Overwintering performance of juvenile temperate estuarine fish

Abstract Estuaries, rich in biodiversity and economically valuable species, are increasingly threatened by climate change-induced factors that challenge fish resilience and survival. This study compared the performance of estuarine fishes between water temperatures reflecting two scenarios: current Sydney winters (16°C) and future winters under climate change (20°C), and at two food levels, for three estuarine fish species (eastern fortescue, Centropogon australis , common silverbiddy, Gerres subfasciatus, and eastern striped trumpeter, Pelates sexlineatus ) Overall, as expected from metabolic theory, fish performance was generally higher at higher temperatures, with growth rates higher at 20°C for G. subfasciatus and C. australis. Bite rates and aerobic scope were generally higher at the higher temperature for all species. G. subfasciatus and P. sexlineatus exhibited increased escape responses at 20°C, with P. sexlineatus also showing greater boldness. Boldness was positively associated with bite rates in P. sexlineatus , potentially indicating foraging advantages under future warming for this species. The order of temperature treatment (20°C then 16°C, vs 16°C then 20°C) affected boldness for G. subfasciatus and growth rate, total length, bite rate and burst speed for P. sexlineatus . Contrary to expectations, food had no effect on fish performance either directly or interacting with temperature, and all three species generally performed better at 20°C than 16°C, suggesting this study was conducted below the species’ thermal optima. Future climate change may therefore favour temperate estuarine fishes at winter temperatures, with potential benefits differing among these species.