Data on 47 otter trawls towed at depths of 30 to 400m in Tosa Bay show demersal fish fauna changes by depth. Results of cluster analysis of the data suggest that three distinct faunal zones exist on the continental shelf (30 to 100m), upper continental slope (165 to 400m), and between them (100 to 165m). Rapid changes of the species recruitment curve mean the species composition of the faunae extensively changes at the two boundaries. Common species among adjacent zones occupy 30 to 40% of the total number of species in each zone. However, the rate is less than 10% between the shelf and the upper slope. These results suggest that the zone from 100 to 165m is the transitional region between the shelf and the upper slope.
Value of the species diversity is high through the shelf and transitional region, and low on the upper slope. The species diversity is largely affected by species evenness within the faunae, and the low species diversity on the upper slope is mainly attributed to a dominant species, Chlorophthalmus albatrossis.

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