Short- and long-term restrictions in feeding rations are commonly used in farmed fish, but the relationships between reduced feeding and social interactions in groups of fish are not fully understood.

In order to monitor food intake, aggression and the formation of hierarchies, 30 groups of Arctic charr, Salvelinus alpinus, were observed before, during and after a 14-day period of feed restriction. During the experimental period, each group of fish (weight range: 100-200 g, n = 6) was fed either 125%, 100%, 75%, 50%, 25% or 0% of the calculated food ration. Individual food intake was measured using radiography. The fish were individually tagged, and video recordings during the feeding period enabled a behavioral scanning of aggressive acts, such as nips, chases, attacks and displacements. Using The Observer Video-Pro and MatMan, a behavioral analysis of the conflict matrix within each group was conducted.

The study revealed a large variation in aggression. After the formation of hierarchies, the highest ranked fish took most of the food and achieved the highest growth rate, even if total food for the group was in surplus. Maturation was the most important factor explaining which fish become dominant - initial fish size did not affect dominance. Aggression clearly affected the level of plasma cortisol, and dominant fish had a significantly lower cortisol level compared with subordinate fish. After a period of reduced feeding, the fish on a restricted ration increased their food intake and aggression in order to compensate for the period with a low food intake.

In conclusion, the study demonstrated that aggression in farmed fish was affected by several factors, including short-term compensation and long-term life history traits.

Paper presented at Measuring Behavior 2002 , 4^th International Conference on Methods and Techniques in Behavioral Research, 27-30 August 2002, Amsterdam, The Netherlands

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