Ecological implications of harp seal Phoca groenlandica invasions in northern Norway

9 1995 Elsevier Science B.V. All rights reserved Whales, seals, fish and man A.S. Blix, L. WallCeand 12l.Ulltang, editors

545

Ecological implications of harp seal Phoca groenlandica invasions in northern Norway Tore Haug and Kjell Tormod Nilssen Norwegian Institute of Fisheries and Aquaculture, Tromsr Norway. Abstract. In the years since 1978, Barents Sea harp seals Phoca groenlandica have appeared in large numbers in Finnmark, North Norway, in February-May. The size of the "seal invasions" increased dramatically in 1987 and 1988 when large seal herds were observed along the coast of North Norway in January-August. The seal invasions gave rise to seal-fisheries conflicts. In addition to consuming fish (capelin, cod, saithe and haddock), the seals caused substantial damage to gill-nets and gill-net catches. The presence of seals may also have resulted in the emigration of commercial species from traditional fishing grounds to deeper strata or areas unsuitable for fishing. Reduced recruitment to the seal population seems to have prevailed during most of the seal invasion period with particularly dramatic effects in 1986-1988, when first-year-mortality may have been almost total. Food shortage, particularly the two important prey species capelin and herring, is discussed as a possible factor contributing to the seal invasions.

Key words: seal/prey, seal/fisheries, recruitment, food shortage

Introduction

Two populations of harp seals Phoca groenlandica inhabit the northeast Atlantic Ocean. These populations whelp off the east coast of Greenland (the Greenland Sea population) and in the White Sea (the Barents Sea population), respectively [1 ]. The annual migration pattern of the Barents Sea population of harp seals is usually characterized by a north-bound feeding migration in spring and early summer (MayJune) and a south-bound breeding migration during winter [2]. In summer and autumn the seals are found in open waters and along the pack-ice in the northern parts of the Barents Sea, and they move southwards in November. In winter and early spring (December-May), the seals are usually concentrated at the southern edge of the range, primarily in the southeastern parts of the Barents Sea and in the White Sea where breeding and moult occur [2,3]. From 1978 onwards, harp seals started to appear in large numbers along the coast of Finnmark, North Norway, in winter and spring. Although recaptures of tagged seals indicated that some of the immature seals were from the Greenland Sea population [4], it seems reasonable to assume that the majority of the seals arose from the Barents Sea population. In the current paper we attempt to discuss the following questions" What were the consequences of the changes in the migratory patterns of

Address for correspondence: T. Haug, Norwegian Institute of Fisheries and Aquaculture, P.O. Box 2511, N-9002 Tromsr

Norway.

546 the seals - for the prey species of the seals, for fisheries, and for the seal population itself?

The Seal Invasions A new, and apparently aberrant, migratory pattern of the Barents Sea harp seal population persisted throughout the 1980s, and there were dramatic increases in numbers of animals observed along the Norwegian coast in 1987 and 1988 [5]. The losses imposed by the invading seals on coastal fisheries in northern Norway [6] led the Norwegian authorities to introduce compensatory bounty payments for seals taken as by-catch in gill nets. The numbers of harp seals caught increased from 500 to 2,000 animals during the first half of the 1980s to more than 56,000 in 1987 (Fig. 1). Numbers caught were lower in 1988, and from 1989 onwards the numbers seem to have returned to the level of the early 1980s. The numbers of seals returned for compensation purposes are, however, lower than the total numbers drowned, which may have been at least 10,000 per year throughout the early 1980s [4,7] and perhaps as many as 100,000 in 1987 [8]. It should also be stated that the number of seals recorded for compensation purposes cannot be used uncritically as an index to estimate the size of the invasion because of the large geographic and seasonal 60 000

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547 variation in fishing effort. For example, the effort in inshore fisheries was reduced from 1987 to 1988, at least in part because of the presence of the seals. Thus, there were fewer nets in the sea in 1988 than previously [4]. In the absence of a direct census of seals along the coast of Norway, the actual size of the invading population remains unknown. Wiig suggested [8] that a by-catch mortality of 25% might be a reasonable estimate, and this would indicate that 300,000-400,000 seals would have been found along the coast of Norway in 1987. The Finnmark invasions in the late 1970s and early 1980s were confined to the period February-May, with pregnant females and immature animals being the first to arrive at the coast [7]. Females left the coast in early March but reappeared in April, whereas males were present from mid-March onwards. The 1987 and 1988 invasions, which occurred over a longer period (January-August) and comprised a mixture of immature and adult seals, were not confined to the coast of Finnmark but occurred also further south along the coast of Norway (Fig. 2). Thus, substantial num-

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548

bers of seals were taken as by-catches in the gill-net fisheries along the coast of the counties of Troms and Nordland [5], and some seals were taken as far south as Skagerak [8]. In recent years the invasions have again been confined mainly to Finnmark, in particular the Varangerfjord area, although it is known that some immature seals have been seen as far south as the Lofoten and Vester~len areas. The harp seal invasions to Finnmark in the 1990s seem to be related to feeding migrations performed by adult females in March-April, i.e. the period between lactation and moult [2,9].

