Spatiotemporal Response of Fish Aggregations to Hydrological Changes in the Lower Pearl River, China, during the Main Spawning Season

The Pearl River is a global hotspot of fish biodiversity, yet has the most threatened endemic fish species in China. Since the establishment of the Changzhou Dam in the lower reach, changes in hydrological rhythm have negatively impacted fish downstream of the dam, but their spatiotemporal distribution in response to flood alteration has received little attention. In this study, hydroacoustic surveys were undertaken monthly in 2016 to monitor the distribution and behavior of fish. Fish densities were higher during the water discharge rising stage than during the falling stage, indicating that the fish aggregate during flooding (coefficient of variation [CV] > 100%) and depart after flooding (CV < 100%), especially aggregations of large fish. The target strength (TS) was allocated to two groups as per their frequency distributions, defined as small fish (-55 dB < TS < -40 dB) and large fish (TS > -40 dB). The sizes of both groups were significantly larger during the rising stage when compared to those during the falling stage (p < 0.01). Comparatively more fish were present with a greater average TS, and a substantially greater proportion of large fish was detected during rising stages. Hydrological variation importantly influences fish aggregations, including the numbers and sizes present, with the differences being particularly pronounced between the rising and falling stages. Combined with relevant studies, it is suggested that water releases from the Changzhou Dam should be regulated to satisfy fish spawning and migration demands during the main breeding season.