Alternative competitive strategies in juvenile Atlantic salmon (Salmo salar):: evidence from fin damage

Dorsal fin damage in salmonid fishes is primarily caused by aggression. While undesirable in fish culture, it can nevertheless be a useful tool to study social interactions in large groups of fish where it is difficult to study the behaviour of known individuals directly. We used low temperature treatment to manipulate the growth rates of juvenile Atlantic salmon (Salmo salar) in the spring and followed the development of fin damage in tagged individuals. Fin damage did not develop until mid-summer, possibly because of a qualitative change in the nature of aggressive attacks. The probability of having fin damage was strongly related to relative body size (fork length) within each group of fish: the largest fish in a tank were up to six times more likely to have damaged fins than the smallest fish. While studies of small groups of salmonids have demonstrated that subordinates are the main recipients of fin damage, the results of this study indicate that the reverse is true in larger groups. We propose that this is because dominant fish compete aggressively amongst themselves and incur fin damage, while less aggressive individuals adopt alternative feeding strategies that result in lower levels of food intake and growth, but reduce the risk of injury. Similar studies could be used to assess the success of feeding regimes in reducing the level of aggression in cultured populations. (C) 2000 Elsevier Science B.V. All rights reserved.