS

Figure 3. Means (+ 1 S.E.) relative monthly grey seal counts. Datesof 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.

seasonal occurrenceTo defme the seasonal occurrence of the grey seals in the colony, the actual monthly

counts were expressed relative to the maximum count in that particular year. Those relativemonthly counts were averaged for all four years 1985, 1986, 1990 and 1991 and given inFig.3. The highest numbers are generally found during April, the peak counts in the fouryears were obtained between 30 March and 8 May. The 10west numbers are obtained inJanuary and December. During March and April, about twice as many animals haul outcompared to the rest of the year. This is probably accounted for by the moult of first thecows and then of the bulls. This pattern of seasonal occurrence is similar 10 the averagemonthly counts obtained at the Farne Islands in the UK (Hewer, 1974). The difference intiming between both groups is related to the difference in the timing of the breeding season,which is about two months earlier at the Farne Islands.

Factors of influence

The three factors most likely to have concurrently influenced and furthered theincrease in numbers of the grey seal colony between Vlieland and Terschelling, areimmigration, release of rehabilitated animals and pup production.

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immigrationThe grey seal stocks in Great Britain exhibited an exponential increase prior to the

1960s, when more accurate and regular surveys were started (Bonner, 1972; Harwood,1987). Dispersal of young grey seals from the British stocks occurred before 1980 (Bonner,1972). Tagging experiments showed that all the recovered tags in the Wadden Sea originatedfrom animals tagged at the Farne Islands (Hewer, 1974). The conservation orientatedmanagement of especially the Farne Islands stock, caused a strong increase in the residentstock (Bonner & Hickling, 1971). It was suggested that the increased seal stock, incombination with the burrowing of puffins and the tramp1ing and pulling of plant materialby nesting gulls, contributed to the destruction of vegetation and consequent erosion of thesoil caps of the breeding islands (8onner & Hickling, 1974). A culling programme to reducethe stock had only a marginal effect. The policy since then has been to disturb breedingcolonies. That caused animals to leave the area and new colonies were established atforrnerly uninhabited islands, like the Isle of May in the Firth of Forth, Scotland (Harwood,1987). This increased dispersal, no doubt promoted the immigration of grey seals into theWadden Sea.

NUMBERS

30

2S

20

15

10

5

1_1~1~1~1*1_1_1~1_1_1_1~

YEARS

Figure 4 Annual number of rehabilitated young grey seals released atthe colony between Vlieland and Terschelling.

rehabilitation and release80th the Seal Research and Rehabilitation Centre at Pieterburen and the Ecomare

Centre at Texel (both in The Netherlands), have released young rehabilitated grey seals intothe Dutch Wadden Sea. Before 1980, the animals were released dose to the only existingsmall colony near Amrum (Germany). However, from 1980 onwards they were released inthe western Dutch Wadden Sea, predominantly between the islands of Vlieland andTerschelling. The number of released young animals is shown in Fig.4. It is unknown howmany of those animals stayed in that specific area. Therefore it is not possible to accuratelyassess the contributing role of this activity. However, the rate of increase in numbers counteddid not markedly change from 1981 to 1985, whereas the number of re1eased animals

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considerably dropped. Therefore it is concluded that during those years, the effect ofreleasing animals was minor in comparison to the effect of immigration. The situation mightbe different from the end of the 1980s onwards. The size of the colony is larger now and thatmight favour the decision of the released animals to stay.

NUMBERS

25

20

15

10

5

O.l-.----.----.---_.---_.---_~-~-.,.._

11185 1_ 1887 1_ 1ll8ll 11190 11181 11182

YEARS

Figure 5 Annual maximum number of grey seal pups counted at thecolony between Vlieland and Terschelling.

pup productionIt was only in 1985 that the first suclding grey seal pups were observed in the colony

between Vlieland and Terschelling. The chances that all pups born there are observed, canvary considerably. When the weather conditions are favourable, i.e. no storm and no springtide, the chances are rather high. The chances of spotting pups are lower in years with stormsand also when there is a combination of spring tide and strong winds. In such conditions, thatspecific tidal flat is flooded and the newborn pups are washed away, often loosing theirmothers. The number of observed pups (Fig.5.) should therefore only be considered asminimum estimates of numbers born.

There has been a shift in the timing of the pupping season, from the middle ofJanuary to early January ('t Hart et al., 1988; Vedder et al., 1992). They suggested thatmost of the whelping fernales were primiparous in the mid-1980s. This fits with the conceptthat the young grey seals released in 1980 are becoming fully mature (usually at the age of7) around 1987 and from then onwards start to contribute to the population. That could partlyexplain the increase in numbers of pups born since 1987. The extra increase in total numbersin 1992 is partly explained by the increase in numbers born in that year. The data series isobviously too short to demonstrate this conclusively.

morphodynamic changesMorphodynamic changes of tidal flats occur in this particular area. The height of the

tidal flat the seals use to haul out, has decreased during recent years. This has already causedthe seals to disperse to other tidal flats in the vicinity. However, the exposure time of these

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flats is generally much shorter, which renders the area in fact less suitable as a breedinghabitat for the grey seals. How this will affect the future development of that colony isdifficult to predict, but it will at least slow down its growth.

