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Feeding
The grey seal feeds on local inshore fish species, cephalopods and crustaceans. Bonner (1982) estimated a daily consumption of 7½-12½ kg. Prime and Hammond (1988) investigated the diet of the grey seal population of Donna Nook (UK). They found the diet to consist predominantly of flounder, sole, sand eel, cod and whiting. They estimated the total annual consumption of that population at 863,500 kg.
Population dynamics and life history
Females become mature at ages 4 to 7 and males at ages over 10 years. Gestation lasts 11½ months including a delay of implantation of 3 months. The pregnancy rate is assumed to be between 80 and 90%. The longevity for females is 46 years and for males 26 years. Adult female mortality ranges from 6 to 13½%. First year mortality is, depending on the location, 34-60%.
Trophic relations
There may be some competition for food with the harbor seal, Phoca vitulina. The killer whale can be a predator in some areas.
Interactions
Many of the fish species in the grey seals' diet are commercially exploited, so there is competition for resources with the fisheries. Grey seals have been seen to raid the nets of fishing boats and completely empty them. They also do damage to set nets. Consequently some seals drown in the nets or are shot by the fishermen. Another concern relating to the fisheries is that the grey seal acts as a vector of the cod worm, Phocanema. Since the life cycle of the cod worm is a complex one, the importance of the grey seal therein is hard to determine.
Population size
Several censuses have been carried out of grey seal populations, using land, boat and aerial counts. The West Atlantic stock is estimated at 30,000 animals, of which 17,900 are located in the Gulf of St. Lawrence and 9,400 at Sable Island. For the East Atlantic stocks the following estimates have been derived for some subgroups:
Iceland: 3,000 (Bonner (1981) mentions a population size of 8,000-10,000 for Iceland) Faroes: 3,000 Outer Hebrides: 35,000 (Bonner (1982): 27,000) Orkney: 14,000 United Kingdom (including Hebrides and Orkney): 69,000 Ireland: 2,000 Norway: 2,000-3,000 USSR: 1,000-2,000.
The total for the East Atlantic stock is somewhere between 85,000 and 89,000. Bonner (1979) estimated the Baltic stock to consist of 5,000 animals. For the early 1980's Helle (1983) estimated the population at only 1,000-1,500. Halkka (1987) and Keränen and Soikkeli (1989) mention a populations size of the Baltic stock of 1,500 to 2,000 animals. Helle (pers. comm.) estimates the 1988 Baltic stock at about 2,000 seals. The 1995 Baltic population was estimated to consist of 5,300 seals (Halkka, pers. comm.) The world population of the grey seal is probably between 120,000 and 124,000. The West and East Atlantic stocks are increasing. The Baltic stock has shown a marked decrease, but seems to be recovering again. Exploitation
The grey seal populations in Scotland and Canada are regulated because of the codworm problem. In the United Kingdom on average 1,000-1,500 pups have been taken annually. A management plan has been enacted on the Outer Hebrides and Orkney.