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Collins New Naturalist Library
SOUTHERN HEBRIDEAN GROUP
This group is centred around the southern Inner Hebrides, more correctly known as The Ebudes. The group is a small one but of considerable geographical interest. The breeding rookeries are on the Treshnish Isles (west of Mull) and the islands of Ghaoidmeal and Eilean nan Ron, off Oronsay. Occasional pups are born on beaches on Colonsay. The skerries adjoining Ghaoidmeal called Eilean an Eoin, Sgeir Leathan and Cearn Riobha may also have a few pups born on them but also provide the sites of the ‘reservoirs’, while the pre-breeding assembly occurs on the skerries which lie between Oronsay and Eilean nan Ron. Nothing is yet known about the haul-outs during the non-breeding period.
The geographical peculiarity of these islands is that all are surrounded by erosion platforms which are exposed at low water. These platforms are very extensive and riven by channels so that, although the tides are very small (8–12 feet), within a very short time the area of exposed ‘land’ is vastly increased but cut across by deep water channels which thus connect the true beaches with the open sea. Only in the Treshnish Isles are the platforms narrow.
As will be seen later, this geography has a profound effect on the social organisation and behaviour of the bulls and cows (Pl. 2). The pups are born on the small beaches and, although none have insuperable barriers separating them from the grass-covered tops of the islands, they do not move inland until they have become moulters. The cows make frequent visits to the open sea along the channels and the bulls hold territory along these channels and on the platform to intercept the cows and mate with them. The group is not large numerically, but has certainly increased under the total protection afforded by the late Lord Strathcona and Mount Royal. It is probably limited by the number of available breeding sites. The total population based on these islands is probably about 2,000.
OUTER HEBRIDEAN GROUP
This may well have been more extensive in the past although one of the largest collection of rookeries, on North Rona, must be of comparatively recent date since there is no record of grey seals using that island during the time of human habitation up to the mid-19th century. Breeding on Canna and a few other islands has taken place until the last century but now all rookeries occur on islands in the open Atlantic. Chief of these is North Rona (Pl. 4), but Gaskeir has about 700–800 pups annually (Pl. 6). Shillay (Pl. 3), Coppay, Haskeir, Causamell and St Kilda have small breeding sites and there is some evidence that while the numbers on Haskeir were larger in the past, Coppay and St Kilda are new sites and increasingly used.
All these islands are largely rock-bound, several with precipitous sides which in many places can be dignified by the term cliff. All however have some point (a geo) or points of access to the vegetation-covered top of the island. This is necessary because, situated where they are, they are subjected to continual Atlantic swell. Thus on none are there any beaches in the normal meaning of the word except on the east or leeward side if the geology permits it. Shillay and St Kilda have such easterly bays and, as Shillay is uninhabited, full use is made of it both as a site for pre-breeding assembly and for a rookery (Pl. 3). On all these islands, the main rookeries are developed well above sea-level where the swell cannot reach, and we find a completely terrestrial set-up as opposed to the almost marine one of Pembrokeshire.
Reservoir sites are known for North Rona on Loba Sgeir, the northern skerries of Fianuis such as Lisgear Mὸire and Lice Mhor and on Gealldruig Mhor to the south-east of the island. Reports of seals on Sula Sgeir and on Sule Skerry point to these being used as fishing haul-outs, but moulting haul-outs have not yet been seen because no one has been there at the right time of year. Elsewhere nothing is known of the reservoir and non-breeding haul-outs, although the Flannen Isles are likely to be so used to the west of Lewis.
This group is a large one comprising probably about 15,000+ seals of all ages. There is little evidence as to whether the numbers are increasing or not. The population based on North Rona appears to be fairly stable, although there seems to be room for the expansion of the existing rookeries. However, the recent establishment of breeding rookeries on St Kilda and Coppay may indicate a small increase.
ORKNEY AND SHETLAND GROUP
Until about ten years ago virtually nothing was known about the seal population in this area. Darling writing in 1947 said that ‘this seal occurs in Orkney and Shetland in relatively small numbers’. We know now that this is very far from the truth and that a population of 8,000 or more are based on these two groups of islands. They have, since 1960, been subjected to a great deal of research. All the main and most of the minor rookeries are known and several moulting sites and other types of haul-out places have been identified. The task has not been made easier because common seals are also present in the area so that records and reports are often not clear as to which species is referred to.
The types of rookeries are rather variable. Perhaps this is because, being an archipelago, the Atlantic swell, already somewhat broken by the north coast of Scotland particularly in Orkney, is not an overwhelming factor. In South Ronaldsay, for example, wide and deep beaches are used as rookeries much as the narrower ones of Pembrokeshire are. The bulls however are stationed on the beaches and not offshore. Elsewhere, such as on the Greenholms, the rookeries, beginning on the rocky foreshore (Pl. 5) stretch inland on the grassland in a very Hebridean manner. Perhaps here more than anywhere else one is driven to concede that the grey seal is extremely adaptable and much of an opportunist.
