Characterizing Hydrolicer treatment of Lepeophtheirus salmonis on farmed salmon

The salmon louse, Lepeophteirus salmonis, infects salmonids and has since the beginning of salmon aquaculture been a challenge for fish welfare and growth. To protect the wild salmon stocks and the farmed salmon, the salmon aquaculture industry is obliged by law to keep the infestation below 0.5 adult female lice on average. The salmon aquaculture industry has several methods, both preventative actions and delousing. Hydrolicer is a mechanical delousing method used frequently. During Hydrolicer treatment, the salmon is first crowded together with a swipe net or ball line. When the upper layers of the salmon pen have a high density of salmon, the Hydrolicer system starts to pump it through the Hydrolicer pipeline system. The Hydrolicer creates turbulence and negative pressure to detach lice before waterjet flushing removes the lice. The standard velocity through the system is 2 m/s and the treatment time per salmon is 21-25s. The knowledge about the effect of the Hydrolicer on L. salmonis is limited, and a field study was conducted to investigate the effect of the Hydrolicer delousing method, where data from three different locations (11 net cages in total) were sampled. The salmon was sampled right before treatment (during crowding), and right after Hydrolicer delousing (20 salmon per sample; in total 28 samples before and 28 samples after treatment). L. salmonis were registered based on life stage groups and their placement on the salmon body. The Hydrolicer pressure adjustments, salmon weight and water temperature was also registered. The overall Hydrolicer effect (% reduction of L. salmonis) was investigated, and the pressure adjustments and effect were tested for correlation. The reduction of lice based on L. salmonis placement was also investigated. The results show that Hydrolicer delousing had a significant decrease of lice at all three locations, with a median effect at location 1-3 of 78%, 59%, and 72%, respectively. The correlation between the effect and Hydrolicer pressure adjustment was low. Chalimus was the life stage group with the highest reduction, with a median effect of 100%, and adult female was the life stage group with the lowest and most variable effect, with a median effect of 62%-72%. The Hydrolicer had different effects based on L. salmonis placement. Zone A (the head) had the lowest reduction, the median effect at location 1-3 was 33%, 10%, and 0% respectively, and no treatment was significant. Zone B (ventral part of the body) was the zone with the highest reduction of L. salmonis. The median effect in zone B at location 1-3 was 92%, 91%, 100% respectively, with all treatments being significant. The adult female was found to prefer zone C (from the anal fin and to the caudal fin) during this study. The distribution of other life stage groups was more spread out. The difference in results between each location could be a result of the difference in L. salmonis prevalence and mean intensity before delousing, along with the considerable difference in salmon weight. The results show that the Hydrolicer treatment has different effect on the different life stage groups as well as the placement of L. salmonis during treatment.