Long-term variations in the relationships between the main components of the Barents Sea ecosystem (cod and capelin) and ecological and physiological aspects of their interactions

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Main trends in the dynamics of the cod-capelin relationship were studied using long-term (starting from the 1970s) data and taking into account stock dynamics and climatic variations. The number of capelin per one cod was taken as an indicator of abundance of the main food which is related to such factors as fisheries on capelin, its consumption by predators, and overlap of cod and capelin distribution. Major trophic relationships of cod, diet switch to substitute objects (polar cod, herring, fish fry, crustaceans, benthos), seasonal and interannual fatness variations, and development of feeding patterns were studied for years with different availability of capelin. According to the first feeding pattern, characterised by high abundance and availability of capelin, cod is provided with good food supplies (and has large fat reserves) during the entire feeding period. This ensures stable conditions of wintering, growth and maturation. The second feeding pattern, when cod suffers from food deficiency and has low fatness, entails anomalous growth and maturation. In warm years, however, low abundance of capelin is compensated for by its high availability. The consumption of capelin in such years is therefore close to that observed in the years of high capelin abundance. Presently, the effects of predation (by cod only) on the capelin stock are greater than those related to fisheries.