Economically optimal management of salmon louse requires adapting to their drug-resistance rather than attempting their eradication

The growing global demand for seafood and concerns about overfishing have spurred the rapid expansion of aquaculture. In aquaculture, managing diseases and parasites presents a critical problem, with drug-based solutions being increasingly challenged by the evolution of drug resistance. In this study, we focus on managing salmon louse in the context of open-cage salmon mariculture with potential for the evolution of drug resistance. We devise a model combining parasite dynamics and fish dynamics in a system of fish farms connected to each other by dispersive stages of the parasite and then evaluate the system-wide economic performance of different management strategies involving three parasite-control measures: drug treatment (administering medicine through fish feed), mechanical treatment (pumping fish through a system of water jets and/or soft brushes), and depopulation (emptying a whole farm prematurely). Drug treatment controls drug-sensitive lice at low cost but becomes ineffective in the presence of drug-resistant lice. Mechanical treatment can clear both types of lice but at the cost of diminished fish growth and additional fish mortality. Depopulation removes both the fish and the parasites within the farm but results in prematurely harvested fish that fetch a lower price. Our results suggest that even when the drug is used only once per production cycle and mechanical treatment and depopulation provide the main control of the parasite, the spread of drug resistance is unavoidable in an open-cage system. Furthermore, it is often not economically optimal to drive resistance to the lowest possible level by minimizing drug use: because resistant lice are assumed to have a slightly reduced fecundity, slightly fewer non-drug treatments are needed for controlling drug-resistant parasites than drug-sensitive parasites. Building on these insights, our model predicts that economically optimal parasite management in the presence of drug resistance combines all three parasite-control measures: mechanical treatment is the main measure to reduce louse infestations, depopulation allows shorter production cycles that become optimal under reduced salmon growth and survival that result from frequent mechanical treatments, and the drug is used not only to provide some parasite control but also to keep the resistant parasites prevalent. Our results thus underscore the need for effective parasite management strategies in salmon aquaculture accounting for the unavoidable prevalence of drug resistance. Notably, the economically optimal approach does not involve combating resistance but rather adapting to it and capitalizing on its positive effects.