Aggressive interactions in pure and mixed groups of juvenile farmed and hatchery-reared wild Atlantic salmon <i>Salmo salar</i> L. in relation to tank substrate

Abstract

Total aggression and individual behavioural traits of equal-sized juveniles of farmed Atlantic salmon Salmo salar L. selected for five generations in the Norwegian Breeding Programme for Salmonids and a wild strain originating from the River Rauma, Norway, were compared in pure (10 fish) and mixed (5 + 5 fish) groups. Total aggression was defined as the sum of the following five offensive behaviour patterns: intentional movements, lateral display, frontal display, charge and bite. Experiments were carried out in 2-m2 tanks with either a grey fibreglass substrate (pure and mixed groups) or diagonally divided into a fibreglass and river cobble substrate (mixed groups only). In mixed groups kept on a fibreglass substrate, the aggressive behaviour directed by wild salmon towards farmed salmon was more frequent than the reverse (84.2 and 27.0 aggressive acts h–1, respectively, P < 0.025). Total aggression was not significantly different between wild and farmed juveniles in tanks with divided substrate. The aggressive behaviour of wild and farmed salmon was significantly increased when kept in tanks with fibreglass substrates (437%, P < 0.005, and 296%, P < 0.05 respectively). Pure groups of either wild or farmed salmon kept on a fibreglass substrate exhibited similar frequencies of total aggression (85.7 and 101.6 0 aggressive acts h–1, respectively, P > 0.1) and single behaviour patterns. The aggressive behaviour of farmed offspring was not influenced by vertical distribution in the tanks with a fibreglass substrate. Farmed salmon tended to position pelagically (78%) and used the water column more frequently than wild salmon (19%), whereas wild salmon hid more. It is concluded that social interactions among groups of juvenile Atlantic salmon are influenced by substrate.