Salmon Lice Should Be Managed Before They Attach To Salmon: Exploring Epidemiological Factors Affecting Lepeophtheirus Salmonis Abundance On Salmon Farms

The ectoparasite Lepeophtheirus salmonis has for decades been a serious challenge to the sustainability of salmonid aquaculture and has the potential to disrupt the ecosystem of wild salmonids. To tackle this challenge, efforts have been made to develop effective lice control strategies that deter ectoparasitic infestations on salmon farms. Infestation control can be enhanced by understanding L. salmonis population dynamics that may reveal critical points at which to intervene. Here, we investigate the impact of key parameters affecting L. salmonis population dynamics on salmon farms with a view to informing L. salmonis management. We built a system dynamics model to simulate L. salmonis populations in a hypothetical Atlantic salmon (Salmo salar L.) farm. Our model focused on three key parameters in the life cycle of L. salmonis: external pressure (rate of introduction of planktonic L. salmonis from outside the farm), attachment proportion (proportion of L. salmonis that successfully infest salmon), and treatment efficacy (proportion of L. salmonis that are killed as a result of treatment). By applying various combinations of plausible values for these three parameters, their modelled impacts on L. salmonis management were evaluated in terms of the total number of required treatments and the total infestation burden during a typical production cycle. These two modelled outcomes represent the cost of treatment and a proxy for other likely costs, such as lost growth or mortality caused by the parasite. Our results indicated that overall L. salmonis infestation levels were more sensitive to changes in the proportion of successful attachment, followed by changes in external pressure, and were least affected by changes in treatment efficacy. While attachment proportion and external pressure are involved at the pre-infestation stages in the life cycle of L. salmonis, treatment efficacy is involved in the post-infestation stages. Therefore, these findings suggest that preventing infestation before lice attach to fish is a more effective L. salmonis control strategy than treating already infested salmon. Overall, the presented results provide numerical evidence for the efficiency of prevention and support the development and application of prevention measures that have been on the rise in recent years.