Tissue-specific oxidative responses and recovery following acute thermal stress in juvenile Amazonian fish Colossoma macropomum.

Colossoma macropomum (tambaqui) is an economically important Amazonian fish increasingly exposed to heatwaves in tropical aquaculture systems. However, multi-tissue responses to acute thermal stress and recovery remain poorly understood. Juveniles were exposed to 34 °C for 24 h, with samples collected at baseline and at 1, 24, and 48 h post-recovery (28 °C). Antioxidant responses and oxidative damage markers were evaluated to determine whether tambaqui re-establishes physiological homeostasis following acute heat shock. Antioxidant enzymes showed significant time- and tissue-dependent changes following acute heat shock. Overall, SOD, CAT, GPx, GR, and GST activities were transiently altered during the early recovery phase (1-24 h), but most enzymes returned to baseline levels within 48 h in the brain, gills, intestine (except CAT), liver, and muscle (except SOD) (p > 0.05). Regarding oxidative damage markers (PC and MDA), the intestine was the most affected tissue, with significantly higher levels at 48 h than at baseline. Based on integrated biomarker analysis (IBRv2), gills and muscle showed the smallest deviation from baseline levels after 48 h, indicating a more complete recovery compared to the other tissues. The intestine, on the other hand, showed persistent oxidative damage markers even at 48 h of recovery. Overall, the results indicate that although acute heat shock induces transient, tissue-specific oxidative disturbances, juvenile tambaqui largely restore redox homeostasis within 48 h, with recovery capacity varying among tissues.