Abstract: Accelerating flow, a characteristic flow field that a fish must face during its downstream migration, usually occurs in fishway facilities at hydropower stations. This study examines the effect of accelerating flows on the downstream migration behaviours of juvenile grass carps (Ctenopharyngodon idellus) of different body lengths (5.75 - 9.48 cm). We focus on accumulative pass rate, time of first arrival at the acceleration zone, and time proportion of staying in the non-acceleration zone to acceleration zone, and compare two different flow accelerations (at 0.29 and 0.59 m/s2). The results show that at acceleration of 0.59 m/s2, the fishes of 7.55 - 9.48 cm long have the highest pass rate; while at 0.29 m/s2, the fishes of 5.57 - 6.48 cm long have the highest rate. In the former case, the time of first arrival is the shortest, indicating stronger downstream migration motivation. And under different flow accelerations, passing the upstream non-acceleration zone always causes a delay effect on downstream migration.

Key words: accelerating flow, body length, Ctenopharyngodon idellus, downstream migration, fishway, ecohydraulics