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

This study addressed two hypotheses: (1) that individual recognitionis used to reduce the cost of contesting resources in sea troutand (2) that the hatchery environment selects against individualrecognition. Predictions from these hypotheses were tested inlaboratory experiments where behavior and growth were studiedin juvenile sea trout. In tact groups of familiar fish werecompared with groups that contained both familiar and unfamiliarindividuals. In general, the results were in agreement withthe first hypothesis. Familiar fish in intact groups had morestable dominance ranks, higher food intake, and better utilizationof food. Familiarity also reduced the distance to the nearestneighbor. Furthermore, initiators of conflicts were more likelyto win against familiar fish than against strangers. These resultssupport game-theory-based hypotheses explaining the dear-enemyphenomenon as an effect of familiarity A picture emerges inwhich familiarity stabilizes the hierarchical structure of agroup and governs behavioral modifications that will promotefeeding and growth, in turn leading to higher fitness. The secondhypothesis, which predicted a reduced effect of individual recognitionin sea-ranched trout, was not strongly supported because familiarityaffected sea-ranched and wild trout similarly in most respects.However, familiarity was not beneficial for growth in sea-ranchedtrout, whereas it increased growth rate in wild fish. In addition,sea-ranched trout tended to maintain larger distances to theirnearest neighbors than did wild trout.