Abstract: Against the backdrop of global climate change and intensive human activities, fish resources in the Yangtze River Basin have continued to decline. To protect aquatic biodiversity and restore the ecosystem, a comprehensive fishing ban has been gradually implemented since 2018 in 332 aquatic reserves across the Yangtze River Basin, including the National Germplasm Reserve for the "Four Major Chinese Carps" in the Chongqing section. By the end of 2019, Chongqing had taken the lead in completing the withdrawal of fishermen from its aquatic reserves, fully prohibiting productive fishing activities. This study aimed to understand the changes in fish resources within the reserve following the fishing ban. From April to May (fish spawning season) and December (fish overwintering season) of 2023, the research team conducted an 8-day hydroacoustic survey using an EK80 split-beam echosounder across an approximately 125 km river section of the reserve, along with a 52-day catch survey. The hydroacoustic survey revealed that the average fish density during the spawning and overwintering periods was (19.44±29.04) and (5.07±3.93) ind./1 000 m³, respectively, with no significant difference. However, the average target strength differed significantly, measuring (-60.66±6.63) dB and (-57.60±6.44) dB, respectively. During the spawning season, fish tended to aggregate near urban areas, whereas in the overwintering period, they were more dispersed in waters farther from urban zones. In both surveys, the highest fish density was observed at depths of 6-9 m, and the target strength generally increased with water depth. The catch survey identified 47 fish species (5 orders, 12 families) during the spawning season, with Hemiculter leucisculus, Pseudobrama simoni, and Squalidus argentatus being the most abundant. In the overwintering period, 48 species (6 orders, 13 families) were recorded, dominated by Saurogobio dabryi, Megalobrama pellegrini, and Coilia brachygnathus. Compared to pre-ban data (2017—2019), fish aggregation patterns in the reserve have shifted significantly. The density in the southern Changshou river section was notably higher than before the ban, but the average fish size has decreased. This study elucidates the post-ban distribution characteristics and changes in fish populations within the reserve, providing valuable insights for research on the evolution of fishery resources and regional fisheries management following the fishing ban.