About half the annual pup production was accounted for in the Muckle and Little Greenholms (1,100–1,500).* Several other islands in this central area of the archipelago, such as Wart Holm, Rusk Holm, Holm of Fara, Fara, Holms of Spurness, Little Linga and Gairsay provide another total of about 1,000. North Ronaldsay probably has only about 100 pups a year, but south of Mainland on South Ronaldsay, Swona and Little Skerry (in the Pentlands) another 250–300 are born. The research work and a local intensification of sealing has recently reduced the population a little and disturbance in one breeding season has been reflected by a change of site or a redistribution of numbers in the next.
Many other skerries, such as Auskerry, Taing Skerry, Wyre Skerry, Eynhallow, Boray Holm, Holm of Birsay, Damsay, the Barrel of Butter and others are used as haul-out sites during the non-breeding period, some of the breeding islands such as Rusk Holm, Holms of Spurness and Little Greenholm are also used in the early months of the year. It is, however, very noticeable that non-breeding haul-outs are never seen on the major breeding rookeries. This differentiation of site use is particularly well shown on Rusk Holm where the two southern skerries are used as reservoir haul-outs during the breeding season, the rookery is on the eastern pebble beach and north-eastern rocks, while the moulting haul-outs are either on the north-western rocks or on the south or south-eastern rocks according to wind and tide.
The numbers in Shetland are much fewer. This must be attributed as much to the rock-bound nature of the coast-line as to the scarcity of small uninhabited islands. In the southern part Lady Holm, Horse Holm and the geos or narrow beaches under Fitful Head provide small rookeries. Only in the north does the lie of the islands provide any lee so that small rookeries are found around Yell and Fetlar. The beaches under Ronas Hill are important breeding sites. In total there are probably only 200–300 pups each year.
In the non-breeding season the major haul-outs in the south and west are on Horse Holm, Lady Holm, around Fitful Head and on the Ve Skerries, west of Papa Stour. Small though these last are they sometimes hold up to 2,000 seals not all of which can be considered as Shetland grey seals in the sense that in the breeding season they will be found on the Shetland rookeries. Fair Isle, halfway between Orkney and Shetland, also has haul-outs and as a number of young seals marked in Orkney have been found in Shetland there is little doubt that the adults move freely between the two groups of islands. The picture here too is somewhat complicated by the presence of common seals and there is no doubt that much more is to be learnt about their distribution here, particularly in the northern islands.
In Orkney and Shetland there has persisted a tradition of sealing which has never been so strong in the Hebrides, possibly by reason of the Norse connections. The grey seal population here, therefore, appears to be fairly stable, fluctuating largely as a result of variations in sealing effort dependent, in turn, on the market price of seal-skins.
NORTH SEA GROUP
There is little difficulty in deciding that the centre of this group is the Farne Islands (Pl. 5). The North Sea is peculiarly free of islands and it may as well be stated straight away that only on the Island of May in the Firth of Forth and on Scroby Sands off Great Yarmouth are there any breeding rookeries other than on the Outer Farnes. Elsewhere there is evidence of their fishing inshore along the east as far south as the Wash, and the coast of Holland, and as far north as the Moray Firth. Fishing haul-outs have been recorded from the Abertay Sands to the Wash, but there is always a haul-out on the outermost islands of the Outer Farnes. At certain times of the year, notably the spring, they consist of very large numbers indeed. The population has been rising steadily in recent years. Annual counts of pup production have been made with considerable accuracy over the past fifteen years or so and an annual increase of about 7% has been calculated. Undoubtedly at the turn of the century the population was at a very low ebb. The islands at present involved as rookeries are the North and South Walmses, Staple and Brownsman, but in the more distant past others were used as rookery sites such as the Wide Opens and other islands of the Inner Farnes. At times of the year, other than the breeding, haul-outs are found on the Longstone and Longstone End, the Harcars and on other skerries of the Outer Farnes. All of these islands are low with at least one or two shelving rock beaches so that access is easy and sheltered within the archipelago. The interiors of the islands are used as well as the ‘beaches’ for rookery sites, but usually only after the more shoreward zones have become congested.
At the time of writing the number of pups produced annually had reached 2,011 (1971) making the Farne Islands group the third largest known in the British Isles with an estimated total population of almost 6,500–7,000.