Discussion

The fact that grey seal remains in prehistoric discoveries were more numerous thanharbour seal remains, does not necessarily indicate that the grey seal was more abundant.The difference is more likely due to the fact that the grey seal is in general easier to huntthan the harbour seal. As grey seal populations decreased, the hunt probably shifted graduallyto the harbour seal, which may explain the increasing ratio of harbour to grey seal remainsin the discoveries.

Because the species is considered to belong to the indigenous fauna of the WaddenSea, it would be possible to promote the recovery of a grey seal stock in the Wadden Sea,by setting up reserves in uninhabited areas that are not flooded during high tide. The releaseof rehabilitated animals in those areas would act as a catalyst for the founding of newcolonies. However, this type of management needs extremely careful consideration, since anincreasing grey seal stock certainly has the potential to come into conflict with othercomponents of the Wadden Sea ecosystem, such as the recovering harbour seal populationand many seabirds which require an undisturbed breeding biotope.

We would like to thank the sta./f 0/ the lighthouse "Brandaris" and the crews 0/ the mv."Zeekoet" and the mv. "Terschelling" who provided us with additional counts 0/grey seals.The Seal Research and Rehabilitation Centre in Pieterburen and the Ecomare Centre atTexei, kindly enabled us to use their data on released animals. It is appreciated that SophieBrasseur, Mike Fedak, Karen Gardiner, Edith Ries & Wim Woljfimproved an earlier version0/ this paper.

References

Bonner, W.N. (1972). The grey seal and common seal in European Waters. Oceanogr. Mar. Biol. Ann. Rev., 10, _461-507. ..,

Bonner, W.N. & Hiclding, G. (1971). The grey seals of the Farne Islands. Trans. not. Hist. Soc. Northumb., 17,141-162.

Bonner, W.N. & Hickling, G. (1974). Seals ofthe Farne Islands, 1971-1973. Trans. nato Hist. Soc. Northumb., 42,65-84.

Clark, J.G.D. (1946). Seal-hunting in the Stone Age of north-western Europe: a study in economic prehistory.Proceed. Prehist. Soc., 12, 12-48.

Clason, A.T. (1988). De grijze zeehond HaJichoerus grypus (Fabricius, 1971). In Terpen en wierden in het Fries-Groningse kustgebied, ed. by M. Bierma, A.T. Clason, E. Kramer & GJ. de Langen. Wolters-Noordhoff/Forsten, Groningen, 234-240.

Deinse, A.B. van (1926). Fossiele cetacea and Pinnipedia in Nederland. Versi. Kon. AJcad. V. Wetensch., 35, 1036-1044.

Drescher, H.E. (1979). Biologie, Ökologie und Schutz der Seehunde im schleswig-holsteinischen Wattenmeer. Beirr.Wildbiol. I, 1-73.

Hart, L. 't, Moesker, A., Vedder, 1. & van Bree, P.l.H. (1988). On the pupping period ofGrey Seals, Halichoerusgrypus (Fabricius, 1791), reproducing on a shoa! near the Island of Terschelling, The Netherlands. Z.Sdugetierk., 53, 59-60.

Harwood, l. (1987). The status and management of seal stocks in Great Britain. Proceed. Coastal Seal Symposium,Oslo, 23-36.

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Lozan, E. Rachor, B. Waterman & H. von Westernhagen. Paul Parey Verlag, Berlin.Requate, H. (1956). Die Jagdtiere in Nahrungsresten einiger frühgeschichtlicher Siedlungen Schieswig-Hoisteins.

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229.Requate, H. (1962). Über nacheiszeitliche Säugetiere und die Geschichte der Haustiere Schleswig-Holsteins. Z.

nerz. Züchtungsbioi., 77, 242-254.Scheibei, W. & Weidei, H. (1988). Zum Vorkommen von Kegelrobben (HaJichoerus grypus Fabricius, 1791,

Phocidae, Pinnipedia) in Schieswig-Hoistein. Zool. Anz., 220, 65-70.van Giffen, A.E. (1913). Die Fauna der Wurten (Diss. Leiden). T. Ned. dierk. Vereen., 2, 1-166.van Giffen, A.E. (1916). Over de terpen in Groningen en Friesland. T. Ned. dierk. Vereen., 2, XI-XVI.van Haaften, J.L. (1974). Zeehonden langs de Nederlandse kust. R.I.N. meded. 104.Vedder, L., 't Hart,L. & van Bree, P.J.H. (1992). Further notes on the pupping period in a recently founded colony

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