. Biology of the seas of the Marine biology -- Soviet Union; Hydrology -- Soviet Union. 218 BIOLOGY OF THE SEAS OF THE Cladocera, Chaetognatha, Euphausiacea, Mysidacea, Amphipoda and the eggs and larvae of different invertebrates and fish. Herring feeds also on fry, mainly its own. Its food varies greatly with the season (Fig. 92). The White Sea herring fattens up in May: during the rest of the year its feeding is not intensive. From September onwards herring practically stops eating, and the percentage of empty stomachs in October may reach 40. In the spring its main food is
. Biology of the seas of the Marine biology -- Soviet Union; Hydrology -- Soviet Union. 218 BIOLOGY OF THE SEAS OF THE Cladocera, Chaetognatha, Euphausiacea, Mysidacea, Amphipoda and the eggs and larvae of different invertebrates and fish. Herring feeds also on fry, mainly its own. Its food varies greatly with the season (Fig. 92). The White Sea herring fattens up in May: during the rest of the year its feeding is not intensive. From September onwards herring practically stops eating, and the percentage of empty stomachs in October may reach 40. In the spring its main food is Calanus finmarchicus, in summer and autumn other Copepoda. Spawning herring do not stop eating, but eat less. The rapacity of herring is demonstrated not only by the fact that it prefers to devour great numbers of PORVYA INLET MAY 304 PORVYA INLET JUNE 216 NISHCHEVA INLET JULY 99. Fig. 92. Food ranges of the White Sea herring in different months (Chayanova, 1939). Repletion indices denoted by associated numerals. 1 Calanus finmarchicus; 2 Small Crustacea; 3 Euphau- siaceae; 4 Mysidacea; 5 Amphipoda; 6 Sagitta; 7 Pteropoda; 8 Fish larvae. its own young, but also by its obvious preference for the larger forms of plankton. The young navaga, like herring, feeds mostly on different small plankton crustaceans. The food of the adult navaga is also greatly varied; however, contrary to that of the herring, its main food consists of benthos, chiefly worms and crustaceans (up to 70 per cent of its food). One-fifth of navaga's food consists of fish-smelt, caplin, launce, Boreogadus saida and others, including navaga itself. Navaga grows more rapacious with age, often swallowing a prey almost as big as itself. While spawning in January, navaga eats very little; once its spawning is over it once more falls greedily upon its food. Seals and porpoises are the navaga's chief enemies. Pollack or Polar cod also form an essential link in the food-chain of the White Sea. The Arctic seas conceal an
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