Before going into the details of the life history of the grey seal, it is necessary to give a brief description of the yearly cycle of both adult males and females. Until recently it was really quite unknown. Even worse, statements appeared in the literature which cannot have been founded on observation because they were so wildly wrong. Until the early 1950’s this species had only been studied during the breeding season. It is not difficult to find the reasons for this. During the late autumn and winter months the weather makes visits to the islands a very chancy affair and few people feel inclined to camp on these isolated spots during this time of year. Yet these are the only ways in which information can be gathered. During the summer months the weather is generally good but on the other hand very few seals will be seen. For the spring however, there is no excuse and it is a very important season indeed. Dr K. M. Backhouse and myself started visiting the Ramsey Island sites in 1952 and soon every month except December and July had been covered. Later all months in Orkney had been covered by Mr E. A. Smith and in the Farnes by Dr J. C. Coulson and Mrs Hickling. The work of Craggs and Ellison throughout the year on West Hoyle Bank has already been referred to. (Oddly enough it was undertaken quite independently of the other work in Pembrokeshire, Orkney and Farnes on which there was considerable interchange of knowledge and information. An example of this interchange will be given below when dealing with the annual moult.)
The best time to start describing the annual cycle is during the summer months of June, July and August. This is a very active feeding period for both bulls and cows. The bulls have to put on sufficient blubber to sustain them during their period of starvation, while they are maintaining territory. The cows also have to put on blubber to provide the fat for the rich milk suckled by the pups, but in addition, must eat sufficient to permit the foetus within to grow. (During the summer the foetus may increase by more than a pound a week.) Towards the end of August there is a tendency for both bulls and cows to move towards their breeding areas. This has been shown by J. Morton Boyd by the ingenious analysis of observations made at a number of points along the coastline of north-west Scotland. In the south-western group this drift is probably a little earlier, but newly arrived cows have been seen by me in early September on Ramsey Island. It is difficult to be precise in an animal whose breeding period extends over at least 2 months. The most that can be done is to note when such a tendency is at its peak and therefore most conspicuous. Other individual seals will still be at sea feeding, others still may already have arrived at the breeding rookeries and have pupped.
Bulls and cows must now be studied separately because the purpose of their assembling in the rookeries is quite different and consequently their behaviour and timing is different. Put very briefly and perhaps in an over simplified way, bulls assemble for mating, cows for pupping and only secondarily for mating.
Bulls seem to predominate at the beginning of the breeding season on or near the rookeries. Here they appear to sort themselves into a dominance order, those having previously bred clearly taking precedence over the younger ones. Gradually they take up station on a territory which they defend against all comers (Pl. 7). Although they immediately approach an invading cow, their agonistic behaviour dies down as soon as they recognise her sex. Invasion by new and younger bulls (Pl. 6) continues for a considerable time and for at least a fortnight or so defence is the dominant activity of each bull holding territory.
Meanwhile cows begin to arrive at the rookeries (Pl. 7) and shortly give birth to their pups. Suckling commences within the hour and is repeated several times a day for at least a fortnight, sometimes for nearly three weeks. During this period the cows resent any approaches by the bull, who is repelled by hooting and rapid waving of the fore flippers (‘flippering’). Eventually, some time in the third week after giving birth, the cow accepts the bull in mating. Mating is often repeated several times at intervals. The cows desert their pups and leave the rookeries. Whether they go immediately to sea or remain in nearby waters is not yet certain, although I am inclined to believe the former is more likely.
The deserted pups moult their first or ‘puppy’ coat of white hair and appear in their ‘moulter’ coat which is similar to the adult pelage. In a healthy pup this process takes about a week; the fourth week of their life. When moulted the pups may, as in the south-west, immediately leave their natal beach, or as in the north and north-west remain on land and in fact move farther inland. Hunger will however eventually drive them to sea within a week or two.
The bulls remain on station for five to six weeks before becoming exhausted. They then move away from the rookeries to sea and recommence feeding to restore the blubber they have used during their enforced fast.
While this is the time-table of individual cows and bulls, it must be remembered that the time-table of the rookery is more extended. Thus a fortnight after the first pups have been born matings will begin, but this will probably be before the peak of the pupping. Consequently while the peaks of the various processes, pupping, mating, moulting, desertion by cows and leaving by bulls will be separated by periods corresponding to the normal time-table of individuals, this central period will be preceded by two or three weeks when arrivals of bulls and cows and some pupping are the only phenomena and will be followed by several weeks in which mating and desertions will predominate. The overall period will be about
Now follows a period about which not so much is known. The nearby haul-out sites are used but not to a great extent. Probably the answer is that both bulls and cows are feeding as much as possible to replace their lost blubber, to fit themselves for the rigours of the winter. One of the most remarkable facts of this period, which covers about
I have not mentioned the actual dates through this breeding and post-breeding period because there are differences between the geographical groups and generalisation is therefore difficult. What makes precision even more difficult is that the full story is not known in detail for all of the groups. However, some attempt may be made to give an overall picture if we omit the Farne Islands group. In the south-western group the peak of pupping is probably in the first week in October, and by mid-November all the beaches are completely deserted. In the Hebrides and North Rona the dates are about 10–14 days later, although the much larger size of the rookeries results in more early and late puppings so that the season appears to be longer and a few adults may still be found in the breeding grounds in early December. In Orkney and Shetland a further 7–10 days should be allowed, so that mid-December would be a reasonable date to compare with mid-November in the south-west. January to March may be counted as the months of the cow moult, although few Orcadian cows will have begun in January and few, if any, Pembrokeshire ones will remain unmoulted in March.
