Combining different treatments to delay resistance in controlling salmon lice in fish farms

We investigate various approaches to reduce the problem of salmon louse for the salmon farming industry. Salmon lice are parasites that can be spread through water from one fish farm to wild fish and to neighbouring fish farms. Various measures have been used to control salmon lice in fish farms, but frequent use of the same type of treatment may cause resistance, especially for medicinal treatments and possibly for non-medicinal treatments (e.g. freshwater).
Our work focuses on resistance development caused by genetic selection. This means that some genotypes of lice have a higher probability of surviving a given treatment than other lice, and consequently the proportion of lice of resistant genotypes tends to increase by using that kind of treatment.
We consider treatment policies involving two types of treatments with similar efficacy properties, but with independent selection mechanisms. The investigation is done by simulating the population of lice in neighbouring fish farms using a population model for lice. When the density of lice on a small sample of fish exceeds a given threshold, a treatment, which type depends on the strategy, is applied. For each simulation we record the number of required treatments and the levels of resistance. Regarding delaying resistance our results suggest that applying the two treatment types in combination is more effective than any of the considered strategies with separate use of the two types.
This is joint work with researchers at the Norwegian Veterinary Institute, Aqualife R&D, Pharmac and the Norwegian University of Life Sciences.