Longitudinal changes in fish assemblage were studied in the Ogata River and djoining tributaries, northeastern Kyushu, southern Japan, in October 2003, and February nd August 2004. A total of 18,015 individuals (14 species/subspecies representing 7 amilies) were recorded at 15 stations, by observers using snorkels. A major fish community ransition was apparent from headwater stations with a simple assemblage dominated by mago salmon Oncorhynchus masou ishikawae and Chinese minnow Phoxinus oxycephalus, o downstream stations with more complex assemblages dominated by cyprinid fishes, such s Japanese dace Tribolodon hakonensis and dark chub Candidia temminckii. Multivariate nalyses separated the 15 stations into three groups: headwater stations with a salmon/innow-dominated assemblage (415–820 m altitude), a transition zone with a dace/chub/innow-dominated assemblage (260–397 m), and downstream stations with a cyprinidominated assemblage (232–255 m). The fish assemblage structures were considered to e influenced by longitudinal environmental gradients, being significantly correlated with ix environmental variables (topographic type and altitude, and stream gradient, mean etted width, discharge and minimum daily mean water temperature) showing monotonic hanges over the length of the stream investigated (16.3 km). On the other hand, cyprinid opulation densities did not show such changes, but declined within the transition zone ownstream from erosion-control dams, suggesting that such densities were affected by maller spatial scale factors, including dam-induced habitat degradation.
