Aspects of dorsal fin rot in farmed Atlantic salmon parr were investigated and the associated pathology described. Substantial evidence was obtained which suggested that the condition was caused and maintained by repeated bites from other parr. The gross histological and scanning electron microscopic appearance of the lesions were consistent with bite wounds and the typical pathology was reproduced by repeated simulated bites. During behavioural
experiments the parr were observed to both bite and damage each others dorsal fins. The typical nodular lesions were more prevalent and took longer to heal at lower temperatures. Dorsal fin rot was found to occur in the absence of damage to the other fins and was more severe in smaller fish. The bacteria associated with the natural lesions and following controlled damage were studied, but not found to play a significant role in the aetiology. They were capable of neither initiating nor maintaining the lesions. The main site of bacterial colonisation appeared to be on exposed fin rays. It was demonstrated that the natural lesions started to resolve as soon as the fish were placed in isolation. A limited study failed to demonstrate a definite relationship between dorsal fin rot and increased susceptibility to Aeromonas salmonicida infection. The implications of all these findings for control of the condition are discussed.