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 , , and the Farne Islands (there is a large population in the 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, , the east coast of , 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 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

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 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 ) 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. Figure 224 shows the general upwards trend in the size of the Orkney stock which now appears to be in excess of 12000 animals. Since 1962 an annual crop of pups (750, Orkney increased to 1000 in 1971) has been taken under licence by local sealers. One of the effects of this 3000 exploitation, which has been restricted to particular islands, is that the proportion of the Orkney stock which breeds on protected islands is greater now than in 1962 when hunting first started. Some of these islands have become breeding localities only recently.

FARNE ISLANDS 1960 WU This is one of the best documented seal populations in the world and certainly the best known grey seal Figure 224. Trends in grey seal pup production, 1960-1974. stock. Key papers are those by Coulson and Hickling Plot for total North Sea obtained by summing data from (1964), and Bonner and Hickling (1971a, 1974). The Orkney (SRD unpublished report, 1975), Farne Islands (Bonner and Hickling, 1974), North Rona (Summers et al., population increased steadily from the 1920’s until 1975), and Shetland (constant 800 pups per year assumed). 1972, when about 2000 pups were born and the stock Changes in the seal populations of the North Sea 281 was reduced as part of a management plan. The until recently, shown almost no change at all, while objectives of the plan (Bonner and Hickling, 1971b), the Monachs have shown an increase in production one of which is to reduce the stock to 1000 breeding from about 50 pups annually in the first half of the females, are expected to be achieved in 1975 when a 1960’s to well over 1000 pups annually in the past further stock reduction is proposed. The stock, which few years. Although this increase at Monachs is clearly now numbers between 6000 and 7000 animals, is not endogenous the source of the immigrating stock protected except for these management operations, is not easy to determine. The notion that Gasker had and some control measures taken in 1963, 1964, and become saturated and spilt over to swell the size of 1965 to protect fisheries (when 990 female young, or the Monachs assembly no longer provides an adequate nearly 50% of the total production of females for equation. However, the assembly at North Rona these years, were killed; Bonner, 1975). might also be considered to be saturated because of Small groups of grey seals occur at the , its stable population size and very high neo-natal (in the Forth Estuary), at Donna Nook (Lincoln­ mortality, but as Summers, Burton, and Anderson shire), and at Scroby Sand (Norfolk). These haulouts (1975) point out, no seals branded at North Rona are considered to consist largely of Farne Islands seals, have yet been reported to breed in the Outer He­ although all the sites are recorded as breeding locali­ brides. ties (total pup production is probably less than 25 per Since 1966 a small am ount of pup hunting (less annum). than 100) took place in some years at the islands of Gasker, Coppay, and . The hunters’ catch in­

NORTH RONA creased to 386 pups in 1973 and 868 pups in 1974. The Monach Isles, where the most spectacular in­ The island of North Rona, which lies about 75 crease in productivity has occurred, are totally pro­ kilometers northwest of the Scottish mainland, was tected. created a National Nature Reserve because it sup­ ports the largest single island breeding concentration of grey seals in the world. The assembly has been SHETLAND studied annually since 1959 (Boyd et al., 1962; Boyd There are no reliable estimates of the Shetland grey and Laws, 1962; Boyd and Campbell, 1971 ; Summers, seal stocks. From a pup production estimate of 800, Burton, and Anderson, 1975). The latest account of Smith (1963) suggested an all-age population of 3000, the stock suggests an average pup production of 2200 but later (Smith, 1966) suggested the more con­ to 2500 annually with no clear indication of an up­ servative figure of 2000 (quoted by Bonner, 1972). ward or downward trend. The population is totally Anderson (1974) reported 855 pups born in 1973 and protected. drew attention to the considerable fluctuations in pup survival from year to year as a result of rough weather

