Interspecific relationship by composition of stomach contents of fish at Akkeshi-ko, an estuary at eastern Hokkaido, Japan

To investigate the food habits of fish in an estuary, we examined stomach contents of 32 species of fish collected at Akkeshi-ko at Eastern Hokkaido, Japan, Fish species included in the analysis were Cottidae (Blepsias cirrhosus, Myoxocephalus brandti, M. jaok), Pleuronectidae (Pleuronectes obscurus, P. pinnifasciatus, Platichthys stellatus), Blennioidei (Lumpenus sagitta, Opisthocentrus dybowskii, O. ocellatus, Enedrias crassipina, Zoarces elongatus), Hexagrammidae (Hexagrammos stelleri, H. lagocephalus), Eleginus gracilis Arctoscopus japonicus, Chaenogobis mororanus, Liparis agassizii, and others. Food organisms were classified into Polychaeta, Mysidacea, Flabellifera, Gammaridea, Natantia, Cragonidae, Brachyura, Pisces, and others. Seven analysis methods were applied to the stomach contents: frequency of occurrence, number, weight index of relative importance, trophic level, Simpson's index of diversity, and Schoener's index of intra- and interspecific overlaps for food habit. Fish in the estuary were clustered into six feeding groups based on their prey: group A fed on Mysidacea and Cragonidae (E. gracilis, A. japonicus, C. mororanus, E. crassispina, H. stelleri, B. cirrhosus, M. brandti, P. stellatus, M. brandti (TL 225mm)); group C fed on Pisces and Natantia (L. agassizi); group D fed on Gammaridea (L. sagitta, O. dybowskii, O. ocellatus, Z. elongatus); group E fed on Polychaeta (P. obscurus), group F fed on Natantia and Brachyura (H. lagocephalus). In each group, competition for food organisms was expected. However, since Simpson's indices of diversity for some fishes n the group were large, it was suggested that the competition for food organisms was not severe within the same group. The food chain of Akkeshi-ko was estimated from relationship of these groups. The Akkeshi-ko food chain structure is complex and highly diverse