Chronic thermal stress affects growth, liver morphology and molecular responses but allows recovery at optimal temperature in juvenile zebrafish (Danio rerio).

Climate-driven aquatic heatwaves pose an increasing threat to fish populations by inducing prolonged thermal stress. However, the resilience of teleosts to chronic heat exposure and their capacity for recovery remains poorly understood. This study investigated the effects of chronic high-temperature exposure on juvenile zebrafish (Danio rerio). Fish were exposed for 60 days to one of four treatments: (T1) 28°C control; (T2) 28°C for 30 days followed by 34°C for 30 days; (T3) 34°C for 30 days followed by 28°C for 30 days (recovery); and (T4) constant 34°C. Growth, liver morphology and the expression of 14 genes related to heat-shock response, growth and liver function by qPCR were assessed. Chronic exposure to 34°C (T4) significantly reduced body mass after 60 days. Genes associated with heat-shock response (hsp70, hsp90aa1, serpinh1b), liver function (got1) and growth (igf1, ghra) were consistently upregulated in T4 fish at Days 30 and 60, while expression in recovery fish (T3) returned to control levels by Day 60. Histopathological changes in T3 and T4 fish, including hepatocellular vacuolation, mirrored these gene expression patterns. Together, these findings demonstrate that prolonged thermal stress impairs growth and alters liver metabolism. Moreover, the identified hepatic and molecular responses represent promising biomarkers of chronic heat stress, with potential applicability to other fish species.