OUTER HEBRIDES conditions. Both Smith (1966) and Anderson (1974) considered that the Shetland grey seals may not be This stock has been less frequently observed than a discrete stock because of the recovery there of young those described above. Boyd (1963) carried out the marked animals from other breeding localities. There first complete survey in the autumn of 1961 and does not appear to be any obvious trend in the number reported pup production of about 1800 (excluding of pups produced (800 to 850 annually, equivalent North Rona which has been treated separately here), to an all-age population of about 3000), and the stock while Smith (1966) recorded a figure of 2000 for 1963. traditionally supports a small pup-hunting industry Since the mid 1960’s there has been a considerable (up to 250 pups per year). increase in productivity. Bonner (1972) quoted pro­ duction figures of 2400 and 3300 for 1966 and 1970 respectively. These figures included the smaller pop­ NORWAY ulation of grey seals at the , but the The breeding range of the grey seal in Norway, increases took place chiefly at Coppay, Shillay, the with the exception of one very minor site near Sta­ Monach Isles, and some others, all in the Outer vanger, does not approach the North Sea proper Hebrides. Results from a survey carried out in 1974 (Øynes, 1966). The stronghold of the species is in the (Summers, Vaughan, and W itthames, 1975) suggest areas of Nordland and Trøndelag, which were repor­ total pup production of 5200, equivalent to an all-age ted by Øynes (1964) to produce about 650 pups per population of 18000. year. Øynes stated that the species had been declining This increase represents rather more than a doubling until about 1956 but since then has been maintained in population size since 1966. However, while some by recruits from the expanding British assemblies. islands have shown increases of this order, Gasker had, However, despite records from southern Norway of 282 C. F. Summers - W. N. Bonner - J. van Haaften

Table 134. Summary of data on common seal stocks of the North Sea

Date of Estimated Reliability Status M anagem ent Locality last stock of estimate survey size

Orkney 1972 2000-3000 Moderate Probably static Hunted, quota 200 pups/year Shetland 1973 2000a M oderate Decreasing until 1973 Totally protected since 1973 East coast, Scotland 1973 850-1050 Poor Unknown Sporadic killing of pups and adults to protect salmon-netting stations Wash 1973 6575± 1076b Good Probably static Hunted, quota 400 pups/year (no hunting in 1974) German Bight 1974 3800 M oderate Declining See text Southern Norway 1965 850c Unknown Declining at time Totally protected since 1973 of survey Total North Sea 15000-18350 Norway (total) 1963 4000d Unknown Declining at time Outside S Norway close season of survey M ay-N ov

(a) Anderson (1974). (b) Summers and Mountford (1975). (c) Øynes (1966). (d) Øynes (1964). seals tagged in Great Britain (Hickling et al., 1962), in the early 1960’s but completely protected during Øynes (1966) has not found any new breeding loca­ the breeding seasons of 1974 and 1975). lities. All seals in southern Norway have been totally Small breeding groups of common seals also occur protected since 1973 (Øritsland, personal communi­ at Donna Nook (Lincolnshire), Blakeney (Norfolk), cation) . Scroby (Norfolk), and in the Thames estuary. The total number of common seals observed at these localities is very variable but has not exceeded 700 CHANGES IN THE SIZE OF COMMON SEAL (SRD unpublished report). It is doubtful if these POPULATIONS constitute separate stocks. The principal concentrations of common seals around the North Sea are in the Wash (eastern England), the GERMAN BIGHT German Bight (from the Rhine Delta to Jutland), Groups of common seals are found in the Nether­ Shetland, and Orkney, with smaller groups on the lands, in the Delta area in the southwest, and in the Norwegian coast and the Scottish east coast (see Table 134). Waddensea in the north; in Germany, in the states of Niedersachsen and Schleswig-Holstein; and on the west coast of Denmark. With the possible exception THE WASH of the seals of the Delta it is likely that no distinct By counting seals from a boat, Sergeant (1951) populations are represented here. Tag recoveries in­ estimated a stock of 1000 to 1500 seals in East Anglia, dicate that young seals may move from one area to but this must be an underestimate because a survey another, and it is possible that the large stock of of this kind could not have covered all the sandbanks common seals in the Wash may provide some recruits during a single low-tide period. Vaughan (1971a), to the continental population (Bonner and Witthames, using aerial surveys, found a maximum of 1722 seals 1974). on the sandbanks but, because it is not known what The most detailed information for this region relates proportion of the stock might haul out simultaneously, to seals from Dutch waters, where the Research In­ this count cannot be used to estimate the total pop­ stitute for Nature Management has been conducting ulation size. From marking and recapture, it was an investigation since 1958. In the late 1930’s the estimated that the average annual production of pups number of common seals in the Netherlands was between 1968 and 1973 was 1450 ± 239; this is estimated at about 1400 in the Delta and 2700 in the equivalent to a total population of 6525 -f 1076 Waddensea (Hentschel, 1937). Over-hunting of the (Summers and Mountford, 1975). From the inde­ young seals, stimulated by payment of bounties on pendent evidence of pup-hunting returns, Summers the grounds of fisheries protection, caused a decline and Mountford (1975) estimated a minimum popu­ in this stock. In 1954 seals were included in the pro­ lation of more than 4000. It seems more likely that visions of the hunting law and could only be taken the differences between these estimates reflect im­ under licence. By 1958 it was apparent that in the proved methods of estimating, rather than an increase preceding few years not all the licences issued had in the stock (which was exploited heavily beginning been used. A survey to investigate this situation was Changes in the seal populations of the North Sea 283

