Data from: Evidence for the evolution of resistance to non-chemical parasite controls: salmon lice from submerged cages produce larvae that swim deeper

Salmon lice (Lepeophtheirus salmonis) 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 the first generation of 122 adult
female lice carrying egg strings from commercial salmon farms using either
standard cages (0–20 m) or submerged cages (20–40 m). 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
(χ² = 278.85, df = 1, p < 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.