The salmon farming industry faces a persistent challenge: the salmon louse (Lepeophtheirus salmonis), a parasite with a significant economic impact. For decades, antiparasitic drugs have been a key tool, but their effectiveness is decreasing due to the evolution of resistance.
A team of researchers from the Okinawa Institute of Science and Technology Graduate University (OIST), the Université de Pau et des Pays de l’Adour (UPPA), the International Institute for Applied Systems Analysis, The Graduate University for Advanced Studies (Sokendai), and the University of Bergen published a study in the journal Aquaculture that models the complex interaction between salmon louse dynamics, salmon production, and economics, evaluating management strategies combining pharmacological and mechanical treatments, and depopulation in open-pen systems.
The results suggest a paradigm shift: instead of fighting to eradicate resistance, the most economically viable strategy might be to adapt to it and even capitalize on some of its effects.
The Growing Problem of Drug Resistance in Salmon Farming
The intensification of aquaculture has exacerbated problems such as diseases and parasitosis. The salmon louse is particularly problematic in open-pen mariculture systems, where parasite dispersal between farms is almost inevitable.
Furthermore, the continued use of drugs creates strong selective pressure favoring the evolution and spread of resistant lice, causing treatments to rapidly lose effectiveness. This situation necessitates seeking and combining alternative management options.
Evaluating Control Strategies: A Bioeconomic Approach
To find economically viable solutions, the researchers developed a bioeconomic model simulating a system of salmon farms interconnected by louse dispersal. The model considers:
- Fish Dynamics: Growth, natural mortality, and the negative effects of the parasite and treatments on both.
- Parasite Life Cycle: Includes eggs, juveniles, and adults, with separate dynamics for drug-sensitive and drug-resistant lice. Resistance is assumed to carry a cost for the parasite, specifically lower fecundity.
- Control Measures
- Pharmacological Treatment: Administration of medication (e.g., emamectin benzoate) via feed. Low cost against sensitive lice, but ineffective against resistant ones.
- Mechanical Treatment: Physical removal of lice (e.g., water jets, brushes). Effective against both types, but costly, stressful for fish, reduces growth, and increases mortality.
- Depopulation: Premature emptying of the farm. Removes fish and parasites but involves harvesting fish below optimal weight, reducing the sale price.
- Production Economics: Costs of smolts, feed, treatments, harvesting, and revenue from fish sales (considering penalties for low weight).
The study’s objective was to identify the combination of treatment and depopulation thresholds that maximizes the system’s long-term profitability.
Key Results: The Inevitability of Resistance
The study yields important conclusions for salmon louse management:
Resistance is Inevitable in Open Pens
Even limiting drug use to once per production cycle (to assess its effectiveness) and relying primarily on mechanical treatments and depopulation, the spread of resistant lice cannot be avoided in open systems.
Minimizing Resistance Is Not Always the Most Profitable
Contrary to intuition, strategies actively seeking to minimize the proportion of resistant lice (through minimal drug use) do not turn out to be the most profitable.
Why? Because of the cost of resistance. If resistant lice have a biological disadvantage (lower fecundity, as assumed in the model), fewer non-pharmacological (and costly) treatments are needed to control them compared to sensitive lice.
The Optimal Strategy Combines All Three Measures
The model predicts that economically optimal management integrates the three sea louse control tools, each with a specific role:
- Mechanical Treatment: The primary measure for reducing active infestations.
- Depopulation (threshold-based): Allows optimizing production cycle duration. Shorter cycles become optimal when growth and survival are reduced by frequent mechanical treatments. Using depopulation as the main control measure results in high resistance and low economic performance.
- Pharmacological Treatment: Used not only for some control (especially early or when resistance is low) but strategically to maintain a prevalence of resistant lice. This allows ‘leveraging’ their lower fecundity and reducing the need for more expensive mechanical treatments.
Adapting vs. Fighting Resistance
The most profitable approach is not to fight resistance at all costs, but to adapt to its inevitable presence and, in fact, capitalize on its biological cost (lower fecundity). Maximum profitability is often achieved when the proportion of resistant lice is significant (e.g., around 50% in the model’s base scenario), not when it is minimal.
The Animal Welfare Factor
The optimal management predicted by the model relies heavily on mechanical treatments, which raise animal welfare concerns due to the stress, reduced growth, and increased mortality they cause.
The study shows that imposing restrictions to improve welfare (e.g., limiting the frequency of mechanical treatments or requiring a higher survival rate) has a considerable economic cost, significantly reducing profitability.
Implications for the Salmon Industry
These findings have direct implications for the industry:
- Acknowledging Resistance: Accepting that drug resistance is an inherent feature of open-pen systems is the first step.
- Integrated Strategies: The future of salmon louse control lies in the intelligent and optimized combination of multiple tools (pharmacological, mechanical, biological – though not modeled here -, and management tools like strategic depopulation or shorter cycles).
- Importance of Resistance Cost: Investigating and better quantifying the actual biological cost of resistance in L. salmonis is crucial, as it directly impacts the optimal management strategy. If the cost is low, the benefits of ‘adapting’ decrease. If it is high, they are accentuated.
- Monitoring: Being able to directly estimate the proportion of resistant lice on a farm (without needing to apply the drug to ‘test’) could improve profitability, especially if resistance carries a high cost.
- Balancing Economics and Welfare: The industry and regulators face the challenge of balancing economic profitability with growing demands for animal welfare, given the cost associated with reducing reliance on intensive mechanical treatments.
Conclusion: Towards Adaptive Management of Salmon Lice
The study suggests a necessary evolution in the approach to managing drug-resistant salmon lice. Instead of a war of attrition against sea louse resistance, a more sustainable and economically viable strategy appears to be intelligent adaptation.
Intelligent adaptation involves using drugs strategically, decisively integrating non-pharmacological treatments despite their costs and challenges, and optimizing production cycles through tools like threshold-based depopulation based on economic and biological factors. Understanding and potentially utilizing the biological cost of resistance could be a key piece in the future of parasite management in aquaculture.
Contact
Duo Xu
Complexity Science and Evolution Unit, Okinawa Institute of Science and Technology Graduate University (OIST)
Onna, Okinawa 904-0495, Japan.
Email: xuduo.1111@163.com
Reference (open access)
Xu, D., Dieckmann, U., & Heino, M. (2025). Economically optimal management of salmon louse requires adapting to their drug-resistance rather than attempting their eradication. Aquaculture, 742578. https://doi.org/10.1016/j.aquaculture.2025.742578
