The effects of environmental complexity upon the aggressive behaviour and growth of juvenile brown trout (Salmo trutta L.) and the carrying capacity of the environment

Abstract

Experiments were performed in stream tanks and aquaris to investigate the effects of environmental complexity upon the aggressive behaviour and growth of juvenile brown trout (Salmo trutta L.) and the carrying capacity of the environment. Natural materials and various structures were used to provide environmental complexity. Aggressive interactions were found to be significantly more frequent in simple environments than in complex ones. The different types of structures used to modify the complexity of the experimental environments had different effects upon the observed levels of aggressive behavioral structures providing overhead cover were associated with the lowest levels of aggression. Increasing fish density resulted in relatively greater increase in aggression in simple environment. The wigwag display of the fish was analysed and found to be a highly ritualised, defensive manoeuvre, the efficiency of which was determined by the social ranks of the fish involved. Carrying capacity of complex environments was significantly greater then that of simple environments. There was a significant inverse correlation between aggression and resident population size in the environment types used. In the growth experiment, different relationships between individual fish's specific growth rates and their size rank position were found in populations of fish kept in simple and complex environments. The differences were probably associated with the different levels of aggression observed in the two environment types and the effect of environmental complexity upon feeding behaviour.