Consequences for Prey Stocks During 1978-1981, the harp seals occurring in Finnmark appeared to feed largely on spawning capelin Mallotus villosus and demersal capelin eggs [7]. Similar observations were made during the winter of 1984, when cod Gadus morhua was also eaten

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549 Stomach contents of harp seals taken in Norwegian coastal areas in 1986-1988 have been analysed (Fig. 3). Capelin were poorly represented in the stomach contents, and the diet of the harp seals comprised 24 prey species. The stomach contents consisted mostly of fish, but prawns and squid were also present [5]. Among the fish species, herring Clupea harengus dominated the diet of seals taken on the MOre coast, while gadoids (in particular cod, saithe Pollachius virens, haddock Melanogrammus aeglefinus, and Norway pout Trisopterus esmarckii) was most numerous in Lofoten (Nordland) and Troms. Gadoids (particularly cod and saithe) and herring occurred frequently in the stomachs of seals captured along the coast of Finnmark. Similar data are presented by Ugland et al. [ 10]. The extended stay of large numbers of harp seals in Norwegian coastal waters in 1987 and 1988 is expected to have had some impact on the stocks of prey species, but in the absence of adequate estimates of the numbers of seals involved, it is impossible to give an accurate assessment of this impact. Some attempt at quantitative evaluation was, however, made by Ugland et al. [10]. Based upon the assumption that a stock of 400,000 harp seals stayed in Norwegian coastal waters for 135 days during the first half of 1987, they calculated that a total of 215,000 _+50,000 tonnes of prey biomass was removed from Norwegian coastal waters by the seals. Despite the uncertainty of their estimates, Ugland et al. suggested [10] that predation from the invading seal herds may have contributed significantly to the sudden declines in, and subsequent failure of recruitment to the fisheries of the 1985 year-class of cod and both the 1985 and 1986 year classes of saithe. The Barents Sea capelin spawning grounds stretch from the White Sea to the coast of northern Norway. Spawning occurs during March-April when harp seals are present along the coast of Finnmark and Kola [11]. The collapse of the Barents Sea capelin stock in 1985/1986 [12] and subsequent very low population level until 1990 [13] probably resulted in a reduced abundance of capelin in coastal waters off North Norway and Russia. The observed reduction in importance of capelin as prey for the invading harp seals in 1986-1988 probably reflects this decline. This view is supported to some extent by observations made on the stomach contents of seals caught in Finnmark during the winter of 1991. Capelin made up approximately 50% of the prey biomass recovered from the stomachs. Stomach contents analysis performed in 1992 showed that capelin constituted nearly 99% of the seal diet [9]. The recovery of the Barents Sea capelin stock in 1991 and 1992 [13] probably contributed to this reappearance of the species as the most important food item for the harp seals during winter in North Norway.

Consequences for Fisheries From 1978 onwards, when harp seals started to follow capelin into Norwegian coastal waters, seal-fisheries conflicts arose, because the spawning migration of the capelin is accompanied by a feeding migration of fishable cod stocks. Fisheries after cod are conducted with gill-nets, hand jigs, long-lines and Danish seines in Nor-

550 wegian coastal waters. The first and most obvious sign of a seal invasion was entanglement and drowning of large numbers of seals in gill-nets. This was originally largely confined to eastern Finnmark, in particular the Varangerfjord area [7], but in 1987 and 1988 many seals were caught in gill-nets in other areas along the coast of northern Norway (Fig. 2). These latter invasions were also of longer duration than previously, something which caused conflicts with the traditional fisheries directed towards cod migrating from the Barents Sea to the coast of Norway to spawn. The harp seals are thought to have given rise to substantial losses in the coastal fisheries of northern Norway, and the Norwegian authorities introduced compensatory payments of NOK 300--400 per landed seal in 1981 [5,7]. The harp seals caused severe damage to both the gill-nets via entanglement and to the catches by tearing and eating pieces (usually the ventral soft parts) of large fish caught in the nets [6]. Another effect, that may also have affected fisheries using other gear types, was an apparent change in the behaviour and availability of several commercial fish species: fishermen claimed that when seals were present, the commercial fish species disappeared from the traditional fishing grounds. Studies performed in a North Norwegian coastal fjord (Ullsfjord) tended to support this claim, since when the harp seals were present in shallow and/or pelagic waters fish such as large cod and haddock were only to be found in deeper water, or in areas unsuitable for fishing using traditional gears [6].

