Spatio-temporal fish distribution differences between closed-fishing and ecological-stocking in Shangyoujiang Reservoir based on hydroacousticassessment

In order to investigate the current status of fishery resources in the no-fishing zones and ecological enhancement zones of the Shangyoujiang Reservoir in the Poyang Lake Basin during the initial stage of the ten-year fishing ban in the Yangtze River, and to clarify the temporal and spatial distribution characteristics of fish and their differences, we conducted two surveys on the reservoir using a combination of hydroacoustic detection and traditional net surveys. A total of 5 816 fish from 47 species, 5 orders and 10 families were caught during the surveys. The dominant species were Aristichthys nobilis, Tilapia zillii, Sinibrama macrops, Xenocypris davidi, Megalobrama skolkovii and Hypophthalmichthys molitrix in 2023; and A. nobilis, T. zillii, S. macrops, T. galilaea and M. skolkovii in 2024. The results of hydroacoustic detection show that in the closed-fishing area, the mean fish densities were 0.126 02 and 0.116 48 ind·m⁻² in 2023 and 2024, corresponding to the estimated total biomass of 138.98 and 115.82 t, respectively. In the ecological-stocking area, the mean densities were 0.182 53 and 0.099 86 ind·m⁻², corresponding to biomass of 1 208.87 t and 961.4 t, respectively. Significant spatial heterogeneity was observed in the ecological-stocking area, characterized by fish aggregations and larger average body sizes, whereas the closed-fishing area showed more uniform distribution with smaller-sized individuals. Overall, the distribution showed a trend of first increasing and then decreasing with water depth. The study indicates that the fishing ban and stock enhancement activities in the Shangyoujiang Reservoir have affected fish composition, resource quantity, distribution, and individual size. To maintain ecosystem stability and fish diversity, further research is needed on the mechanism of how these measures influence fish community structure and spatiotemporal distribution patterns.