Abstract
Hydroacoustic surveys were performed to clarify the density and spatial distribution of fish upstream and downstream of Changzhou Dam in the Pearl River, China, with regard to the condition of the spillways. Fish were densely distributed in the forebay of the upstream dam and the average fish density was 0.22 individuals/<TEX>$m^3$</TEX> under open spillways on 24 June 2010, but when the spillways were closed on 25 June 2010, the fish in the upstream dam dispersed and the average fish density decreased to 0.007 individuals/<TEX>$m^3$</TEX>. Meanwhile, the average fish density in the downstream region was 0.28 individuals/<TEX>$m^3$</TEX> before the spillway was opened on 24 May 2010, but it decreased to 0.08 individuals/<TEX>$m^3$</TEX> on 26 June, just after the spillway was closed. The vertical distribution of fish upstream of the dam was not consistent. The target strength (TS) of fish upstream of the dam was larger than that of fish in the downstream region, although the distribution of TS was similar between the upstream and downstream regions. Therefore, we concluded that while numerous fish could swim to the upstream region while the spillways were open, closed spillways obstructed fish migrating upward from the downstream region.
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