Summer diet and feeding strategy of Pacific cod (Gadus macrocephalus) inhabiting the southern coast of Hokkaido, Japan

The ontogenetic and interannual variations in the diet and feeding strategy of Pacific cod (Gadus macrocephalus) were examined based on 1302 specimens collected by a trawling net off the southern coast of Hokkaido from late June to early July during 2016-2020. The diet comprised 63 species (13 taxa). Fish was the most important prey, followed by decapod crustaceans. In 2016, walleye pollock (Gadus chalcogrammus) was the dominant fish prey species, while amphipods, decapods and Japanese lanternfish (Notoscopelus japonicus) were prominent in the diet in 2017. Japanese sardine (Sardinops melanostictus) were more predominant during 2018-2020. An ontogenetic comparison of diet indicated that smaller individuals (<= 400 mm standard length, SL) preyed mainly on small prey items, such as decapod crustaceans and polychaetes, whereas larger individuals (>400 mm SL) preyed mainly on fish. Moreover, prey size exhibited a significant increasing trend with predator body length. These results suggest that Pacific cod has a flexible feeding strategy that differs by life stage. The updated information on predator-prey relationships provided herein improves our understanding of energy flow in the marine ecosystem and could facilitate ecosystem-based fishery management strategies.