Swimming performance and thermal limits of early-life Brachymystax tsinlingensis under environmental stressors: implications for conservation stocking of an endangered salmonid.

The endangered cold-water salmonid Brachymystax tsinlingensis, endemic to China, faces ongoing threats from habitat degradation, yet conservation stocking efforts lack a robust physiological basis. To address this gap, we quantified swimming performance and kinematics in larvae and juveniles of this species, and assessed the effects of starvation, light intensity, and acute temperature change on aerobic (critical swimming speed, Ucrit) and anaerobic (burst swimming speed, Uburst) capacities. A pronounced ontogenetic shift was observed: juveniles exhibited higher absolute and relative swimming speeds than larvae, supporting the selection of juveniles for stocking. Starvation exceeding three days significantly reduced both Ucrit and Uburst in juveniles, whereas larvae experienced high mortality (>40%) before measurable declines in swimming performance, revealing stage-specific vulnerability. Juvenile Uburst was optimal under intermediate illumination (320-340 lx), suggesting a behavioral preference that may inform release site selection. Temperature emerged as the dominant abiotic filter: swimming performance (Ucrit) followed a unimodal response, peaking at 20-23°C and declining markedly at 26°C, while Uburst showed a progressive decline with increasing temperature, defining a critical thermal limit for this species. These findings demonstrate that swimming behaviour and performance in early-life B. tsinlingensis are shaped by ontogeny, energy status, light environment and water temperature, with interactive effects that define critical tolerance thresholds. By linking behavioural and physiological responses to environmental stressors, this study provides a quantitative basis for informing conservation and management strategies for this endangered species.