The use of a general linear model to identify epidemiological factors affecting the abundance of chalimus stages of the sea louse (Lepeophtheirus salmonis) on Scottish salmon farms

In this communication chalimus lice numbers in the second, third and fourth quarters of the production cycle are examined in relation to a range of prevailing risk factors using an approach based on that already taken for mobile stages (Revie et al. in press). A total of 54 two-year production cycles from 29 farms on the West Coast of Scotland were analysed. Chalimus levels in the second six-month period of the first year of the production cycle were principally explained (adjusted R2 = 56% of the variation) by:the number of veterinary medicine treatments administered, the site hydrography and current speed characteristics. Sites with a high chalimus abundance attracted more treatments as did fish located in sites with low current speeds and an oscillatory hydrography. In contrast, chalimus levels in the third six-month period of the production cycle were principally explained (adjusted R2 = 65% of the variation) by: the abundance of mobile and chalimus stages in the preceding six-month period and treatment. It would appear that there are both management and environmental factors which affect the number of chalimus sea lice on fish in Scottish salmon farms and these have an impact on overall sea lice abundance.