A Comparative Study of Otolith Microchemical Characteristics of Coilia nasus in Poyang Lake Under Extreme Climate and Normal Periods.

Poyang Lake, China's largest freshwater lake, is a critical spawning ground for the anadromous fish Coilia nasus. In 2022, extreme drought and upstream regulation caused historically low water levels, exposing lakebeds and desiccating spawning grounds, with spawners found dead. To assess impacts on migration, we collected 11 desiccated and decomposed specimens from the 2022 exposed lakebed and 18 fresh specimens from the same area in 2024 (normal year). Otolith Sr/Ca ratios reliably recorded lifetime salinity. Despite degradation, the 2022 otoliths preserved complete habitat records, confirming method reliability. Most specimens were the migratory long-jaw form of the species; two were resident short-jaw specimens, with uniformly low Sr/Ca ratios. All migrants showed a three-phase anadromous pattern: freshwater hatching (Sr/Ca < 3), marine feeding (>3), and spawning migration into freshwater (<3). The outermost freshwater zone (SIII) was significantly wider in the 2022 migrants (144 ± 30 μm) than in the specimens from 2024 (33 ± 24 μm, p < 0.01), indicating that the extremely low water levels severely slowed upstream migration. Critically, driven by natal homing instincts, the fish reached the desiccated sites and did not seek alternative locations, causing mass mortality-revealing high site fidelity and low adaptability. For the planned Poyang Lake Water Conservancy Hub, we recommend maintaining minimum ecological water levels to prevent spawning ground desiccation and ensuring river-lake connectivity during migration and larval dispersal to sustain C. nasus and other aquatic animals under warming and engineering pressures.