A multi-disciplinary effort to improve topical treatments in salmon louse control

Topical treatments of salmon lice are carried out in large sea cages or in well boats. During delousing in cages, these must be efficiently shielded with a tarpaulin. Currently either a full tarpaulin or a semi-enclosed tarpaulin is fitted on the outside of the net. A recent survey showed that the practice regarding the use of topical therapeutants varied considerably, and that suboptimal treatments were frequent. Major challenges observed were i) to safely fit and control the tarpaulin, ii) to efficientl y release the therapeutant into the enclosed volume, producing an homogenous distribution of the chemical, iii) to manage the fish behaviour in this environment to produce an efficient treatment and iv) to accurately evaluate the efficacy of treatments. W ithout further knowledge and expertise to each of these points, suboptimal treatments will continue and the force that drives the resistance development will not decrease. This project aims to optimise topical cage treatments by building a model system wh ere topical treatments in sea cages can be studied, evaluated and further developed under predictable conditions. The model will be based on physical and behavioural data from tank studies with and without fish, and will yield theoretical means to evaluat e delousing methods. The simulation model will be tested at an active production site. Well boats may be the only option for topical delousing in adverse weather conditions. This project will examine the therapeutant dispersion, physical environment and behaviour of the fish during delousing in well boats, and suggest improvements. Current evaluation of the efficiency of a treatment involves sea lice counts before and after treatment, and estimating mean abundances. This statistic requires very large sam ples at low prevalence, and is thus labour intensive. In this project, methods which estimate prevalence will be explored theoretically, and suitable counting protocols subsequently field tested.