Growth hormone increases aggressive behavior in juvenile rainbow trout.

The aim of the present study was to clarify the role of growth hormone in social interactions in juvenile salmonids. Growth hormone increases the metabolic demands and feeding motivation in teleost fish. As a consequence, growth hormone may increase aggression levels and/or fighting ability. To test these hypotheses we observed agonistic behavior in pairs of juvenile rainbow trout (Oncorhynchus mykiss) consisting of two control fish (C/C pairs), two growth hormone-treated fish (GH/GH pairs), or one growth hormone-treated and one control fish (C/GH pairs). The initiator and the winner of each act of aggression were registered. Aggression was lowest in the C/C pairs, intermediate in the C/GH pairs, and highest in the GH/GH pairs, with the difference between the C/C pairs and the GH/GH pairs being significant. This supports the hypothesis that GH increases aggression levels. However, in the C/GH pairs, the number of conflicts won by GH-treated and control fish did not differ significantly. Thus, because social status was not increased, GH did not appear to affect fighting ability. We suggest that growth hormone affects aggression indirectly by increasing the swimming activity, and/or by inducing defense of a larger territory, thereby increasing the encounter rate between opponents. Since increased aggression can incur energetic and mortality costs, there may be selection against high GH levels in natural populations.