Collins New Naturalist Library. H. Hewer R.

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      THE relationships of the grey seal, Halichoerus grypus (Fab.) have been discussed in the previous chapter. It clearly emerges that this species occupies a unique position being the only species of the genus.

      The world distribution of the grey seal is peculiar and unlike that of any other seal species. There appear to be three distinct populations (Fig. 7). To prove that there is never any interchange of individuals is practically impossible, but all the available evidence suggests that no real exchange takes place. The three populations are centred on the Baltic Sea, on the eastern north Atlantic and in the western north Atlantic. Davies (1957), in a very interesting paper which dealt with the possible geographical and historical reasons for this separation, suggested that these three populations should be called the Baltic, eastern Atlantic and western Atlantic respectively. There are also very good biological reasons for the separation. The eastern Atlantic grey seals, most of which are present in British waters, breed in the autumn;* the other two populations breed in late winter to early spring. Similarly the eastern Atlantic seals breed on land, either on beaches or on the landward slopes; the others tend to breed on ice and only if it is an exceptional year and there is little ice do the western Atlantic ones breed on the adjacent shore. Consequent upon these differences the social structure of the breeding seals is different. The eastern Atlantic seals tend to form large rookeries in which the cows outnumber the bulls by 5 to 10 times; in the ice-breeding forms a much closer approximation to equality is found in the western Atlantic region and such information as is available for the Baltic seems to suggest that the same applies there – isolated cows with their pups with attendant bulls over a wide area of ice floe.

      Davies points out that all three populations must have been united during the last Inter-glacial period, occupying the seas from northern Labrador to Greenland, Iceland, Norway and the White Sea, on the average about 15° north of the present range (Fig. 8). As the species is a land-breeder as opposed to an ice-breeder the succeeding Glacial period would have forced the grey seals to separate into two populations, one occupying the seas from Newfoundland to Florida and the other the seas from the ‘British Isles’ (which were not separated from the continent at that time) to the Moroccan coast. With the first retreat of the ice and glacial conditions covering the ‘British Isles’ and northern Europe, the North Sea and Baltic Sea came into existence again broadly joined in the region now known as southern Sweden and Denmark. The grey seals of the eastern Atlantic had meanwhile moved northward again occupying the seas from the Bay of Biscay round the ‘British Isles’ and into the Baltic. However after this changes in the land and sea levels caused a land connection to appear between Sweden and north Germany (Pomerania) thus cutting off a lake comprising the area of the present Baltic, including the Gulf of Bothnia and most of present-day Finland. This is known as the Ancylus Lake and in it were isolated grey seals which formed the origin of the present Baltic population. Much more recently the North Sea and the Baltic have again been joined but only by very narrow channels at the southern end of the Kattegat. As we shall see later there is evidence that only the very northern end of the Kattegat is entered very occasionally by young grey seals. The main eastern Atlantic population of grey seals no longer use the southern shores of the North Sea which are sand and mud only, quite unlike the preferred rocky situations in Britain and Norway.

FIG. 7. World distribution of the grey seal. There are three distinct populations in: 1. the Western Atlantic, 2. the Eastern Atlantic and 3. the Baltic.

      Until recently there has been a paucity of museum material. While this has been largely overcome for the eastern Atlantic form and some is being collected in Canada of the western Atlantic seals, practically nothing is yet available from the Baltic. In any case no one has yet brought what material there is together for comparison. When this is done it is quite likely that sufficient difference will be found to warrant the recognition of 3 sub-species. The geographical isolation, which is normally required for such a recognition, appears to be complete and biologically reinforced. The Baltic grey seals occur in the Baltic itself and in the Gulf of Finland, but do not penetrate westward into the Bornholm area or the Kattegat. The eastern Atlantic seals occur from the west coast of Brittany, all round the British Isles and on the Norwegian coast, but few records exist of their appearance as adults on the Dutch coast or Frisian shores. Grey seals are certainly present in the Faeroes and Iceland region and it is thought that these are eastern Atlantic from their autumnal breeding season, but it is a far cry from Iceland to the western Atlantic breeding grounds off Nova Scotia.

FIG. 8. The origin of the three populations of grey seals. Gross-hatched areas = glacial ice; diagonal lines = permanent sea (polar) ice; dotted line = approximate limit of seasonal drift ice; dotted areas = distribution of grey seals. A. shows the grey seals in one population close to the Arctic Circle in the Last Inter-glacial Period. B. shows the grey seals split into two populations, east and west Atlantic, by the maximum glaciation of the Last Glacial Period. C. shows the northward movement of the eastern Atlantic grey Seals following the retreating ice. The Baltic Sea is open to the North Sea across Denmark and southern Sweden, allowing entry of the grey seal. D. shows further northward movement of both ice and both populations of grey seals. England is still united to the continent but a bridge of land from Denmark to southern Sweden has locked off the Baltic population of grey seals. Since the period of the Ancylus Lake, Denmark has become separated from Sweden and England from the continent. (Redrawn from Davies 1957)

      In Canada the Arctic Unit has for a number of years been investigating both common and grey seals. While some of their results parallel our observations for the eastern Atlantic, others tend to emphasise the differences. Dr Mansfield and his collaborators have of course an interest in several other species of arctic seals but for our two species contact is maintained with British workers.

      For the Baltic grey seal the story is not so satisfactory. Mr Oliver Hook has for a number of years been into the Baltic to track down the pups and to relate the ice movements with the probable centres of breeding. So far, however, except for willing co-operation in his work, the Swedes have not undertaken much research on their own. Political difficulties are probably the cause since further investigation of breeding areas and so on would almost certainly involve Finnish and Soviet coastal waters. There seems, however, some urgency for work in the Baltic since the indications are that the population is decreasing considerably. In an enclosed sea this could lead to near extermination, unless international co-operation is established and this is difficult in the absence of facts.

      The species has a number of characteristics not normally taken into account by systematists which emphasise its uniqueness. All the other members of the Phocinae are either much more aquatic in their normal behaviour, or are confined to colder waters and associated with ice, or both. In British waters Halichoerus grypus is a truly temperate seal and always breeds on land. Both physical and behavioural (or ecological) characteristics tend to show that Halichoerus grypus diverged from the main Phocine stock early on. Its habit of breeding on land with the establishment of a social system may thus have evolved quite separately and does not indicate connecting links with other Pinnipede species in the Otariidae.

      For many years the grey seal of Britain was thought to be the bearded seal, Erignathus barbatus, and it was not until 1825 that it was firmly established that it was Halichoerus grypus. This may appear to the layman as either merely a matter of words or else plainly stupid. But there is some excuse for this confusion. Normally when a species is named and described its name and description are tied to a specimen which СКАЧАТЬ