Size-dependent thermal limits in Australian hybrid abalone: implications for productivity shifts with ocean warming

Ocean warming and extreme heatwaves threaten marine species supporting commercial fisheries and aquaculture. Predicting the responses of these industries to chronic and acute warming depends on understanding which life stages are most vulnerable, the potential for stocks to adapt to changing thermal environments, and the availability of thermally adapted genotypes to help enhance stock resilience through strategic interventions. Here, we shed light on some of these knowledge gaps by quantifying the critical thermal maximum (CTmax) of similar to 10-210 g hybrid abalone (Haliotis rubra x H. laevigata) from two farms representing contrasting thermal environments from south-eastern Australia. CTmax was not dependent on body size or provenance (farm) when heating rates were rapid (1 degrees C per h), but a significant relationship between CTmax and body size was observed when heating rates were slower and more ecologically realistic (1 degrees C per 12 h). Histological analyses revealed a negative relationship between CTmax and the stage of gonadal development when abalone were exposed to chronic thermal stress conditions. These results suggest that marine heatwaves and ongoing ocean warming might favour smaller, less fecund animals in natural and farm settings. This could potentially impact future harvestable biomass, recruitment and population dynamics in wild-capture fisheries, and production of larger, high-value animals in farm settings. This study adds to a growing body of literature pointing to complex and often negative effects of climate change on commercial fisheries, and the potential need for interventions aimed at bolstering fisheries resilience against the effects of ocean warming.