Familiarity confers anti‐predator and foraging advantages on juvenile brown trout

Many fish species preferentially associate with familiar conspecifics rather than unfamiliar individuals with whom they have no previous experience. The function of discriminatory ability, however, is not well documented. In territorial fish, observations of lower aggression levels among pairs of familiar neighbours lead to two predictions. First, that increased time available for vigilance among familiar fish will allow a faster response to a predator attack, and second that after an attack familiar fish resume foraging more quickly than unfamiliar fish. We conducted an experiment to test these hypotheses using wild juvenile brown trout from Sweden. Our results confirm hypothesis one by demonstrating that familiar brown trout juveniles respond 16% more quickly to a model predator than individuals held in unfamiliar groups. Hypothesis two, that familiar fish resume feeding more quickly after a simulated predator attack was not supported. Nevertheless familiar fish consume a greater number of food items than fish in unfamiliar groups. To the best of our knowledge our results provide the first evidence that familiarity confers advantages through the direct fitness benefits afforded by faster predator evasion responses, and through the long‐term benefits provided by increased feeding opportunities. We discuss the possibility that these advantages promote the formation of stable association patterns and improve growth and survival in wild populations.