BACKGROUNDSalmon lice (Lepeophtheirus salmonis) are the most important parasite of farmed salmon. Infective larvae position themselves in the upper part of the water column to increase encounter probabilities with potential hosts. Previous studies have shown that a 'snorkel' sea-cage technology protects salmon from infection in surface waters. We tested whether deep snorkels would more effectively reduce lice infestation than shallow snorkels and still uphold adequate conditions for the fish. Five sea-cages (12 m × 12 m) each holding approximately 3000 Atlantic salmon (Salmo salar) (53 ± 10 g) were fitted with snorkels that gave protection from infection for 0, 4, 8, 12 or 16 m. We tested if reductions in the settlement of new salmon lice copepodids were consistent among 4 separate infection periods.
RESULTSLice infestation decreased exponentially with depth in all time periods. Infection levels in shallow snorkels (0 and 4 m) were consistently four to ten times higher than those in deep snorkels (12 and 16 m). Key welfare and production performance indices were similar across all snorkel depths.
CONCLUSIONDeeper snorkels dramatically and consistently reduced infection levels of salmon lice compared to shallow snorkels without consequences for fish welfare and production performance. Therefore, reducing salmon sea lice encounters using a depth-based barrier is a powerful management tool for salmon farming.
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