Fluctuation and variation in stream-fish assemblages after a catastrophic flood in the Miyagawa River, Japan

In the autumn of 2004, a typhoon caused a catastrophic flood of the Miyagawa River in Japan. Based upon snorkeling surveys conducted every autumn from 2005 to 2009, we monitored the post flood fluctuation of the local fish assemblages at nine sites of both the main stream and subsidiary streams of the river. Results revealed that species richness significantly increased from 2005 to 2009. In addition, the fish densities of eight species significantly increased over the same period, whereas the density of one species decreased, and that of eight others remained unchanged. Categorization based on Euclidean distance revealed five main clusters from the nine sites. Among these sites, fish assemblages within subsidiary streams were stable as they remained within the same clusters while those in the main stream were dynamically variable through time as they changed cluster membership. In addition, the Euclidean distance between two arbitrary fish assemblages was positively correlated with environmental distance (the Euclidean distance calculated based on river width, depth, velocity and pebble size), time distance, and spatial distance along the river. In conclusion, the fish assemblages were dynamically and regularly altered and varied in the five years after the flood, except for those in the subsidiary streams, and such variation was related to environmental, temporal and spatial variation.