Consequences for the Seal Population Although there are many uncertainties about the accuracy of population censuses made after the mid-1980s [14-16], there seems to be little doubt that the seal invasions, particularly those in 1987 and 1988 in which several thousands of seals were captured in gill-nets, have influenced the status of the Barents Sea population. Traditionally, this population has been harvested by Soviet/Russian and Norwegian sealers in the pack-ice areas of the White Sea and the southeastern Barents Sea [ 1720]. The population was heavily exploited after World War II and was probably reduced from 1.25-1.5 million individuals in the early 1950s to less than 500,000 by the mid- 1960s [21 ]. Quotas for Soviet catches were introduced in 1955 (100,000 seals) and quotas were gradually reduced thereafter. In 1965 a quota of 34,000 seals was imposed upon the total catch. From 1963 onwards, adult females were protected in whelping areas, and Soviet catches of 1-year-old and older seals ceased in 1965. In 1977 the total catch quota was increased to 50,000 seals [20]. It was estimated that the population had increased to ca. 800,000 animals by 1978, and that the annual production of pups was approximately 170,000, leading to a rate of population increase of about 5% per year [20]. Soviet aerial surveys conducted in 1980 confirmed that pup production was similar to the 1978 estimates [14]. It was thought that the population would continue to increase if subjected to an annual removal of 50,000 animals. Catch quotas were set at 60,000 in 1981, 75,000 in 1982

551 and 82,000 in 1983, and followed by a decrease to 80,000 animals which was maintained in the period 1984-1987. Age composition data collected by Russian sealers, and surveys carried out during the second half of the 1980s, indicated a reduction in recruitment to the seal population after 1985 [14,15]. Age composition data from Norwegian catches support the Russian observations, and suggest that there may have been poor recruitment to the population in the years since 1981 [22]. The 1986-1988 cohorts are particularly weak, and recruitment of 1-year-old seals did not improve significantly until 1992 (Fig. 4). The complete recruitment failure in 1986-1988 occurred concurrently with the large harp seal invasions to Norwegian coastal waters. It is possible that large catches of harp seals in Norwegian coastal gill-net fisheries [5] may have been a contributory factor leading to the poor recruitment to the population. Catch quotas were reduced to 70,000 seals in 1988, and then to 40,000 seals in 1989, and this quota has been maintained up to the present [ 15]. Aerial surveys, conducted in 1991, provided an estimate of approximately 140,000 females breeding in the White Sea that year [15]. This suggests that recruitment to the stock in the early 1990s was lower than that in the late 1970s, when it was assumed that the management regime imposed would allow the population to increase at a rate of 5% per year [20]. Thus,

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552 the present population size appears to be considerably smaller than estimated in the late 1970s prognosis.

Why Did the Seal Herds Move Westwards? Changes in harp seal migrations that have resulted in invasions of seals to coastal areas of northern Norway have been recorded on several previous occasions, e.g. at the beginning of this century (1901-1903) and during World War I (1916-1919). The reason for the invasions is by no means fully understood, but it has been pointed out that the invasions have coincided with periods of low temperatures and salinities, extensive ice cover and a westerly distribution of producers, grazers and predators in the Barents Sea [4,8]. The years 1902 and 1903 were both particularly cold and there was extensive ice-cover in the Barents Sea. It is also probable that the numbers, age composition, and geographical and temporal distributions of the invading seals in these years were comparable to the 1986-1988 situation [8]. In contrast to previous harp seal invasions, which ceased after a few years, the recent invasions have persisted throughout the period 1978-1994, but with variable intensity (Fig. 1). The climatic conditions in the Barents Sea have been quite variable in the period from 1978 until the present [23]. The period 1977-1982 was very cold, but from 1982 onwards there have been both warm (1983-1984 and from 1989 until present) and cold (1985-1988) periods. Thus, both the long duration of the recent invasions (1978-1994), and the large size of the 1987 and 1988 invasions do not seem to completely comply with the cold-climate hypothesis. Changes have occurred in the Barents Sea population of harp seals, with possible population growth up to the early 1980s [20,22], and there have also been substantial changes in the marine ecosystem of the Barents Sea during the course of the past

