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 s..., Louse Collection and Larval Production
From early April to late May 2024, we collected female Lepeophtheirus salmonis from five salmon farms during routine lice counts within all cages at the sites (Fig. 1). We gathered 25 pairs of egg strings from adult females at two standard Atlantic salmon cages in Smørdalen and Brattavika, and 22 pairs from a standard cage in Uforø. For the submerged farms at Gjengane and Hestabyneset, we collected lice from recently deceased fish, as live fish sampling before harvest is difficult in submerged technology. The dead fish sank to the bottom of the submerged cages, where a lift-up suction pump brought them to the surface. The fish were then placed into a bin with a grid to allow water drainage, making it easy to collect still-attached lice. We obtained 25 pairs of egg strings from Gjengane and 11 pairs of egg strings from Hestabyneset, as well as 14 females and 4 males for laboratory reproduction.
After collection, lice were transported in ch..., # Data from: Evidence for the evolution of resistance to non-chemical parasite controls: salmon lice from submerged cages produce larvae that swim deeper

Dataset DOI: [10.5061/dryad.b5mkkwhs6](10.5061/dryad.b5mkkwhs6)

## Description of the data and file structure

### Files and variables

### **Column.csv: data file**

### **Variables:**

* **rep**: Number of replicate column tests conducted. Each replicate tests the behaviour of copepods under controlled conditions.
* **depth**: Depth of the column in centimeters (range: 10–80 cm in 10cm intervals).
* **pressure**: Pressure applied in the column (constant at 5 atm) to simulate deeper water conditions.
* **lice**: Number of lice observed within a specific 10 cm interval of the column.
* **total.lice**: Total number of lice within a column replicate (sum over 10–80 cm).
* **prop**: Proportion of lice in a 10 cm interval relative to the total lice in that column.
* **fail**: Number of lice not in the 10 cm interval (`total.lice - lice`...,