Table 135. Estimates of common seal numbers in appear to be reliable stock assessments for non­ Dutch waters consumption flatfish in the Waddensea. High con­ centrations of pollutants have been found in dead Year Waddensea Delta area seals recovered from the Waddensea. The concentra­ tion of polychlorinated biphenyls varied from 385 to 195 9 ...... 900 800 2530 p.p.m. in the blubber of adult seals and from 196 0 ...... 1 000 250 1961 ...... 1 000 350 47 to 660 p.p.m. in the blubber of seals less than two 196 2 ...... 1 100 355 years old. Mercury concentrations in liver in 1969 1963...... 1 300 355 and 1970 varied from 36 to 765 p.p.m .; in 1972, 196 4...... 1 400 350 26 to 1111 p.p.m .; in 1973, 13 to 922 p.p.m., and in 1965 ...... 1 400 350 1974, 155 to 1002 p.p.m. Brain mercury has varied 1966...... 1 450 325 196 7...... 1 450 300 from 0-5 to 15 p.p.m. for adult seals. Research is 196 8...... 1 500 300 continuing on whether these concentrations of pol­ 196 9...... 1 200 100 lutants may play a part in the observed fall in the 1970 ...... 900 less than 100 1971 ...... 700 less than 50 birth rate or in the decline in the condition of the 197 2...... 600 less than 50 seals. 197 3...... 500 less than 50 In Niedersachsen, which directly adjoins the Dutch 197 4 ...... 500 about 25 part of the Waddensea, a similar but less abrupt decline in seal numbers has been observed (Table 136). initiated and revealed only 800 seals in the Delta In 1963 there were estimated to be 1900 seals and in and 900 in the Waddensea in 1959. 1974 only 1250. Since 1972 the seals have been totally Common seals were totally protected in the Nether­ protected. lands in 1962, and this was followed by an increase Information on seal numbers in Schleswig-Holstein in the W addensea from 1100 in 1962 to 1500 in 1968 has not been readily available, but what there is (Table 135). There was no similar increase in the indicates a fairly stable population of about 1500 Delta where the “Delta works”, the building of dikes seals. and bridges, destroyed much of the habitat and About 500 common seals are found on the west caused great disturbance. Only about 25 common coast of Denmark. There is no evidence of a serious seals are now believed to inhabit the Delta. The decline in numbers. Hunting is permitted from Waddensea population has, despite total protection, 1 September to 31 May, and about 75 seals are taken declined since 1968 to a present total of about 500. each year. The reasons for this are not clear. There is now very The current population (1974) of the German great disturbance at the time of pupping and suckling Bight is thus about 3800; ten years ago it was about caused by yachtsmen and the “wadlopers” who walk 5500. at low tide from one sand-bank to another or even to some of the islands. Drilling operations for gas and SHETLAND oil and low-flying aircraft also cause disturbance. Shetland has a long tradition of local seal hunting, There has been a decrease in pup production, which and it was from an analysis of the hunters’ catch that is now only about half what is normally expected. Tickell (1970) first demonstrated an increase in Furthermore, parasite infestation of lungworms, Para- hunting pressure in the late 1960’s when up to 90% filaroides gymnurus, tapeworms, Diphyllobothrium spp., of the pups of the year were taken. After surveys of and lice, Echinophthirius horridus, are much more severe the stock showed that the adult population was being than ten years ago. depleted by hunting outside the breeding season a The declining birth rate and the poorer condition of the seals (which is the basic cause of the heavy parasite infestation) may be the result of under­ Table 136. Estimates of common seal numbers in nourishment or, perhaps, poisoning by pollutants. Niedersachsen With regard to food it seems that shrimps, Crangon No. crangon, have decreased very considerably since 1965, Year No. Year as the combined result of increased predation by cod, 1963...... 1 900 1969...... 1 350 too intense fishing and a long series of mild winters 1964...... 1 700 1970...... 1 400 (Boddeke, personal communication). Stocks of flound­ 1965...... 1 650 1971...... 1 300 ers, Platichthys flesus, which constitute 30% of the diet 1966...... 1 750 1972...... 1 450 1973...... 1 300 (Havinga, 1933), appear to have been stable in the 1967...... 1 650 1968 . .. 1 550 1974 1 250 last five to ten years. Unfortunately there do not 284 C. F. Summers - W. N. Bonner - J. van Haaften

