Introduction and methods
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 to reduce exposure to infective stages that concentrate near the surface. Preventative management strategies can impose sustained selection on parasite populations, but evolutionary responses depend on both the strength of selection and the availability of heritable variation. While resistance to chemical treatments is well documented, less is known about whether parasite behaviours can evolve in response to non-chemical control methods.
In this study, we investigated whether vertical swimming behaviour in salmon lice larvae is heritable and responsive to selection. First, we sampled 122 adult female lice carrying egg strings from commercial 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 assessed in pressure columns simulating a depth of 10 m. In a complementary multi-generational experiment, larvae originating from approximately 500 lice collected from five farms were subjected to directional selection for surface swimming, bottom swimming, or random selection across three generations.
Results and discussion
Larval vertical distribution was significantly influenced by parental environment, with a strong interaction between depth behaviour and cage type (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, indicating that vertical swimming behaviour is at least partly heritable. Experimental selection further demonstrated that this behaviour can evolve: surface-selected larvae increasingly occupied upper layers, whereas bottom-selected larvae showed consistent shifts towards deeper swimming over successive generations. By the third generation, bottom-selected larvae were 11% more likely to occur at depth than the founding population.
Together, these findings provide evidence that vertical swimming behaviour in L. salmonis is both heritable and evolutionarily responsive to sustained selection. This reveals a potential pathway for adaptation to depth-based farming strategies. Although the rate of behavioural change appears gradual, the continued use of submerged cages may select for deeper-dwelling lice over time. These results highlight the importance of incorporating evolutionary principles into parasite management, suggesting that the long-term sustainability of submerged systems will depend on mitigating selection pressure, for example through the maintenance of refugia.
Figure 1. Mean proportion of copepodids across 10–90 cm depth by generation. (A) Founding generation (F1), before selection. (B) Second generation (F2), showing three lines selected for vertical swimming behaviour — top 10% (red), bottom 10% (green), and random (purple). (C) Third generation (F3), following another round of selection.
Acknowledgment
We thank the Institute of Marine Research for assistance with the initial lice collections, and the technical staff, 2025 summer interns, and apprentices at Matre for their support with infection procedures and maintaining lice across generations. We are grateful to Velimir Nola and Kris Oldham for help with lice collection, and to Stein Carlos Lopez Fyllingsnes, Torfinn Aga and Martin Matrefor maintaining the pressure columns. We also acknowledge technical support with fish husbandry, particularly Thea Svendsen and Tone Vågseth for assistance with fish sedation.