The feeding ecology of juvenile (< 140 mm TL) and adult (> 140 mm TL) capelin (Mallotus villosus) in the St. Lawrence River Estuary and western Gulf of St. Lawrence was studied between October 1974 and October 1975. Both juveniles and adults fed almost exclusively on zooplankton. Three prey totally dominated the diet. Copepods were numerically most abundant but euphausiids were dominant in terms of biomass. Appendicularians ranked third in both number and biomass. This differed from the diet of capelin in the Canadian Atlantic only to the extent that amphipods replace appendicularians in the diet in the Atlantic and large copepods are more abundant in the diet there. A shift in diet occurred during growth. Premetamorphic capelin (< 75 mm TL) consumed small prey exclusively: invertebrate eggs, cyclopoid copepods, and diatoms predominated. The diets of juvenile (75–139 mm TL) and adult (> 140 mm TL) capelin overlapped considerably but a pronounced shift from small copepods to adult euphausiids occurred at ~ 140 mm TL. Early morning and evening feeding peaks occurred between May and August. A single midday feeding peak prevailed in October–November. Feeding of adult capelin was active in the spring prior to spawning but virtually ceased during spawning. Juvenile capelin fed actively throughout spring and summer. The daily ration of zooplankton by capelin was estimated to be 5% of wet body weight in summer May–September and ~ 2.5% in October–November. Year-to-year variations in average daily consumption in excess of 100 000 t have occurred between 1972 and 1978 and average daily consumption has declined by up to 310 000 t during the same interval. This significant year-to-year and long-term release of zooplankton from predation by capelin may be an important regulator of changes in year-class strength in other species of the area, notably arctic cod, Boreogadus saida and short-finned squid, Illex illecebrosus.Key words: capelin, Mallotus villosus; Gulf of St. Lawrence, ecology, food habits, daily ration, estuaries, zooplankton, multispecies interactions, arctic cod, Boreogadus saida; squid, Illex illecebrosus
Size class segregation of Arctic cod (Boreogadus saida) in a shallow High Arctic embayment