Sea lice (Copepoda: Caligidae) on wild sea trout (Salmo truttaL.) in Scotland

Abstract

Lepeophtheirus salmonis infection occurred on 30% of 622 sea trout examined from Scottish waters, with intensities up to 258 per fish. Caligus elongatus was rarer, being recorded on only 3% of trout, at a maximum intensity of 14. Geographical and temporal variations were found in the pattern of infection with L. salmonis. Most lice recorded were chalimus stages. On each fish, the same developmental stage of lice predominated, but this stage varied, even between fish caught on the same location at the same date. Haemorrhage at the base of dorsal fins infected with chalimus infection was the only skin damage found associated with lice. Analysis of the population structure of lice at two locations showed a progression of developmental stages with time, but also evidence of continual re-infection up to the end of sampling in late July. A positive relationship was found between sea trout condition and the intensity of infection with chalimus III+ larvae but a negative relationship was found with the intensity of chalimus II infection, possibly related to the length of time in, and adaptation to, sea water. The heaviest infection of sea trout with lice was recorded in the salmon farming areas of the west and north-west of Scotland. However, no clear pattern emerged with levels of infection at single locations in relation to the proximity to the nearest salmon farm or with the pattern of infection (and larval production) within an adjacent salmon farm. ? 1998 International Council for the Exploration of the Sea