The importance of being familiar: individual recognition and social behavior in sea trout (Salmo trutta)

This study addressed two hypotheses: (1) that individual recognition is use d to reduce the cost of contesting resources in sea trout and (2) that the hatchery environment selects against individual recognition. Predictions fr om these hypotheses were tested in laboratory experiments where behavior an d growth were studied in juvenile sea trout. Intact groups of familiar fish were compared with groups that contained both familiar and unfamiliar indi viduals. In general, the results were in agreement with the first hypothesi s. Familiar fish in intact groups had more stable dominance ranks, higher f ood intake, and better utilization of food. Familiarity also reduced the di stance to the nearest neighbor. Furthermore, initiators of conflicts were m ore likely to win against familiar fish than against strangers. These resul ts support game-theory-based hypotheses explaining the dear-enemy phenomeno n as an effect of familiarity. A picture emerges in which familiarity stabi lizes the hierarchical structure of a group and governs behavioral modifica tions that will promote feeding and growth, in turn leading to higher fitne ss. The second hypothesis, which predicted a reduced effect of individual r ecognition in sea-ranched trout, was not strongly supported because familia rity affected sea-ranched and wild trout similarly in most respects. Howeve r, familiarity was not beneficial for growth in sea-ranched trout, whereas it increased growth rate in wild fish. In addition, sea-ranched trout tende d to maintain larger distances to their nearest neighbors than did wild tro ut.