Seasonal Responses in Water Velocity and Water Depth Preferences of the Cyprinid Fishes ＂Acrossocheilus paradoxus＂ and ＂Varicorhinus barbatulus＂ in Choushui River of Taiwan

The cyprinid fishes ＂Acrossocheilus paradoxus＂ and ＂Varicorhinus barbatulus＂ in Choushui River, Taiwan were sampled with electro-fishing at 93 stations, each with 10 grids (1.5 m × 3.0 m). The occurrences were related with the water velocities and water depths, and compared between the summer monsoon season and winter dry season. Also, the seasonal responses were modeled with regression function equations with added interaction terms and a dummy variable for the seasons. For ＂A. paradoxus＂ the occurrence increased significantly with the decrease in water velocity but with the increase in water depth. No seasonal difference was observed. The fish preferred slow flowing pools than fast flowing riffles in both seasons. For ＂V. barbatulus＂ the occurrence decreased with the increase in water velocity in both seasons but it was higher in winter than in summer, suggesting the seasonal additive effect of water velocity on the fish occurrence. Also, ＂V. barbatulus＂ preferred deeper pool in summer, but in winter its optimal depth preference was at the depths of around 40 cm, corresponding to the upstream areas of pools receiving tail-water of upstream riffles. The seasonal differences in the water velocity and water depth preferences between the two species were considered to be the adaptation strategies of the species in response to seasonal changes in the environmental conditions.