Temporal dynamics of fluvial fish community caused by marine amphidromous species in the Shubuto River, southwestern Hokkaido, Japan

We investigated the fluvial fish communities at 16 sites that cover the entire Shubuto River in northern Japan in July and September 2010. A total of 5,552.9 g of biomass and 980 individuals were identified as produced by and belonging to 11 species, and 7,079.7 g of biomass and 2,587 individuals were pinpointed as produced by and belonging to 17 species. The data on the 11 and 17 species were recorded in July and September 2010, respectively. The July and September data exhibited significant differences in individual and biomass compositions (G-test; both P < 0.001). The number of recorded amphidromous Gymnogobius opperiens and Rhinogobius sp. (a species complex of Rhinogobius brunneus) individuals was significantly affected by the interaction term between watercourse distance from the sea and sampling season (July or September). By contrast, no such significant influence was found for the biomass produced by the aforementioned species. Fisher’s exact tests revealed that the G. opperiens and Rhinogobius sp. compositions during the growth stage (larva–juvenile vs. post-juvenile) significantly differed between July and September 2010. These results are probably attributed to part of the seasonal trend exhibited by the fish communities in the Shubuto River; that is, the spatial pattern of the fluvial fish communities from July to September is probably caused by an increase in the species occurrence and larval and juvenile recruitment of G. opperiens and Rhinogobius sp. migrating from the sea to the river.