Protection and pitfall: barrier nets reorganize fish communities in an ex situ reserve and compromise Yangtze finless porpoise prey after the fishing ban

Introduction The 2020 Yangtze River ten-year fishing ban represents a landmark conservation intervention, yet its ecosystem-level effects within ex situ reserves for critically endangered aquatic megafauna remain largely uncharacterized. Methods We compared pre-ban (2016) and post-ban (2024–2025) fish surveys in He-wang-miao Oxbow, a designated ex situ reserve for the critically endangered Yangtze finless porpoise ( Neophocaena asiaeorientalis ) and a germplasm resources bank for Chinese sturgeon ( Acipenser sinensis ). Results We documented rapid fish community reorganization. The surveys revealed a net loss of 4 species, with 15 historically recorded taxa undetected and 11 new taxa recorded after the ban. Total fish biomass increased 5.5-fold, alongside numerical increases in species diversity and a shift toward community stability. However, these aggregate gains coincided with a catastrophic collapse in small-bodied fish biomass from over 76.6% to 1.4% of total catch, the primary prey base of the Yangtze finless porpoise, while sturgeon-preferred demersal and benthopelagic assemblages remained stable. Community dominance shifted from small-bodied fish to large river-lake migratory species, a transition driven by barrier net retention, intensified top-down predation and interspecific resource competition. Discussion We conclude that physical barrier nets, rather than the fishing ban alone, restructured the aquatic trophic web and compromised the porpoise’s prey base, requiring targeted adaptive management to align ecosystem recovery with endangered species conservation goals.