Acoustic assessment of fish resources in subtropical run-off-river hydropower plant in the Pearl River

As the utmost downstream dam in the main stream of Xijiang River, Changzhou Hydro-Junction has changed the original hydrological characteristics and ecological environment of Xijiang River. In order to grasp the fish distribution and standing crops after reservoir impoundment, a split beam echo-sounder EY60 (120 kHz) was used in the river channel from Guiping to Wuzhou in April 2014, 2015, and 2016. The average fish density in the survey area was 0.1215±0.4198 ind.m^-3 in 2014, 0.0190±0.1232 ind.m^-3 in 2015 and 0.0107±0.0186 ind.m^-3 in 2016. According to the horizontal distribution data, fish density showed significant difference in different area and the density in the upper area was significantly higher than that in other areas. Moreover, fish density was negatively correlation with water depth. As a whole, the Target Strength (TS) decreased in the past three year, TS was significantly different among the three areas and showed a decreasing tendency from the dam area to the upper area. Fish resources decreased gradually, as indicated by the decreased fish density and miniaturized fish sizes in the past three years.