Agonistic behavior and growth in newly emerged brown trout (Salmo trutta L) of sea‐ranched and wild origin

Selection experiments and game theory models have revealed that the changes in agonistic behavior following selection for rapid growth rate of fish depend on the access of food. If food is spatially restricted and in excess of demand, the intensity of agonistic behaviors will decrease. This prediction was tested in an experiment on wild and sea‐ranched brown trout (Salmo trutta L.) fry originating from a common stock. Agonistic behavior, activity, and specific growth rate were studied in tanks (56 × 56 cm) at high (159.4 fry/m²) and low (9.6 fry/m²) stocking densities given either a large or small food ration (3.0% and 1.5%, respectively, food per total body weight and day). Observations were done during 4 days in each trial. Generally, fry of sea‐ranched origin had a higher growth rate. No differences in activity were found. Intensity of agonistic behavior was higher among wild groups. There was a tendency for interaction between density and strain, e.g., wild fish were relatively more aggressive at higher densities. Food ration had no effect on level of agonistic behavior. These results indicate that the selection for rapid growth in hatcheries may indirectly select for reduced aggressiveness. Genetic integrity and diversity of wild populations may be threatened when interbreeding occurs between wild and hatchery fish. Aggr. Behav. 28:145–153, 2002. © 2002 Wiley‐Liss, Inc.