278 Rapp. P.-v. Réun. Cons. int. Explor. Mer, 172: 278-285. 1978. CHANGES IN THE SEAL POPULATIONS OF THE NORTH SEA C. F. S u m m e r s Institute for Marine Environmental Research, Seals Research Division,* c/o British Antarctic Survey, Madingley Road, Cambridge CB3 OET, England W. N. B o n n e r British Antarctic Survey, Madingley Road, Cambridge CB3 OET, England J. VAN HAAFTEN Rijksinstituut Voor Natuurbeheer, Kemperbergerweg 11, Arnhem, The Netherlands The distribution and changes in abundance of the grey seal, Halichoerus grypus, and the common seal, Phoca vilulina, in the North Sea are described. Principal concentrations of grey seals occur at North Rona, Orkney, and the Farne Islands (there is a large population in the Hebrides but this is unlikely to in­ fluence the North Sea). The total North Sea population of grey seals is between 29000 and 32000. Principal concentrations of common seals in the North Sea are at Orkney, Shetland, the east coast of Scotland, the Wash, and the German Bight; the total population is between 15000 and 18350. Changes in stock size are thought to be closely related to predation by man. Protective legislation has often been followed by increased numbers of seals, but habitat destruction, disturbance by human activities, and possibly contamination of the environment have prevented recovery of the common seal stocks in the Netherlands. INTRODUCTION the North Sea. The seals of Iceland and the Faroe Six pinnipede species have been recorded from the Islands, though numbering some thousands (Smith, coastal waters of the North Sea: the grey seal, Hali­ 1966; Lockley, 1966; Arnlaugsson, 1973) are not dealt choerus grypus Fabricius; the common seal, Phoca with as it is thought unlikely that they penetrate to vitulina L.; the ringed seal, Phoca hispida Schreber; the North Sea proper. the harp seal, Pagophilus groenlandicus Fabricius; the In the absence of other significant predators in hooded seal, Cystophora cristata (Erxleben) ; and the recent times, seal numbers have been controlled walrus, Odobenus rosmarus (L.). Except in the north of mainly by human hunters. Southwell (1881) in an Norway, which is outside the area considered here, early account of seals in British waters claimed that the ringed seal, harp seal, hooded seal, and walrus common seals occurred in “greatly reduced numbers” occur only as rare visitors, and their breeding range as a result of hunting but were nevertheless “still is confined almost entirely to the Arctic. This account, abundant” in Orkney, Shetland, and the Hebrides therefore, is restricted to the grey seal and the common and that “many young ones” were born in the Wash. seal. The distribution of these two species in Europe He reported that the “chief places of resort” of grey has recently been reviewed by Bonner (1972). Although seals were Orkney, Shetland, the Hebrides, and the their breeding ranges extend from Ireland to Finn­ west coast of Ireland, although they were known to mark and from Iceland to the Baltic, this paper breed at the Fame Islands also. He gave little in­ deals only with the populations of the eastern and dication of the size of these populations but quoted a northern seaboard of the British Isles and the west suggestion that the seals of Haskeir in the Outer coast of Europe, from southern Norway to the English Hebrides were in danger of being hunted to extinction. Channel, excluding the Kattegat and the Baltic (see A similar situation was reported by Collett (quoted Fig. 223). The remote island of North Rona, which in Smith, 1966) concerning the grey seals of Halten has a large breeding population of grey seals, is (in Sør Trøndelag, Norway), where hunting had regarded as being in the North Sea, and the grey allegedly reduced the stock from 500 or 600 in 1870 seals of the Outer Hebrides are also briefly considered to about 200 in 1910. Only Evans and Buckley (1899), because, during feeding dispersals, they may enter when reporting that the common seal was much more * Since incorporated into the Sea Mammal Research Unit (NERC), address as above. Changes in the seal populations of the North Sea 279 about this time a few workers started independently to collect, for the first time, accurate information about the size of grey seal stocks in Great Britain. Their findings are discussed below. The common seal, which was given no legal protection in British waters until 1970, has not yet shown evidence of an increase in numbers but has, with some notable exceptions, Orkney supported uncontrolled hunting in many parts of its range. Southern <Q, ^60 Norway M . F5?i T METHODS Hebrides The difficulty of counting adult seals in the sea, and the lack of samples representative of all age classes ashore has caused most workers to use pup production as an index of population size. Such an index can, if necessary, be converted to an estimate of the all-age population by using a life table for the species in question (e.g. Hewer, 1964; Bigg, 