For the Farne Islands group all these dates must be delayed by about a further month. There the first pups are in late October and the peak about 3–4 weeks later in November, pupping continuing actively well into December.
Our knowledge of the growth of the foetus is derived solely from northern material from Orkney. The mean date of the recommencement of the growth of the embryo is the middle of February. Thereafter the foetus grows apace in about 80% of the cows, the others are non-pregnant. So far as can be made out they continue to fish in the adjacent seas periodically coming ashore in fishing haul-outs for short periods at low tide. In the summer months the haul-outs must be both few and short because the numbers seen are very few indeed compared to those at other times of the year.
We have left the bulls at sea after the breeding season and it is not until mid-February that we have any evidence that large numbers come ashore. Their annual moult appears to be timed about two months after that of the cows. It was first observed in April in Pembrokeshire, but by then many were completely moulted and few remained unmoulted early in May. Dr Backhouse and myself, having found the comparatively large moulting haul-outs of bulls in Pembrokeshire, were able to direct attention to this in other groups, such as the Farne Islands, where the same phenomenon was then seen by other observers. These moulting haul-outs of the bulls are very remarkable and often number thousands at a time in the large northern groups. The bulls, like the cows, largely disappear in the summer months from all the inshore sites and we can only conclude that they too are feeding at sea to prepare themselves for the breeding season.
We can now turn to some of the distinguishing characteristics of this species, both in structure and habits.
It is always extremely difficult, if not impossible, to estimate the length or size of seals in the field since there is rarely, if ever, any standard for comparison. Measurements therefore, while of use on a carcase, cannot really be used by an observer in the field. Any deductions based on claims to be able to ‘age’ seals in the field must therefore be regarded as highly dubious and probably very misleading. The pup is born at a length of about 33 ins. and 32–33 lbs. Bull pups appear to be very slightly heavier, but the difference is not truly significant having regard to the numbers which have been weighed. In any case it could only amount to a few ounces. It is very difficult indeed to speak about moulters since by that time all the differences in nutrition have taken effect and weights can vary between 100 lbs. or more and little more than the birth weight, if the moulter has been starved and is likely soon to die. The growth in length is quite small so that few moulters reach 40 inches until well after they have left their natal beaches however great their weight. Some of these very heavy moulters (over 100 lbs.) are so bloated with blubber that they can hardly turn their heads and movements are quite lethargic. There is some evidence that pups which do not reach the weight of 90 lbs. before being deserted by their mothers and moulting, have little or no chance of surviving their first year of life. Certainly all those below 60 lbs. appear to die before the first six months is out and all the recorded weights of grey seals in their first year of life are about 90 lbs. or less. Further, yearlings which have been weighed on the Farne Islands appear to fall more or less within the limits shown by the largest moulters. In other words there is little, if any, increase in weight in the first year of life.
These conclusions appear to be confirmed by the known weights of older bulls and cows. 3–4 year old bulls only weigh 170–190 lbs. while the same age group of cows weigh 130–150 lbs.; 5–6 year old cows may reach 170–180 lbs. but records for bulls are very few, although they suggest that by that age they may attain 2 cwt. There are practically no weights of fully mature bulls or cows to justify generalisations. All that can be said is that probably the older well-established breeding cows are over 2 cwt. and the older territorial bulls probably top 3 cwt. or even reach 4 cwt. In any case the bulls vary much more than the cows and in both sexes changes in weight amounting to
Much more data is available about lengths. Although little, if any, weight is acquired during the first year, the moulters grow considerably in length, an average of 55–60 ins. being normal at the end of the first year for both sexes. Thereafter some differences can be considered significant between the two sexes. The rate of growth is fairly steady until puberty at 5–6 years when cows will average about 6 ft. and bulls 6 ft. 6 ins. During the following 3 or 4 years the rate of growth of cows declines so that at 10 years of age they have attained their near maximum of 80–84 ins. (7 ft.). Some growth appears to continue throughout life but it is very small in the cows.
The years following puberty in the bulls account for the great difference between the older bulls and cows, for their rate of growth falls off much more slowly and is continued at the higher rate for a longer period. By 10 years of age the average is 90 ins. and 96 ins. is reached about 2 years later. Thus in round figures mature cows are about 7 ft. long and territorial bulls average about 8 ft. but may vary about 6 ins. more or less.
