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Infestation patterns of the sea louse Lepeophtheirus salmonis from 44 salmon farms in the Hardangerfjord on the south-west coast of Norway over the period 2004 to 2007 were assimilated to create 20-month production cycle profiles for spring and autumn stocked generations. The timing and frequency of in-feed and bath treatments to control sea lice associated with these profiles was considered. Spring and autumn stocked farms were observed to have different patterns of sea lice counts on salmon during the first and second years of production. Spring stocked sites experienced increasing infestation toward the end of the first year and on average counts remained elevated thereafter, whereas autumn stocked sites averaged lower sea lice counts throughout most of the production cycle until the latter part of the second year when these escalated rapidly. In-feed treatments were the predominant form of sea lice control in the first half of the production cycle on spring stocked farms, whereas bath treatments were used exclusively in the second half of the production cycle; a very similar pattern of therapeutant use was observed on autumn stocked farms. Results using the SLiDESim (Sea Lice Difference Equation Simulation) infection model and a range of biological and production parameters showed that modelling resulted in a better fit to the mobile lice profiles for autumn stocked farms compared to spring stocked farms. Some features of the mobile lice profiles were not captured by the infection model such as the oscillation of the population between months 11 and 18 of the production cycle on spring stocked farms, and a large peak observed in month 19 on autumn stocked farms. Before modelling can be used to evaluate optimal treatment strategies or other management interventions there remains a need to more clearly understand the underlying biological processes associated with the dynamics of sea lice infestations in the Hardangerfjord.

At a time when sea lice control is a major issue for salmon aquaculture worldwide it has become imperative that scant veterinary medicinal resources for the treatment of fish on farms should be conserved and used effectively. This communication reports the use of the mathematical simulation model SLiDESim to investigate how best to administer cypermethrin bath treatments on Scottish salmon (Salmon salar L.) farms to control the challenge from lice (Lepeophtheirus salmonis) during a two-year production cycle. It was found that these topical treatments are most effective when administered in pairs approximately six weeks apart. Timing of treatment is critical and depends on the number of treatments administered over the production cycle. For 4, 5 or 6 treatments during a two-year production cycle, SLiDESim indicated that the first pair of treatments is best initiated in autumn of the first year of production with the second pair starting between 13 and 18 weeks later. This strategy can produce considerable gains in the predicted reduction of sea lice levels when compared with those historically observed when using cypermethrin on Scottish farms. The effects of altered efficacy were also explored using the model and indicate that even a moderate reduction in treatment efficacy can have considerable impacts on lice control over a production cycle. The SLiDESim computer model provides a framework to explore the more efficient use of veterinary treatments for the control of sea lice on salmon farms.