The gray seal is a highly successful marine predator of the Northern Hemisphere. This precocious and inquisitive marine mammal has had a changing relationship with humans, being exploited for its fur and blubber in the early last centuiy but now being protected under national and international legislation. It continues to be a controversial species in relation to its interactions with fisheries where it often comes into conflict with humans for resources. However, it has sustained the vagaries of this relationship, and its population is increasing in almost eveiy area it is found.
I. Diagnostic Characteristics
The gray seal is the only member of the genus Halichoerus. Its species name, grypus, means hook nosed, referring to the Roman nose profile of the adult male. Halichoerus means sea pig in Greek. This species exhibits sexual dimorphism with the mature males weighing between 170 and 310 kg and adult females between 100 and 190 kg. Individuals from the population in the western Atlantic are significantly larger than those from the eastern Atlantic; males can weigh over 400 kg and females over 250 kg. Genetic studies suggest that the western and eastern Atlantic populations are distinct and diverged approximately 1 million years ago (Boskovic et al, 1996).
II. Selected Morphological and Physiological Data
Morphological differences between the sexes can be seen in Fig. 1. The neck and chest of the male are wrinkled and often scarred, whereas females are much sleeker. Both have the convex nose and wide muzzle, which are very pronounced in the male. Many of the females are gray in color with a distinctive cream/off-white background and markings, particularly around the neck, with generally a dark back and light underside. Males are more uniformly dark when mature, but subadults can have similar cream-colored patches on the neck and the side of the face. Females mature at between 3 and 5 years old and males around 6 years, although they are probably not socially mature until 8 years old.
Figure 1 Adult male (top) and female (bottom) gray seals. The female has distinct markings on the fur, whereas the male is more uniform in color and larger than the female.
III. Distribution and Geographic Variation
Figure 2 shows the geographic range of the gray seal. Breeding rookeries are on remote uninhabited islands or on fast ice. The single biggest island-breeding colony is on Sable Island (85,000, increasing at almost 12% per annum). Other major sites in the western Atlantic are in the Gulf of St. Lawrence (69,000). The northeast Atlantic population in Iceland was estimated to be 11,600 in 1987 with approximately 3000 in Norway. 2000 in Ireland, and between 1000 and 2000 in the White Sea. The Baltic population is estimated at approximately 5000 animals. The British population is surveyed annually and is currently approximately 110,000 animals, increasing at about 6% per annum.
IV. Life History and Ecology
The females give birth, on land or on ice, to a single white-coated pup between September and March. The earliest breeding colonies are those in the south of the United Kingdom and Ireland. Further north around the British Isles the breeding season is later, between October and November. In Canada, peak pupping is not until January and in the Baltic it occurs in late February-early March.
At birth the pup weighs between 11 and 20 kg and, over the lactation period, lasting an average of 18 days, can quadruple in weight to over 40 kg. The mothers’ milk is very fat-rich (around 50-60% lipid) and is mobilized from her blubber stores. The pup’s white coat, known as the lanugo, is shed at weaning. The pup then undergoes a postweaning fast on land for a period lasting between 10 and 28 days, dining which it loses approximately 0.5 kg per day. The reason for this fasting period is not fully understood, but physiological changes during this time suggest that it is related to the development of diving ability.
Toward the end of lactation the female comes into estrus and mates. On some colonies there may be as many as 10 females to 1 male, whereas on rookeries, where access is not restricted by narrow gullies, the sex ratio may be 2 females to 1 male. Males compete for access to females but do not defend discrete territories, and matings may occur in the water as females return to the sea, as well as on land. Females fast during the breeding season and may lose up to 40% of their initial body weight during the breeding season, as they do not feed during this time. The gestation period is 8 months, and to achieve a 12-month breeding cycle the fertilized egg is not implanted until 4 months after conception. This occurs around the time of the annual molt when animals spend longer time hauled out on land. Gray seals generally return to their natal site to breed and show a high degree of site fidelity, often returning to within meters of their previous pupping sites (Pomeroy et al, 1994).
Gray seals feed on a variety of fish species and cephalopods (Hammond et al, 1994a,b). However, a large proportion of their diet is sand eels or sand lance (Ammodi/tidae). which can make up over 70% of the diet at some locations and in some seasons. Other prey include whiting, cod, haddock, saithe, and flatfish (plaice and flounder). Thev are largely demersal or benthic feeders, and foraging trips lasting between 1 and 5 days away from a haul-out site are frequently focused on discrete areas that are within 40 km of a haul-out site (McConnell et al, 1999). On average. gray seal dives are generally short, lasting between 4 and 10 min with a maximum recorded duration of about 30 min. Typically, in the United Kingdom, animals dive down to the sea bed, which is largely 60 m in depth, falling to 200 m in some areas, although dives at depths >300 m have been recorded.
Figure 2 Map showing the geographic distribution of the gray seal.