Effects of social environment and energy efficiency on preferred swim speed in a marine generalist fish, pile perch Phanerodon vacca

Energy efficiency is a key component of movement strategy for many species. In fish, optimal swimming speed (U_opt) is the speed at which the mass-specific energetic cost to move a given distance is minimised. Additional factors may, however, influence an individual's preferred swimming speed (U_pref). Activities requiring consistent sensory inputs, such as food finding, may require slower swimming speeds than U_opt. Further, while the majority of fish display some form of social behaviour, the influence of social interactions on U_pref remains unclear. It is unlikely that all fish within a group will have the same Upref, and fish may therefore compromise individual U_pref to swim with a conspecific. This study measured the U_opt, U_pref, and U_pref in the presence of a conspecific (U_pair) of pile perch Phanerodon vacca, a non-migratory coastal marine generalist. U_opt was significantly higher than, and was not correlated with, U_pref. Fish therefore chose to swim at speeds below their energetic optimum, possibly because slower swimming allows for greater awareness of surroundings. Mean U_pair was significantly lower than the U_pref of the faster fish in each pair but did not differ significantly from the U_pref of the slower fish. Therefore, faster fish appear to slow their speed to remain with a slower conspecific. Our study suggests that environmental factors, including social surroundings, may be more important than energetic efficiency for determining swim speed in P. vacca. Further studies of fish species from various habitats will be necessary to elucidate the environmental and energetic factors underpinning U_pref.