Collins New Naturalist Library. H. Hewer R.

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СКАЧАТЬ Sassenach!) landed there on October 1st to stay for at least three weeks and make periodic censuses of the pups and other observations. Similar expeditions have gone almost every year since with varying personnel to keep a check on the numbers. Dr Boyd also organised over several years observations from scattered points over the west coast of Scotland to see whether data could be collected indicating movements of seals to or from the breeding centres. Some interesting facts have emerged from this too and from aerial photographs of some islands to which access is extremely difficult and rarely possible.

      But all this was field work and without a background of knowledge based on the examination of material in the laboratory many of the deductions which one would like to have made could only be very tentative. One of the outstanding problems related to the time-scale of the grey seal’s life. Several views had been put forward in the past without any firm basis. This is very dangerous because such ideas are often repeated as though they were well established facts. Here was such an instance. Davies accepted the view that a 12-year life span was normal for the female grey seal and had based his calculations of the total south-western group population on this. There neither was then, nor is now, any way of determining the age of a seal in the field. At that time there was not any way of doing so even if the carcase of the animal was available for examination. ‘How long do they live?’ or ‘How old is it?’ are two of the commonest questions asked about animals and they are almost the most difficult to answer. To find a method was, therefore, of first priority.

      Researches on the Pribilov fur-seal and several other mammals had shown during the post-war years that the deposition of some of the hard parts of the teeth was periodic and could be interpreted in terms of years, just as one can determine the age of some fish from their scales. By the late 1950’s I had satisfied myself that the layers of cement on the outside of the root of the teeth could, after some practice, be so used in the grey seal with reasonable accuracy. All that was now wanted was more material.

      During these years complaints from the salmon fishers, both rod and net, had been growing. Also from the white fish industry came reports that the occurrence of cod-worm was very much on the increase and the grey seal was blamed as the vector of this parasite while the Farne Island group were thought to be the centre of the culprits. Consequently the grey seal became of economic importance. The carcases of seals killed at the salmon nets along the east coast of Scotland were then made available and a great deal of material began to come in. It was not very well preserved of course and it was highly selective consisting only of those seals whose bodies could be recovered; if they were too heavy to handle they were not brought ashore. However, it was a start. Added to this an investigator, Mr E. A. Smith, was appointed in 1959 jointly by the Development Commission (on the Fisheries side) and by Nature Conservancy; he soon found that there was a group of grey seals (as well as common seals) in Orkney. Tagging soon showed that they were as guilty as the Farne Islands ones for depredations on the salmon of the Scottish east coast. Moreover the group was a very large one and could supply material in sufficient quantity for research without fear of seriously depleting the stocks. Of the other two known large groups, Farne Islands and North Rona, one was a sanctuary under the National Trust and the other a National Nature Reserve. Orkney and later Shetland then supplied most of the really valuable material. Further, when some degree of cull was decided upon during the breeding season, the statutory protection afforded the grey seal since the early 1930’s was lifted and for the first time we were able to obtain a sample of a breeding population in October 1960. Using the already discovered method of age determination an entirely new light was thrown on the age structure of the population, on the sexual life of both bulls and cows and many of the field observations previously made began to fall into place and to make sense.

      Many more minor problems have been worked on since, particularly by Mr Smith in the field and by Dr Backhouse and myself in the laboratory. None of us however has failed to take part in the other type of work involved, since cross-fertilisation of ideas between field and laboratory is essential for orderly progress to be made. There remained of course a number of outstanding puzzles which will be mentioned in the pages which follow.

      Within the last few years however, there have been developments which promise a continuation of systematic and sophisticated research, not only on the grey seal, but also on the common. The establishment of the ‘Seals Research Unit’ at Lowestoft under the National Environment Research Council with Mr Nigel Bonner in charge assisted by a small team of very active and resourceful zoologists has already shown what can be done using more modern equipment and the importance of prolonged co-ordination in work which must of necessity be scattered over a wide geographical area. Some of these results will be referred to later on.

      It is now necessary to define certain words which will be used to describe collections of grey seals when found on land. In the past the term ‘colony’ has been used in so many different senses that it has become almost meaningless and I shall not employ it. Before appropriate alternative words could be found it was necessary to know something about the seals and how the world population was organised. Already it has been shown that there are three major units and I propose to refer to these as ‘forms’. This term does not have the precise meaning taxonomically that ‘sub-species’ would carry but it does suggest that we are dealing with sub-units of the world population which are distinct on biological grounds as well as geographical. So little is known as yet of the break-down of the Baltic and western Atlantic forms that it is only possible to supply terms to describe the composition of the eastern Atlantic. Here first we must distinguish between the centre of breeding and places where, in the non-breeding season, the seals may haul out. The breeding centre will consist of ‘rookeries’ where pups are born and mating takes place. In addition there are collections of seals associated with the rookeries to which reference will be made later. The important biological fact of the rookery in relation to seal populations is that it is the origin of the population. If therefore the females may use one of several adjacent rookeries, the products of these rookeries must be considered as a single population arising from a ‘group’ of breeding centres. Here I propose the use of the word ‘group’ to describe a subdivision of the eastern Atlantic population which appears to behave as an entity and to possess some definable characteristics, either geographical or biological or both, which separate them from other ‘groups’.

      In some instances we still do not know enough to be certain whether we are dealing with one or two groups. For example, a number of grey seals breed on the north Cornish coast, there is an occasional birth on the Isle of Lundy and many more breed on the islands and coast of Pembrokeshire and Cardiganshire. Are we justified in thinking of these as a single working unit or ‘group’, or are there two? To add to the difficulty, just across the St George’s Channel, grey seals breed on the Irish islands of the Saltees and farther north on Lambay Island off Dublin. Should these be separated or included? What sort of evidence is there? We know that there are no breeding sites in the northern part of the Irish Sea but in the area of the Mersey Bight many can be seen in the non-breeding period of the year as well as on Bardsey Island off the Caernarvonshire coast and around Holyhead Island. Their numbers wane during the period when the Pembrokeshire seals are breeding but some, instead of going south, might well go west or south-west and breed on the Irish coast. Many a young seal in its first grey coat, only a month or so old, marked in Pembrokeshire, has turned up on the coast of Ireland even as far west as Galway Bay. Do they come back to the Welsh coast when they are mature, or do they join the Irish breeding seals? So far we have no definite answer and I prefer to think of a south-western group which would include all those I have already mentioned on the west and south coasts of Ireland, the Irish Sea, west Wales and Cornwall, the Isles of Scilly and Ushant off the Brittany coast.

      Other ‘groups’ for which there is some evidence are: Southern Hebridean, comprising breeding areas on the Treshnish Isles (off Mull), in the islands around Oronsay such as Eilean nan Ron and Ghaoidmeal and the occasional birth recorded on Rathsay Island off Antrim, Northern Ireland; Outer Hebridean, covering Gaskeir, Shillay, Coppay, Haskeir, Causamell, St Kilda and North Rona, including Sula Sgeir; Orkney and Shetland; North Sea comprising the Farne Islands, Isle of May and Scroby Sands (Figs. СКАЧАТЬ