Temperate predators and seasonal water temperatures impact feeding of a range expanding tropical fish

Range expansions of species due to climate change threaten the function and composition of marine ecosystems globally, yet factors constraining or facilitating this redistribution are poorly understood. Native predators may constrain prey shifting poleward through consumption, or by restricting their feeding activity. However, the extent that native predators impact range-expanders will likely be structured by physiological mismatches between these groups, associated with water temperatures. We examined how temperate predators and seasonal water temperature affected foraging of the planktivorous tropical reef fish, Abudefduf vaigiensis, within temperate southeastern Australian waters, an emerging new range. Foraging excursions of A. vaigiensis were compared between predator-rich marine reserves and predator-depauperate fished reefs during summer and winter (~18 and 22 °C water, respectively). A. vaigiensis foraged with shorter excursions in marine reserves than fished reefs and higher excursions during summer than winter. Effects of predation risk and water temperature on A. vaigiensis foraging were isolated in an aquarium experiment. Groups were held at 18 or 22 °C and visually exposed to a temperate predator, a predator control (temperate herbivore) and an empty tank. Foraging excursions and feeding rates were reduced when exposed to predators at 22 °C, but did not differ between predator and the predator control or empty tank at 18 °C. Results suggest temperate predators may restrict range expansions of A. vaigiensis by reducing its’ food intake during summer months. But winter water temperatures may limit feeding, independent of predation risk. Protection of predators from fishing should improve resistance of some marine ecosystems to impacts of range expanding prey.