Overwintering performance of three juvenile temperate estuarine fishes

Abstract

Estuaries, rich in biodiversity and economically valuable fish species, are increasingly threatened by climate change-induced factors that challenge fish resilience and survival. This study compared the juvenile performance of three estuarine fish species (eastern fortescue, Centropogon australis, common silverbiddy, Gerres subfasciatus, and eastern striped trumpeter, Pelates sexlineatus) at two water temperatures reflecting two scenarios: current Sydney winters (16 °C) and future winters under climate change (20 °C). The fishes' total range of acclimation duration was 16 days at each temperature level, and the experiment included two food levels (low and high), along with two different treatments that accounted for the orders of temperature exposure (20 °C then 16 °C, vs 16 °C then 20 °C). We used a broad range of behavioural and physiological metrics such as growth (change in mass and length), metabolic rate, bite rate, burst speed, boldness, shelter and predator escape response. Overall, as expected from metabolic theory, fish performance was generally greater at higher (future) winter temperatures than at lower (current) winter temperatures, with growth rates, change in total length, bite rate and aerobic scope higher at 20 °C than 16 °C 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 affected boldness for G. subfasciatus and growth, bite rate and burst speed for P. sexlineatus. Overall, and surprisingly, performance metrics were largely unrelated.

Contrary to expectations, food had no effect on fish performance either directly or interacting with temperature. Our findings suggest this study was conducted below the species' thermal optima, and future climate change may therefore favour temperate juvenile estuarine fishes at winter temperatures, with potential benefits differing among these species.