Diel Feeding Habits of Sockeye and Chum Salmon in the Bering Sea during the Summer.

Abstract

Diel feeding habits of sockeye salmon Oncorhynchus nerka and chum salmon O. keta in the Bering Sea during the summer were investigated by four parameters: the frequency of empty stomachs, stomach content weight, digestion stage, and pH of the inner wall of the blind sac in the stomach. Stomach samples were obtained at various times of day from gillnet surveys. Sockeye and chum salmon exhibited similar diel feeding patterns, although each of the four parameters of sockeye salmon fluctuated more distinctly than in chum salmon. Based on the present results together with previous knowledge, it is hypothesized that the two species obtain metabolic energy in different ways; sockeye salmon feed on more nutritious food organisms than chum salmon, but are inferior to chum salmon in digestion abilities. Chum salmon can utilize peculiar or poorly nutritious food organisms such as jellyfish, on which other salmonid fish seldom feed, with the help of physiologically and morphologically greater digestion abilities.