1. Introduction
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In the wild, Atlantic salmon Salmo salar spend at least a year and usually more in fresh water, where they are territorial and engage in frequent bouts of agonistic
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behaviour Kalleberg, 1958; Keenleyside and Yamamoto, 1962 . In aquaculture, densi- ties are much higher than in the wild and thus, the physical damage caused by
aggression is more frequent and severe: so much so that fin condition can be used to Ž
. distinguish between farmed fish and wild stocks e.g., Bosakowski and Wagner, 1994 .
Aggression has a negative impact on growth and welfare in farmed fish, as it can result in physical damage, which may lead to secondary infections with pathogens such as
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Aeromonas salmonicida Schneider and Nicholson, 1980; Turnbull et al., 1996 . Inequal- ities in food intake resulting from social hierarchies maintained by aggression can lead
Ž to growth depensation where initial small size differences within a group become more
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pronounced as time goes on Jobling, 1985; Jobling and Wandsvik, 1983 . Social
subordination is also associated with chronic stress, which can have detrimental effects Ž
. on health and growth Schreck et al., 1997; Wedemeyer, 1997 .
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In small groups of fish fewer than 20 , there are usually pronounced social
hierarchies dominated by one or two aggressive individuals that monopolise the food supply and reduce the feeding activity and thereby the growth of their social subordi-
Ž nates Jobling and Wandsvik, 1983; Koebele, 1985; Huntingford et al., 1993; Adams et
. al., 1998 . Dominant fish perform more aggressive acts than subordinates, and the
subordinates usually receive more aggressive nips and exhibit more fin damage than Ž
dominants Fenderson and Carpenter, 1971; Abbott and Dill, 1989; Gregory and
. Griffith, 1996; Moutou et al., 1998 . However, the conclusions reached from such
studies may not necessarily hold true for larger groups of fish. For instance, in paired Ž
. encounters between juvenile Arctic charr SalÕelinus alpinus , the more aggressive fish
of the two usually acquired more food, but in culture conditions the same individuals Ž
were no more likely to grow well than their subordinate partners Adams and Hunting- .
ford, 1996 . This may be because the social hierarchy is less stable in larger groups Ž
. Fenderson and Carpenter, 1971 . A difficulty in studying aggressive interactions in
culture conditions is the large number of fish involved. This makes it practically impossible to observe the behaviour of known individuals. It is here that the damage
caused by aggressive behaviour can be used as an indicator to shed light on the subject. The best-known physical damage caused by aggression is inflicted on the fins and is
termed fin damage, fin erosion or fin rot. These terms cover a range of symptoms including splitting of the fin rays, tissue loss and pale nodular thickening of the distal
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portion of the fin Turnbull et al., 1996; Winfree et al., 1998 . While other factors can be Ž
. involved see Winfree et al., 1998, for a summary , there is little doubt that the principal
cause of fin erosion in farmed salmonids, especially when it occurs on the dorsal fin, is Ž
aggressive behaviour. In paired encounters between steelhead trout Oncorhynchus
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Abbott and Dill, 1985 , and in small groups of Atlantic salmon Turnbull et al., .
1998 , the dorsal fin was attacked more frequently and incurred more damage than other parts of the body. Fish that had damaged dorsal fins showed immediate improvement in
fin condition when placed in isolation, while injuries similar to fin rot could be produced by simulating bites with the head of a dead salmon parr, but not by other means
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Turnbull, 1992 . Most importantly, scanning electron micrographs of damaged fins from fish-farms showed clear tooth marks and an absence of bacterial infection
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Turnbull et al., 1996 . Fin damage has been used as an indicator of the strength of the social hierarchy by
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Christiansen and Jobling 1990 and Moutou et al. 1998 , providing useful insights into the dynamics of aggression within larger groups of fish than can easily be studied
otherwise. Fin splitting is the primary symptom of fin damage, and repeated splitting eventually leads to tissue loss and thickening of the remaining tissue. Splitting heals
Ž rapidly, whereas re-growth and reduction in thickening take longer to occur Turnbull,
. 1992 . Therefore, splitting is likely to be the best indicator of current levels of
aggression. The aim of this study is to examine the effect of body size on the incidence of fin
damage in large groups of fish kept under culture conditions. Using data from individu- ally marked fish previously subjected to different manipulations of growth rates, we are
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able to compare the effects of both relative to other group members and absolute body size on the timing and duration of fin damage. We demonstrate a strong and consistent
effect of relative body size, which indicates the existence of alternative strategies of aggression and feeding within groups of fish.
2. Materials and methods
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