Objectives: To identify practical, physiology-based indicators of thermal tolerance under ecologically realistic slow warming, and to compare reciprocal hybrids for breeding and management use.
Methods: Juveniles of Acipenser baerii and reciprocal hybrids (A. baerii♀ × A. schrenckii♂, BS; A. schrenckii♀ × A. baerii♂, SB) were ramped at 1 °C per 8 h to loss of equilibrium (LOE). A subset underwent 24 h rapid cooling for recovery. We measured serum cortisol, lactate, and glucose; antioxidant status (SOD, CAT, T-AOC, MDA); tissue LDH activity; and hsp70/hsc70/hsp90 expression in liver, gill, and kidney, with parallel histopathology.
Results: LOE occurred near 33 °C in A. baerii and 34–35 °C in hybrids. BS outperformed SB in survival and functional recovery. Heat stress increased cortisol and lactate, while glucose tended to decline. Antioxidant metrics changed little overall, whereas LDH rose, especially in kidney. HSP induction was tissue- and cross-direction-specific (predominantly liver in BS, gill in SB), aligning with pathology (hepatic vacuolar degeneration vs. gill epithelial lesions). Many variables normalized within 24 h of cooling, revealing substantial physiological reversibility.
Conclusions/Recommendations: Slow-warming plus recovery tests expose resilience that acute shocks miss and can be deployed on farm. We recommend a composite selection index for thermotolerance—LOE temperature + serum lactate/LDH + tissue-specific HSP expression—and targeted husbandry during heatwaves: staged cooling and organ-specific support (hepatoprotectants for BS, gill-protective measures for SB). Breeding programs should incorporate slow-warming screens to identify thermotolerant broodstock and to validate genomic predictors.

slow-warming heat stress; sturgeon; reciprocal hybrids; organ-specific responses; heat shock proteins (HSPs); lactate dehydrogenase (LDH); loss of equilibrium (LOE)