Attack site and resultant damage during aggressive encounters in Atlantic salmon (Salmo salar L.) parr

Fin rot is a term used to describe a range of changes in fins, from splitting and erosion to nodular thickening. It is common among farmed Atlantic salmon (Salmo salar) parr and is of considerable economic and welfare importance. Fin rot is known to commence when fins become damaged, for example, following attacks by conspecifics; however, not all fins are equally affected, the dorsal fin being much more frequently damaged. This study examined the hypothesis that fin rot is more prevalent in the dorsal fin because it is more frequently attacked during aggressive encounters with conspecifics. Behavioural observations showed that the dorsal and caudal fin areas are much more frequently attacked than other areas of the body and significantly more attacks directed at the dorsal fin area resulted in physical contact than attacks directed at other parts of the body. Fin damage, scored as the total amount of fin splitting was highest in the dorsal and pectoral fins despite the pectoral fins being attacked less frequently than the dorsal or caudal fins. This suggests that pectoral fins may have sustained damage through contact with the tank as well as through attacks. We conclude that at least one reason for the observed prevalence of fin rot in the dorsal fin of Atlantic salmon in aquaculture systems is that it is a preferred site of attack by conspecifics and because attacks are more likely to end in physical contact than attacks on; other areas of the body and, thus, they sustain more damage. (C) 1998 Elsevier Science B.V.