Linking physiological indicators and HSC70‑2 gene expression to acute thermal tolerance and stress thresholds in common carp ( Cyprinus Capio )

This study delineated acute thermal thresholds in common carp Cyprinus carpio (39.9 ± 5.27 g; 13.54 ± 0.67 cm) by integrating physiological (cortisol, hematology, serum biochemistry, opercular movement) and molecular (HSC70-2) responses to rapid temperature shifts. Fish were acutely ramped from 25 °C to 13–37 °C at 3 °C h−1 (n = 10 per treatment). Cortisol increased at 25 °C and rose significantly at 37 °C (p < 0.05), indicating heat-induced stress, whereas cold-shock responses were variable. Complete blood count parameters shifted immediately with any deviation from 25 °C, evidencing acute physiological disturbance. Serum biochemical indices changed markedly below 17 °C and above 27 °C, suggesting metabolic disruption at these extremes. Opercular movement decreased with cooling and increased with warming, with pronounced changes below 23 °C and above 29 °C, consistent with respiratory compensation. Hepatic HSC70-2 expression exceeded other tissues and shifted significantly at 17 and 27 °C (p < 0.05), identifying molecular inflection points. Given the acute ramping rate, these findings represent short-term tolerance limits rather than chronic optima for growth or welfare. Future long-term studies incorporating performance and welfare metrics are needed to refine temperature recommendations for aquaculture.