complete ban was imposed on hunting the common CONCLUSIONS seal in Shetland in 1973 (Bonner et al., 1973; An­ TRENDS IN SIZE OF STOCKS derson, 1974). The order prohibiting hunting, under Information on numbers of common seals, as we the terms of the Conservation of Seals Act of 1970, have already pointed out, is necessarily much less is renewable annually and is still in force. Population reliable than that for grey seals. Even at the present estimates from the two surveys were in close agree­ time the best information available for most common ment in suggesting a population of about 2000. seal stocks is a minimum estimate of size obtained by counting directly from boats. Notwithstanding dif­ ferences in the quality of data, there is considerable ORKNEY evidence to suggest that changes in the seal stocks Bonner’s (1972) reference to the population of of the North Sea have been closely related to the level common seals in Orkney is the only published estimate of predation by man. Where seals have been protected of the total stock (less than 2000). However, the result by legislation their numbers have increased rapidly, of a survey in 1972 suggests a population size of 2000 but where protection has been withheld stocks have to 3000 (SRD unpublished report). There is not a been prevented from increasing and in some cases strong tradition of seal hunting in Orkney (the cur­ have been much reduced. Thus the increasing stocks rent quota of 200 common seal pups per year is not of grey seals in Great Britain have been viewed with always taken in full) and, although adequate quanti­ alarm by fishermen, particularly by the Scottish tative evidence is lacking, it seems likely that the salmon netsmen, while the threatened status of seals population has not declined significantly since the in Norway, and of the common seal in Great Britain early part of the present century. and the Netherlands, has been of equal concern to conservationists. The population which has shown the most dramatic EAST COAST, SCOTLAND increase in size, the grey seals of the O uter Hebrides, Rae (1960) quoted estimates of 2000 common seals has an all-age total of about 18000 individuals, over in the Moray Firth and a further 2000 (including half the size of the entire stock of grey seals in the some grey seals) in the Firth of Tay, but Bonner (1972) North Sea proper. Licensed pup hunting, as carried estimated 500 to 1000 for the Firths of Tay and Forth out in Orkney for the past decade, would not appear together. From later survey data it was estimated that to be a very effective means oflim iting such population there were about 550 in the Moray Firth and 350 to increases. 500 in the Tay (SRD unpublished report). There is At the other extreme, protective legislation has not insufficient evidence to suggest that there has been a halted the decline of common seals in the Nether­ real decline in the population during the past fifteen lands where habitat destruction, disturbance by years or so. It would appear unlikely since the species human activities, and possibly contamination of the is not exploited by commercial hunters in this area, environment have prevented recovery of stocks. Cur­ although some common seals are shot to protect local rent legislation in Great Britain and Norway should, salmon fisheries. however, prevent further reduction of depleted stocks of common seals in those countries.

