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

Dorsal fin damage in salmonid fishes is primarily caused by aggression. Whi le undesirable in fish culture, it can nevertheless be a useful tool to stu dy social interactions in large groups of fish where it is difficult to stu dy the behaviour of known individuals directly. We used low temperature tre atment to manipulate the growth rates of juvenile Atlantic salmon (Salmo sa lar) in the spring and followed the development of fin damage in tagged ind ividuals. Fin damage did not develop until mid-summer, possibly because of a qualitative change in the nature of aggressive attacks. The probability o f 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 time s 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 reve rse is true in larger groups. We propose that this is because dominant fish compete aggressively amongst themselves and incur fin damage, while less a ggressive individuals adopt alternative feeding strategies that result in l ower levels of food intake and growth, but reduce the risk of injury. Simil ar studies could be used to assess the success of feeding regimes in reduci ng the level of aggression in cultured populations. (C) 2000 Elsevier Scien ce B.V. All rights reserved.