An increasing number of temperate marine ecosystems globally have become regions of tropicalisation, example being the seasonal displacement of coral reef fishes from natal tropical ecosystems in SE Australia marine habitats. This arrival of low-density populations of juvenile tropical fish (termed ‘vagrant’ fishes) results in changes to local fish communities, as vagrant fish may shoal with temperate fish. Previous research has shown that the vagrant tropical damselfish Abudefduf vaigiensis benefited from shoaling with temperate heterospecifics through the mitigation of predation risk, allowing increased foraging opportunities and attaining larger body sizes. This study aims to compare how varying shoaling compositions and ambient water temperature influence the physiology and behaviour of A. vaigiensis and how this impacts their fitness and survival. Results show that somatic growth is strongly associated with increasing temperature, and that a greater presence of A. vaigiensis in a mixed shoal impacts the growth rates of a heterospecific. Results also indicate that novel predators can induce risk-aversive behaviours that compromise feeding rates, as we observed reduced boldness when exposed to a live predator. Shoaling behaviours may facilitate the range expansion potential of tropical fishes; therefore, it is imperative that we better understand how they function in novel ecosystems.