Detritus and Juvenile Salmon Production in the Nanaimo Estuary: I. Production and Feeding Rates of Juvenile Chum Salmon (Oncorhynchus keta)

Abstract

Theories of food chain dynamics have important implications for the management of marine resources. As yet, however, there are few empirical studies of the food chain dynamics of resource species against which these theories can be judged. This paper compares the food requirements of juvenile chum salmon (Oncorhynchus keta) in the Nanaimo Estuary with the productivity of their principal food species. Chum were present in the estuary from March until June and the estuary population ranged up to 4.1 million in May 1975 and 2.4 million in April 1976. The average weight of chum was 0.66 g in 1975 and 0.65 g in 1976, and their rate of growth averaged ~6% body weight per day in both years. Food intake estimated by three independent methods ranged 4.4–18% body weight per day and was assumed to average 15% body weight per day. Annual fry production was 2381 kg in 1975 and 1122 kg in 1976. Food intake was 6184 kg in 1975 and 2815 kg in 1976. The principal dietary item in both years was harpacticoid copepods. In 1975 Harpacticus uniremis made up 50% of the diet overall, and >80% of the diet when fry were most abundant. The seasonal pattern of abundance of fry and H. uniremis on the estuary was the same, and the fry consumed most of the estimated production of H. uniremis. Juvenile chum production was potentially limited by food supply. Food chain dynamics were, therefore, important in the productivity of the chum population, but since both chum and its chief food were rare and ephemeral elements of the estuarine fauna, their interaction probably had little impact on the dynamics of the estuary as a whole. Key words: chum salmon, estuaries, food chains, populations, feeding rates