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30 years. The most conspicuous changes relate to the rises and falls in the stocks of two pelagic shoaling fish species (Figs. 5 and 6). The Norwegian spring spawning herring stock collapsed in the late 1960s but is now recovering, whereas the Barents Sea capelin stock collapsed in the mid-1980s, recovered to some extent in the early 1990s but is once more at a very low level [13]. Both species are known to be consumed by Barents Sea harp seals [5,9,24,25], and the collapses of the stocks of these important prey organisms combined with increasing numbers of seals within the population have been proposed as being important factors underlying the seal invasions. This may be the case for the extensive invasions of 1987 and 1988 which followed the 1985/1986 capelin stock crash. Observations of effects (decreased growth rate, increased age at maturity and reduced female fecundity) that could indicate density-dependent responses within the seal population [22] may support a hypothesis that food shortage has been a factor contributing to the seal invasions. Both the diet and the intensity of feeding of the Barents Sea harp seals vary seasonally. During early autumn, when the seals feed intensively in the northern parts of their range, the diet comprises mainly crustaceans (particularly pelagic amphipods). As the seals move southwards in late autumn and early winter, the diet changes to fish, particularly capelin and polar cod Boreogadus saida [25], and in the southeastern parts of the Barents Sea, herring appears to be an important food during the winter [3,17,24]. During breeding and moult (March-June), the feeding intensity of most adult seals is substantially reduced, but mature female seals have been observed to feed on capelin for a short period (late March-early April) following lactation [9]. The seals build up energy stores during the period of intense feeding and these stores sustain them over the period of little feeding. This is illustrated by an examination of variations in blubber thickness and condition: the seals are very thin during spring and early summer (May-June), fatten in late summer and autumn (AugustSeptember), maintain this level of fatness until February, and then become thinner in the period March-June as the stores of blubber decrease rapidly during lactation and

554 moult [26]. In 1987 and 1988 the invading harp seals, particularly the subadults, were said to be thin and in very poor condition [4,8]. Adult seals taken in gill-nets in North Norway in February 1988 were also in poor condition, being significantly thinner than animals taken in the East Ice in February 1993 [26]. The observation of seals in poor condition along the Norwegian coast in 1987 and 1988 may indicate that the food resources available to the seals were limited. Given the facts that the late autumnal diet of the seals usually contains considerable quantities of capelin and that the capelin stock crashed in the mid-1980s, it seems likely that the seals may have faced a food shortage in late autumn or early winter at the time of the dietary shift from crustaceans to fish [25]. In 1989, the spawning of the Barents Sea capelin was successful, and there was some recovery of the stock [ 13], but there was a further decline in stock size between 1992 and 1993 (Fig. 5). If there is a link between the abundance of capelin and the migration patterns of the harp seals, the low abundance of capelin in 1993 might have been expected to result in an influx of large numbers of harp seals to waters along the Norwegian coast. However, the numbers of seals observed in coastal waters during 1993 and 1994 were considerably lower than in the years 1986-1988, and influxes were generally restricted to eastern Finnmark (Gunnar Henriksen, Office of the County Governor of Finnmark, Vadsr Norway; personal communication). The reduced size of the seal invasions in the years 1993 and 1994, may be related to a relatively low population size (following the poor recruitment in the 1980s [22]), and the presence of alternative prey species in the southeastern Barents Sea. The polar cod is a key prey species in aquatic ecosystems in the Arctic, including the Barents Sea [27,28]. The stock size of Barents Sea polar cod is not known, but results of annual acoustic surveys conducted since 1986 suggest that the stock is depleted [29]. Although harp seals are known to feed on polar cod during the winter [3,24,25], this species is an unlikely candidate as the alternative winter diet for the harp seal population. On the other hand harp seals have recently been observed to prey upon immature herring which are now very abundant in the southern Barents Sea [24]. The stock of Norwegian spring spawning herring has increased substantially in recent years, and since 1988, when the major part of the strong 1983 year class spawned for the first time (Fig. 6), the southern Barents Sea has served as the main nursery area for the immature fish (0-group and recruits up to 3-4 years old) [30]. Immature herring are now probably the most important winter prey of harp seals in the southern Barents Sea [24], and this may in part help to explain why the seal invasions have been of reduced proportions in recent years despite the continued low size of the capelin stock. Based upon current knowledge, the following scenario can be proposed: a series of cold years in the Barents Sea initially led to a more westerly winter distribution of an increasing population of harp seals, with an increase in occurrence along the coast of northeastern Norway. Food shortage, possibly resulting from the 1985/1986 collapse in the capelin stock, may have exacerbated the problem by forcing large numbers of harp seals to leave their traditional wintering areas in the southeastern Barents Sea in favour of the coast of North Norway in 1987 and 1988. Increased mor-

555 tality, particularly of young animals, appears to have prevailed, leading to reduced recruitment to the harp seal population. Substantial increases in the abundance of immature Norwegian spring spawning herring in the southeastern Barents Sea may have resulted in the establishment of a suitable alternative winter food resource for the harp seals, thereby contributing to the reductions in the size of the seal invasions observed since 1988.

Acknowledgements Studies on harp seal ecology have been funded by the Norwegian Council of Research, project no. 4001-2800.083. Drs. Malcolm Jobling and Nils Oien are thanked for commenting on the manuscript.

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