Hydroacoustic survey of fish density, spatial distribution, and behavior upstream and downstream of the Changzhou Dam on the Pearl River, China

Hydroacoustic surveys were conducted to understand the relationship between fish density, spatial distribution, and behavior upstream and downstream of the Changzhou Dam on the Pearl River, China, and the condition (open/closed) of the spillways. When the spillways were open on 24 June 2010, numerous fish were observed to be densely distributed in the forebay upstream of the dam, with an average fish density was 0.22 fish m(-3). When the spillways were closed on 25 June 2010, the fish upstream of the dam dispersed, and the average fish density decreased to 0.007 fish m(-3). Prior to operating the spillways on 24 May 2010, the average fish density downstream of the dam was 0.28 fish m(-3); in comparison, on 26 June, immediately following closure of the spillways, the average fish density downstream of the dam was 0.08 fish m(-3). Fish were more active on June 24 than on 25 June: they swam faster and their positions in the water column varied greatly. On 26 June, fish did not to swim as freely in the water column as those measured on 24 May. Based on these observations, we conclude that a large number of fish are able to swim to the upstream side of the dam while the spillways are open.