Spatio-temporal changes of demersal fish assemblage in Osaka Bay

Abstract

To investigate the demersal fish assemblage in Osaka Bay, Japan, and the environmental factors affecting it, sampling surveys were conducted using a small bottom trawl at 20 fixed stations within the bay during February, May, August, and November from 2014 to 2016. Based on cluster analysis of fish species composition at each station, the 20 stations were classified into five groups, revealing a gradual change in species composition from the eastern to the southwestern area of the bay. In the group located in the easternmost area, individual density was positively correlated with dissolved oxygen concentration but negatively correlated with both water temperature and acid-volatile sulfide levels. Conver­sely, the group in the southwestern area of the bay showed no apparent seasonality or relationship between changes in individual density and environmental factors. Compared to the results of surveys conducted in the 1990s using methods similar to those applied in this study, the composition of dominant fish species remained largely unchanged, although variations were observed in their relative abundances. Moreover, certain species recorded in the 1990s were scarcely detected in the 2010s. The structure of the demersal fish assemblage in the inner bay area is considered to be the result of adaptation to the sedimentary environment.