Phenotypic divergence of sand flathead (Platycephalus bassensis) between heavily and lightly fished regions in Tasmania, Australia.

Abstract

Fishing is one of the most sustained forms of human-wildlife interaction and can alter trait distributions through selective harvest and repeated disturbance. Such changes, whether plastic or evolutionary, may alter productivity, resilience, and recovery in exploited species. The sand flathead (Platycephalus bassensis), a benthic ambush predator with strong site fidelity, supports lutruwita (Tasmania's) largest recreational fishery and is exposed to contrasting levels of fishing pressure across its range. In southern Tasmania, fishing mortality exceeds natural mortality more than fivefold and biomass has fallen below 20% of unfished levels, while northern regions remain comparatively lightly fished. This regional contrast offers a natural setting in which to investigate whether sustained harvest is associated with regional differences in physiology and behaviour, and whether such variation is more consistent with fishing pressure, environmental conditions, or their interaction. We compared mass-specific metabolic rate, boldness, and size-at-age between sand flathead from heavily and lightly fished regions. Metabolic rate was measured using intermittent flow-through respirometry, and boldness was quantified in a shuttlebox based on exploration latency and bait strikes. Fish from the heavily fished south exhibited smaller size-at-age, a 62% higher mean metabolic rate, and a transient post-capture elevated metabolic rate consistent with greater metabolic reactivity or stress responsiveness, whereas boldness did not differ between regions. Our findings align with other exploited systems and raise the possibility that trait diversity of sand flathead in southern regions of Tasmania have been shaped, at least in part, by fisheries selection. We discuss the relevance of these results for fisheries management and emphasize the importance of assessing trait variation in wild populations, where expression is likely shaped by the interactive effects of fishing pressure and local ecological conditions.