DEPTH-GRADIENT STRUCTURE OF THE DEMERSAL FISH COMMUNITY ON THE CONTINENTAL-SHELF AND UPPER SLOPE OFF SENDAI BAY, JAPAN

The demersal fish community was examined at 15 sites on the continental shelf and upper slope off Sendai Bay, Japan, from 1989 to 1991. The community structure was analyzed along the depth gradient (ca 150 to 450 m). The species compositions at the shallowest site and the deepest site were different from the other sites. The depth range covered not only an upper slope community but also the shallower shelf community and the deeper slope community. The species composition varied annually at each site, but some stable species composition was observed in intermediate-depth waters (ca 240 to 370 m). The intermediate depths were considered to be center depths of the distribution of the uppermost slope community which was characterized by the dominance of Theragra chalcogramma and Gadus macrocephalus, high biomass and low species diversity. Biomass was maximum and species diversity was minimum at the intermediate depths, and gradually changed with depth toward the other communities. The transition depths were located on both sides of the center depths of the upper slope community, where the percentage by biomass of the 2 dominant fish species was low and/or variable and the species composition was heterogeneous compared to the center depths.