Seals feed those species of fish which happen to be the most abundant and the easiest to catch at any time. The grey seal's food consists solely of fish, but the diet of the ringed seal includes also crustaceans in addition to fish.
Seals' nutritional requirements vary by seasons. They eat the least during the moult around the springtime and the most towards the end of the summer and in the autumn. An adult grey seal eats on average 4.5–7.5 kg of fish each day and a ringed seal 2.5-3.5 kg. Seals, when obtaining food from fish traps, cause damage by destroying parts or all of the catch and gear.
It has been found that, in the Baltic Sea, grey seals feed on over 20 different fish species and ringed seals on at least 12 different species. The typical prey fish for the ringed seal is small-sized schooling fish and bottom fish (approximately 10 cm in length on average). The grey seal's prey fish is bigger in size. The seal diet varies by areas. Baltic herring is the most important source of nutrients in the Baltic Sea for both the grey seal and the ringed seal. Other important species in this respect are sprat, whitefish, vendace and other salmon type fish. A specially important prey fish for the ringed seal is the three-spined stickleback.
Over 400 three-spined sticklebacks were found in the
alimentary tract of a certain ringed seal.
Photo: Mia Valtonen
Baltic seals – balancing between sustainable ecosystem management and fisheries (ECOSEAL)
Hylkeen saalistuksen vaikutukset kalakantoihin ja erityisesti lohikantoihin (in Finnish)