Spawning cohort trade-offs of reproductive time and output in cyprinid fish along an elevation gradient

Understanding environmental requirements for fish reproduction in natural habitats is crucial for population conservation and restoration. However, such information is extremely scarce for the highly threatened schizothoracine fishes that cover large elevation ranges on the Tibetan Plateau. This research investigated the spawning time, output and environmental conditions of wild Schizopygopsis thermalis along an elevational gradient (i.e., from 1766 to 4372 m) in the upper Nu-Salween River. It found that the spawning grounds of S. thermalis were mainly located at elevations between 3600 and 4372 m, and spawning habitats mostly consist of still marginal areas (still water <10 cm deep) with gravel substrates. Otolith microstructure analysis revealed, for the first time, that S. thermalis spawns in two seasons (i.e., winter: from December to next February, and spring: from April to May), with the winter spawning period generally longer than the spring spawning period. Winter spawning started earlier and ended earlier at higher elevations, while spring spawning occurred earlier at lower elevations but ended simultaneously at higher elevations. As elevation increases, reproductive output also increases, and spawning cohorts shift from winter-dominant to spring-dominant. Winter spawning cohorts were associated with lower water temperatures but higher discharges than the spring spawning cohort, and both cohorts showed elevation-related environmental preferences. Our results suggest that schizothoracine fish can adapt spawning strategies to elevation-related environmental conditions, which fill the gaps in the reproductive characteristics of schizothoracine fishes, and highlight the importance of conserving schizothoracine habitats at high elevations in the Qinghai-Tibetan rivers.