Open-ocean infestation by salmon lice (Lepeophtheirus salmonis): Comparison of wild and escaped farmed Atlantic salmon (Salmo salar L.)

Abstract

Salmon lice (Lepeophtheirus salmonis) infestation was investigated in 128 salmon (Salmo salar L.) caught on floating long-lines in the Norwegian Sea during November-March 1993-1995. Overall prevalence was 99.2% and abundance 29.5 lice per salmon. Most lice were adults (90%), and 72% of these were ovigerous females. These adult lice were estimated to be at least 3 months old based on prevailing sea-surface temperatures at the sampling site: 7°C in November and 3°C in March. The prevalence and abundance of lice on one sea winter (1SW) salmon were significantly higher on escaped farmed fish than on wild salmon. However, no difference in abundance was observed between 2SW farmed and wild salmon. The average number of lice per surface area of fish (density) was significantly higher in 2SW wild salmon than in ISW wild salmon, indicating an accumulation of lice on the salmon in the oceanic phase. No differences in density were observed between 1SW and 2SW farmed salmon. The prevalence of Caligus elongatus was low (5.5%), with an abundance of 0.9 lice per salmon.