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

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Abstract
This study addressed two hypotheses: (1) that individual recognition is used to reduce the cost of contesting resources in sea trout and (2) that the hatchery environment selects against individual recognition. Predictions from these hypotheses were tested in laboratory experiments where behavior and growth were studied in juvenile sea trout. In tact groups of familiar fish were compared with groups that contained both familiar and unfamiliar individuals. In general, the results were in agreement with the first hypothesis. Familiar fish in intact groups had more stable dominance ranks, higher food intake, and better utilization of food. Familiarity also reduced the distance to the nearest neighbor. Furthermore, initiators of conflicts were more likely to win against familiar fish than against strangers. These results support game-theory-based hypotheses explaining the dear-enemy phenomenon as an effect of familiarity A picture emerges in which familiarity stabilizes the hierarchical structure of a group and governs behavioral modifications that will promote feeding and growth, in turn leading to higher fitness. The second hypothesis, which predicted a reduced effect of individual recognition in sea-ranched trout, was not strongly supported because familiarity affected sea-ranched and wild trout similarly in most respects. However, familiarity was not beneficial for growth in sea-ranched trout, whereas it increased growth rate in wild fish. In addition, sea-ranched trout tended to maintain larger distances to their nearest neighbors than did wild trout.

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