Thermal tolerance in juvenile King George whiting (Sillaginodes punctata) reduces as fish age and this reduction coincides with migration to deeper colder water.

Heat shock proteins (HSPs) are sensitive and readily produced under thermal stress in many fish species and thus serve as a useful stress bio-indicator. Two experiments were conducted to test the hypothesis that King George whiting (KGW) Sillaginodes punctata approaching sexual maturity exhibits a decrease in HSP production and that exposure to high temperatures provokes HSP production in juvenile whiting. Both adult and juvenile whiting expressed significant increases in HSP69 in response to temperature shocks of 24, 26, 28 and 30 °C. Juvenile whiting had significantly higher HSP69 than adults and expressed more HSP69 at 24 and 26 °C. No mortalities were observed in juvenile fish at 30 °C while 50% of adults suffered mortality at 30 °C. Following exposure of juveniles to 24, 26 and 28 °C, HSP69 was measured at 24, 96 and 168 h. HSP69 peaked at 96 h and returned to the 24h level after 168 h exposure. This study indicates that juveniles can cope with high temperatures better than adults, which offers a partial explanation to fish movement patterns in nature where younger fish inhabit near shore waters and then migrate to deep water towards maturation. Further, this work implies that KGW growth and recruitment can be affected by increasing temperatures due to global warming.