1Hokkaido Central Fisheries Experiment Station, Yoichi, Hokkaido 046-8555, 2Marine Biological Research Institute of Japan Co., Ltd, Sapporo Branch, Sapporo, Hokkaido 007-0862, Japan
Seasonal changes in the composition and food habits of fish communities in a Sargassum confusum-dominated bed off the coast of Ishikari, Hokkaido, Japan were examined. Twenty-seven species, a total of 997 individuals, were collected by beach seine surveys from June 2002 to August 2003. Numbers of individuals and species increased in spring to fall compared to winter. A cluster analysis based on stomach contents composition showed that the fish communities comprised eight feeding groups: A) phytal amphipods, B) phytal and benthic amphipods, C) benthic polychaetes and phytal amphipods, D) phytal amphipods and isopods, E) benthic copepods and amphipods, F) planktonic copepods, G) phytal mysids and planktonic copepods, and H) phytal mysids feeders. Ontogenetic diet shifts were recognized in Pholidapus dybowskii, Aulichthys japonicus, Hexagrammos otakii and Syngnathus schlegeli, although Blepsias cirrhosus, Neozoarces steindachneri and Zoarchias veneficus were included in nearly the same feeding groups throughout the year. Sebastes schlegelii and Sebastes taczanowskii seemed to change their food habits with the amount of favorite diet. It is suggested that dietary diversity and stable production in the Sargassum bed contribute to the formation of the feeding ground.