Ontogenetic thermal and metabolic patterns guide physiologically informed, size-based fishery management.

Wrasse species are widely used as cleaner fish in salmon aquaculture but their performance as cleaner fish may be impacted by thermal and ontogenetic changes. Understanding the conditions in which each species of wrasse is most productive as cleaner fish is hugely valuable to make informed decisions on the management of fisheries. To do this, we use a novel geochemical proxy to reconstruct ontogenetic trends in thermal habitat and field metabolic rate for six species of wrasse from two sampling locations from the Isle of Skye (Scotland) and Dorset (southern England). We describe consistent ontogenetic transitions from warmer to cooler thermal habitats across all species, and demonstrate reductions in activity levels associated with ontogenetic habitat shifts after accounting for body size and temperature effects. Field metabolic rates were not significantly different between Dorset and Skye after accounting for body size and temperature. Ontogenetic shifts in thermal habitat and activity level have implications for the effectiveness of wrasse as cleanerfish, as well as providing evidence to support size-based harvesting rules. More broadly, this work highlights the potential of otolith isotope analyses to recover field-based, context-specific data on physiology and thermal ecology, advancing fisheries conservation and management strategies.