NORWAY PREDATOR/PREY RELATIONSHIPS The common seal breeds along the entire length of Seals feed principally (and almost exclusively) on the Norwegian coast. Throughout this area it has repu­ fish, and thus a substantial stock of seals must have a tedly become much depleted in numbers, particularly significant effect on the stocks of fish in the feeding during the postwar era. Øynes (1964, 1966) recorded area. It is known that both grey and common seals a total population of about 4000, of which some eat a wide variety of fish (Rae, 1968). In both species occurred in southern Norway. He claimed that the gadoids are the most important group, and there is stock would be hunted to extinction unless protected, some evidence that common seals eat more pleuro- pointing out that even in the areas of Møre and nectids and clupeoids than do grey seals. Feeding Romsdal, where the species was most numerous, the ranges are not known for either species, but seals are common seal has been eliminated from some fjords. rarely seen far from land and it is generally accepted All seals were given total protection in southern that they are mainly coastal feeders. The amount of Norway in 1973 and a closed season in northern food eaten by seals in the wild is not precisely known. Norway from May to November inclusive (Øritsland, Keyes (1968) found that for a variety of species in personal communication). captivity the amount of food given per day was Changes in the seal populations of the North Sea 285 between 6 and 10% of the seals’ body weight. This Boyd, J. M., Lockie, J. D. & Hewer, H. R. 1962. The breeding would correspond to about 5 to 8-5 kg of food per day colony of grey seals at North Rona. Proc. zool. Soc. Lond., 138: 257-277. for common seals and 7-5 to 12-5 kg for grey seals. Coulson, J. C. & Hickling, G. 1964. The breeding biology of the Havinga (1933) calculated that common seals re­ grey seal, Halichoerus grypus (Fab), on the Farne Islands, quired 5 kg of food per day, and Steven (1934) esti­ Northumberland. J. Anim. Ecol., 33: 485-512. mated that grey seals in Cornwall ate at least 6-8 kg Evans, A. H. & Buckley, T. E. 1899. A vertebrate fauna of the Shetland Is. Edinburgh, 248 pp. of fish per day. However, Ronald et al. (1975) have Havinga, B. 1933. Der Seehund (Phoca vitulina L.) in den Hol­ maintained adult harp seals on a daily intake of only ländischen Gewässern. Tijdschr. ned. dierk. Ver. (Leiden), 3- 3 to 4% of body weight. None of these estimates is 79-111. very certain nor is it known whether seals feed every Hentschel, E. 1937. Naturgeschichte der nordatlantischen Wale und Robben. In Handb. Seefischerei Nordeuropas, 3, (1): day. The number of empty stomachs and guts found 1-54. in specimens shot indicates that phases of feeding and Hewer, H. R. 1963. Provisional grey seal life table. In Grey seals fasting alternate, and grey seals at least do not feed and fisheries, pp. 27—28. HMSO, London. during the breeding season. If it is assumed that seals Hewer, H. R. 1964. The determination of age, sexual maturity, longevity and a life table in the grey seal (Halichoerus grypus). feed 300 days per year and that common seals require Proc. zool. Soc. Lond., 142: 593-624. 7 kg and grey seals 10 kg of food per day, it will be Hickling, G., Rasmussen, B. & Smith, E. A. 1962. Innvandring seen that the stocks of seals in the North Sea remove fra Storbritannia til Norge av havert (Halichoerus grypus). between 120 million and 133 million kg of fish per Fiskets Gang, 13: 1-7. Keyes, M. C. 1968. The nutrition of Pinnipeds. In The Behaviour year. Present knowledge is insufficient to say how and Physiology of Pinnipeds, pp. 359-395. Ed. by J. Harrison this might affect fish stocks or fisheries, though locally et al. Appleton - Century - Crofts, New York. the effect on