The demersal f~s h 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 intvrmediate depths were considered to be center depths of the distribution of the uppermost slope com~l~unity 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 specles composition was heterogeneous compared to the center depths.

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SynopsisFish assemblages at an artificial reef site, a natural reef site and a sandy-mud bottom site, on the shelf (depth 130 m) off Iwate Prefecture, northern Japan, were surveyed by using a bottom trammel net from May 1987 to March 1993. A total of 12 173 fishes of 48 species were recorded. Physiculus maximowiczi was dominant and comprised 69% of the total numerical abundance. Total fish number was lowest in March at all the 3 sites when P maximowiczi migrated to deeper and warmer waters. Assemblage equitability and species diversity also varied seasonally in accordance with the abundance fluctuation of P maximowiczi. P maximowiczi, Alcichthys alcicornis and Hexagrammos otakii were more abundant at the artificial reef and natural reef sites, while Dexistes rikuzenius and Hemitripterus villosus were more abundant at the sandy-mud bottom site; total fish abundance was largest at the artificial reef site mainly due to the large number of P maximowiczi. Species richness was similar among sites, but equitability, and consequently species diversity, was lowest at the artificial reef site. The main effect of the artificial reef seemed the attraction of P maximowiczi from nearby bottoms, especially from natural rocky reefs; its large abundance determined the structure of the artificial reef fish community.

Demersal fish communities were studied on the lower continental shelf and the upper continental slope along the Pacific coast of northeastern Japan. Species composition, number and weight of each species were examined based on otter trawl samples at 45 stations. Mean density and biomass of demersal fishes were 131 ha -1 and 21 kg ha -1 , respectively. The ten most abundant species comprised of about 95% of total number and weight of overall catch indicating simple species composition. Gadiform fishes Theragra chalcogramma, Gadus macrocephalus and Physiculus maximowiczi were the most important species by number, weight and frequency of occurrences, and three main community types represented by the three key species were recognized. Theragra-dominant community showed higher density and biomass, and lower diversity than Physiculus-dominant community did. Species diversity of demersal fish community was negatively correlated to density and biomass. Density and biomass of demersal fish community were high on the uppermost slope, and the high abundance resulted from low-diversity communities dominated by T. chalcogramma and G. macrocephalus.