<b>Thermal generalism shapes tolerance to extreme heatwaves in estuarine fishes</b>

We then experimentally exposed two species from low- and high-environmental variability estuaries to moderate (+2 °C, 14 days) or extreme (+4 °C, 7 days) simulated heatwave events, assessing twelve physiological and behavioural proxies. All three species collected in situ maintained comparable physiological performance across sites representing contrasting thermal variability. Both experimental species behaviourally and physiologically tolerated the moderate and extreme laboratory-simulated heatwave conditions regardless of thermal history, supporting species-level thermal generalism rather than population-specific acclimation.

In the absence of heatwave conditions, <i>S. ciliata</i> from high-variability sites also showed steeper allometric scaling and faster growth at 26 vs 24 °C experimental control temperatures, suggesting this species may benefit from the warmer baseline temperatures projected under gradual climate change. Together, our findings support thermal generalism as the primary driver of tolerance in estuarine fish to climatic extremes, with important implications for the fisheries they support under climate change.</p>