Evidence for the Evolution of Resistance to Non‐Chemical Parasite Controls: Salmon Lice From Submerged Cages Produce Larvae That Swim Deeper

Salmon lice (<i>Lepeophtheirus salmonis</i>) pose a major challenge to the sustainability of salmon aquaculture due to their capacity to rapidly evolve resistance to parasite control methods. As the effectiveness of chemical treatments has declined, the industry has increasingly relied on preventive strategies to limit initial infections. One such approach is depth-based farming, where fish are held deeper in the water column using submerged cages. These systems reduce exposure to lice, which typically concentrate near the surface. However, there is growing concern that such practices may inadvertently select for lice that are better adapted to deeper swimming, potentially enabling resistance to depth-based interventions. In this study, we investigated whether vertical swimming behaviour in salmon lice larvae is influenced by the depth at which their parents were collected. We sampled 122 adult female lice carrying egg strings from commercial salmon farms using either standard cages (0-20 m) or submerged cages (20-40 m). The first-generation larvae were reared under controlled conditions, and the vertical positioning of 11,291 copepodid larvae was tested in pressure columns simulating a depth of 10 m. Our results revealed a significant interaction between larval depth distribution and the cage type from which the parental lice were sourced (<i>χ</i> ² = 278.85, df = 1, <i>p</i> < 0.001). Larvae from standard cages showed a greater tendency to ascend (35% vs. 23%) and were less likely to sink (19% vs. 27%) compared to larvae from submerged cages. These findings suggest that vertical swimming behaviour may be heritable, with submerged cages potentially selecting for deeper-dwelling lice over time. This study provides the first evidence that the depth preference of salmon lice larvae may be influenced by their parents' environment. Understanding this behavioural inheritance is crucial for evaluating the long-term sustainability of submerged cage systems and for developing lice management strategies that anticipate evolutionary responses.