Otolith Strontium Isotope (87Sr/86Sr) Reveals Mixed Life Histories of Coilia brachygnathus in the Middle–Lower Yangtze River Floodplain

Life-history plasticity allows fishes to exploit heterogeneous freshwater networks, and proactive habitat selection enhances their capacity to cope with climatic and human-induced stressors. <i>Coilia brachygnathus</i>, long regarded as a freshwater resident, nevertheless has a poorly understood habitat use and selection. We reconstructed habitat use of <i>C. brachygnathus</i> across the middle-lower Yangtze by coupling water ⁸⁷Sr/⁸⁶Sr baselines with otolith ⁸⁷Sr/⁸⁶Sr transects. We used 60 water samples, compiled published datasets, and measured additional water samples from the mainstem, major tributaries, and connected lakes. In situ ⁸⁷Sr/⁸⁶Sr profiles were smoothed with generalized additive models, revealing significant within-profile variation for all fish. Two modes emerged: (i) residents, whose otolith values remained within local water ranges, consistent with limited movement; and (ii) migrants, whose profiles departed from local baselines, indicating exchanges among lakes, tributaries, and the mainstem. Several fish captured at Anqing and Zhenjiang possess Poyang-like core signatures that declined to mainstem values, while bidirectional shifts were common in the Dongting-Xiang system. These results overturn the residency paradigm, demonstrate flexible partial migration, and indicate proactive habitat selection in the face of environmental change, thereby supporting management that maintains river-lake connectivity and quantifies lake contributions to mainstem stocks.