1969). Methods of determining pup production include direct Schleswig-Holstein £>} and Niedersachsen “V counting through an entire breeding season (Goulson H' and Hickling, 1964; Summers, Burton, and Anderson, ddensea — 1975), direct counting during part of a breeding season which is then related to a season in which total production is known (Boyd et al., 1962; Boyd and Campbell, 1971; Coulson and Hickling, 1964), in­ direct counting from aerial photography (Vaughan, 1969, 1971a), and mark-recapture (Summers and 50 _ Mountford, 1975). These techniques are reviewed in some detail by Bonner (1972). Where pup production cannot be estimated, seals can be counted either hauled out at low tide or swimming in the water. At Grey best this gives an adequate representation of the total stock (van Bemmel, 1956) or at least gives a minimum Figure 223. Seal populations associated with the North Sea. estimate of it (Vaughan, 1971b). Because grey seal pups are usually confined to land for some weeks after birth, it is relatively easy to count them. Common numerous than the grey seal in Shetland, conceded seal pups, on the other hand, are usually born on that this may have been an erroneous impression intertidal banks and skerries and are very difficult to resulting from the more inaccessible haunts of the count. For these reasons estimates of population size grey seal. in the grey seal are usually based on pup production Hunting pressures on seals have been relieved by data and are, therefore, more reliable than estimates legislation in most countries bordering the North Sea. for the common seal which are only rarely derived For example, Norway in 1973 added total protection in this way. for seals in the whole of the southern part of the country to existing local protection (Øritsland, per­ CHANGES IN THE SIZE OF GREY SEAL POPULATIONS sonal communication) ; the Netherlands controlled the hunting of common seals in 1954 and afforded them The main grey seal breeding assemblies affecting total protection in 1962. Great Britain passed laws in North Sea waters are those in Orkney, North Rona, 1914 and 1934 to protect grey seals during the breed­ the Hebrides, and the Farne Islands, with smaller ing season. Rae (1960), in a review of the impact of groups in Shetland (including Fair Isle) and Norway seals on Scottish fisheries, quoted several authors to (see Fig. 223 and Table 133). support his conclusion that the grey seal increased in numbers under this protection. Nowhere was this more ORKNEY strikingly evident than at the Farne Islands where the The first meaningful estimates of the grey seal population quadrupled between 1928 and 1957. At population in Orkney were made by Smith (1963) in 280 C. F. Summers - W. N. Bonner - J. van Haaften Table 133. Summary of data on grey seal stocks of the North Sea Date of Estimated Estimated Reliability Status Management Locality last pup all-age of estimates survey production population N orth Rona 1974 2200-2500a 8000-9000 Good Static Totally protected Orkney 1974 3600 12600 Good Increasing Part hunted (1000 pups/year), otherwise close season Sep-Dee Shetland 1973855d 3000 Poor Unknow n Sporadic hunting Farne Islands 1974 ci655-2010b 5800-7050 Very good Increasing Management objective is stock of until 1972 1000 breeding females, otherwise totally protected Southern Norway 1965 + + U nknown Unknown Totally protected since 1973 Total North Sea 8310-8965 29000-32000 Outer Hebrides 1974 5200g 18000 Good Increasing Part hunted (1000 pups/year). Monachs totally protected Norway (remainder) 1963 650+f 2000-3000 Unknown Unknown Close season May-Nov (a) Summers, Burton, and Anderson (1975). (b) Pre-cull estimate, (c) 1974 estimate, (d) Anderson (1974). (e) Øynes (1966). (f) Øynes (1964). (g) Summers, Vaughan, and Witthames (1975). the early 1960’s. From a pup production estimate of He subsequently recorded a decline in production around 3 000 per year he estimated a total population between 1960 and 1965 (Smith, 1966), while V aughan of 10500 using Hewer’s (1963) provisional life table. (1969) amended and extended the Orkney estimates up to 1968. However, as there is no year for which pup production has been measured directly through an entire breeding season in Orkney, it has been necessary to standardize the method of obtaining an Number of pups born estimate of total production to make all the counts comparable. To do this all the data acquired since 9000' North 1960 have been reworked and a constant conversion factor applied to the maximum number of live pups counted at each site in each year. Since the relation­ Scientific expedition at North Rona and cull at Farne Islands ship between the maximum number of live pups and total production at other assemblies can range from about 1:1-4 (Hickling, personal communication) to Naval expedition about 1:1-8 (from Summers, Burton, and Anderson, 6000 1975), the choice of factor applied to the Orkney counts, 1 -5, was somewhat arbitrary.