Salmon lice Lepeophtheirus salmonis infestation rates higher in exterior net pens

Salmon lice Lepeophtherius salmonis infestations cause major disruptions in marine Atlantic salmon Salmo salar farming operations. Effective mitigation of these parasites in salmon farms is reliant on monitoring and preemptive treatment of lice populations before they become self-propagating. Farms can consist of multiple salmon-rearing cages (net pens) arranged in a grid. Studies that focus on elucidating the infection dynamics of salmon lice often concentrate on a regional scale, incorporating multiple farms for analysis, but have rarely examined the dynamics that occur within a single farm. In this study, the interactive effects that occur between net pens and the environment at the farm scale were investigated. Specifically, we examined whether net pen placement and increased environmental exposure differentially affected lice abundance and/or aggregation on fish. Through weekly sampling of lice burdens, we compared exterior net pens, which occupy the corners of the grid arrangement, and those that are interior, buffered from exposure by adjacent net pens. We followed the establishment and eventual epizootic infestation of salmon lice on a farm over a summer growing season until the infestation required treatment. We found that exterior net pens had consistently higher overall abundances and indicators of aggregation. We also found that exterior net pens harbored more adult female lice. These results have important implications for the adoption of better monitoring strategies and more informed experimental designs.