fisheries, particularly those fisheries using Lockley, R. M. 1966. Grey seal, common seal. Andre Deutsch, set nets, can be severe. London, 175 pp. Øynes, P. 1964. Sei på norskekysten fra Finnmark til Møre. Fiskets Gang, 48: 694-707. Øynes, P. 1966. Sei i Sor-Norge. Fiskets Gang, 45: 834-839. REFERENCES Rae, B. 1960. Seals and Scottish fisheries. HMSO, Edinburgh, Anderson, S. S. 1974. Seals in Shetland. In The Natural Environ­ 39 pp. ment of Shetland, pp. 114-118. Ed. by R. Goodier. Edinburgh. Rae, B. B. 1968. The food of seals in Scottish waters. M ar. Res., Amlaugsson, T. 1973. Selur viö Island. Rannsoknastofnun 2: 23 pp. fiskiönaSarins, 25 pp. (mimeo). Ronald, K., Uthe, J. F. & Freeman, H. 1975. Effects of methyl Bemmel, A. C. V. van. 1956. Planning a census of the harbour mercury on the harp seal. ICES CM 1975/N: 9, 15 pp. (mimeo). seal (Phoca vitulina, L.) on the coasts of the Netherlands. Sergeant, D. E. 1951. The status of the common seal (Phoca Beaufortia, 54: 121-132. vitulina) on the East Anglian coast. J. mar. biol. Ass. U.K., 29: Bigg, M. A. 1969. The harbor seal in British Columbia. Bull. Fish. 707-717. Res. Bd Can., 172: 1-33. Smith, E. A. 1963. The population of grey seals. In Grey seals Bonner, W. N. 1972. The grey seal and common seal in European and Fisheries, pp. 15-17. HMSO, London. waters. Oceanogr. Mar. Biol. Ann. Rev., 10: 461-507. Smith, E. A. 1966. A review of the world’s grey seal population. Bonner, W. N. 1975. Population increase of grey seals at the J. Zool. Lond., 150: 463-489. Farne Islands. Rapp. P.-v. Réun. Cons. int. Explor. Mer, Southwell, T. 1881. Seals and whales of the British Seas, pp. 2-43. 169: 366-370. London. Bonner, W. N. & Hickling, G. 1971a. The grey seal of the Farne Steven, G. A. 1934. A short investigation into the habits, abun­ Islands. Report for the period October 1969 to July 1971. dance, and species of seals on the north Cornwall coast. J. mar. Trans, nat. Hist. Soc. Northumb., 17: 141-162. biol. Ass. U .K ., N.S., 19: 489-501. Bonner, W. N. & Hickling, G. 1971b. Grey seals at the Fame Summers, C. F., Burton, R. W. & Anderson, S. S. 1975. Grey seal Islands : a management plan. Privately circulated report, 21 pp. (Halichoerus grypus) pup production at North Rona: A study Bonner, W. N. & Hickling, G. 1974. The grey seals of the Farne of birth and survival statistics collected in 1972. J. Zool. Lond., Islands 1971 to 1973. Trans, nat. Hist. Soc. Northumb., 42: 175: 439-451. 65-84. Summers, C. F. & Mountford, M. D. 1975. Counting the common Bonner, W. N., Vaughan, R. W. & Johnston, L. 1973. The status seal. Nature, 253: 670-671. of common seals in Shetland. Bio. Con., 5: 185—190. Summers, C. F., V aughan, R. W. & Witthames, S. R. 1975. Bonner, W. N. & Witthames, S. R. 1974. Dispersal of common Increasing numbers of grey seals in the Hebrides. ICES CM seals (Phoca vitulina), tagged in the Wash, East Anglia. J. Zool., 1975/N: 17, 5 pp. (mimeo). Lond., 174: 528-531. Tickell, W. L. N. 1970. The exploitation and status of the common Boyd, J. M. 1963. The grey seal (Halichoerus grypus Fab.) in the seal (Phoca vitulina) in Shetland. Bio. Con., 2: 179-184. Outer Hebrides in October 1961. Proc. zool. Soc. Lond., 141: Vaughan, R. W. 1969. Grey seal numbers in Orkney. M am. Soc. 635-662. Bull., 32: 11-15. Boyd, J. M. & Campbell, R. N. 1971. The grey seal (Halichoerus Vaughan, R. W. 1971a. Aerial photography in seals research. grypus) at North Rona, 1959-1968. J. Zool. Lond., 164: 469-512. In The application of aerial photography to the work of the Boyd, J. M. & Laws, R. M. 1962. Observations on the grey seal Nature Conservancy, pp. 88-98. Ed. by R. Goodier, Edinburgh. (.Halichoerus grypus) at N orth Rona in 1960. Proc. zool. Soc. Vaughan, R. W. 1971b. Aerial survey of seals in the Wash. Lond., 139: 249-260. NERC Occ. Publ., 11 pp.