Identifying Key Factors Associated with the Spatiotemporal Dynamics of Fish Assemblages in Subtropical River-Type Reservoir Ecosystems: Implications for Fishery Management and Conservation

Characterizing the spatiotemporal patterns of fish communities in subtropical river-type reservoirs provides useful baseline information for fisheries stewardship and aquatic ecosystem monitoring. This work investigated fish assemblages across 18 sampling sites within Shanxi Reservoir, Zhejiang Province, using an integrative approach combining gillnet surveys, hydroacoustic detection, and habitat parameter monitoring. Sampling was conducted during two periods: October 2022 (autumn, post-flood, fattening phase) and April 2023 (spring, normal water level, spawning phase). A total of 22 species were documented in spring and 21 in autumn, with Cyprinidae species predominating. Dominant species shifted seasonally, led by Hypophthalmichthys nobilis, Culter dabryi and Hemiculter leucisculus. Planktivores and carnivores were more abundant in spring, while omnivores increased in autumn. Spatially, fish populations were mainly concentrated in the shallow riverine zone (SRZ), predominantly comprising small- to medium-sized individuals. Mantel tests and Pearson correlation analyses identified associations between fish assemblage structure in the deep lake zone (DLZ) and environmental variables including dissolved oxygen, conductivity, chlorophyll a, and phytoplankton density and biomass. This study provides preliminary baseline data on fish community dynamics in Shanxi Reservoir and offers tentative observations that may help inform future zoned monitoring and ecological restoration planning.