Abstract: To investigate the spatiotemporal dynamics of fish resources downstream of the Wudongde Hydropower Station, hydroacoustic surveys were conducted using a Simrad EK80 echosounder (200 kHz) from May 2024 to April 2025. These surveys were integrated with fish catch sampling conducted in May and November and monthly monitoring data from a fixed fish aggregation station located on the right bank downstream of the dam. The fish catch surveys collected 250 individuals representing 31 species across 24 genera and 6 families, with Carassius auratus dominating the assemblage (44.4%), followed by Coreius guichenoti (5.6%) and Hemiculter leucisculus (5.2%). Both species richness and abundance were higher in May than in November. The fixed aggregation station recorded a total of 35,026 individuals representing 55 species from 40 genera and 11 families. The assemblage was numerically dominated by Hemiculter leucisculus (54.90%), Culter alburnus (32.98%), and Lepturichthys fimbriata (5.11%), with daily catches exhibiting pronounced seasonal peaks between June and September. Hydroacoustic target strength corresponded to fish body lengths ranging from 10.1 to 117.4 cm, and monthly fish densities varied between 3.07 and 53.43 individuals per 1000 m3. Temporal trends in fish length and density derived from hydroacoustic data were consistent with those from catch surveys and showed significant monthly correlations with aggregation station data (density: R = 0.73, p < 0.01; body length during May–October: R = 0.82, p = 0.047), demonstrating strong complementarity between the two monitoring approaches in terms of spatial coverage (hydroacoustics) and temporal continuity (aggregation station). Hydroacoustic observations further revealed distinct seasonal spatial patterns of fish distribution downstream of the dam. During spring and early summer (March–June), spawning-driven aggregations dominated, with spawning grounds of different species shifting longitudinally within 1.8–12.0 km downstream of the dam across months. In contrast, during autumn and winter (September–March), fish distributions became increasingly dispersed, characterized by downstream diffusion, low-density conditions, and intermittent return movements. Overall, fish distribution dynamics downstream of the dam can be summarized as a seasonal cycle of reproduction-driven aggregation, dispersal, low-density persistence, and return migration. These findings provide a scientific basis for optimizing the operation of fish aggregation systems and ecological regulation downstream of large dams and highlight the value of integrating hydroacoustic monitoring with fish tracking, hydrological observations, and environmental data to further elucidate underlying driving mechanisms.