Delousing Efficacy and Physiological Impacts on Atlantic Salmon of Freshwater and Hyposaline Bath Treatments

Abstract

A pilot study was conducted to investigate the delousing efficacy and physiological effects of freshwater and hyposaline water in Atlantic salmon ( Salmo salar ) for various treatment durations. There were four primary treatment groups: salmon lice ( Lepeophtheirus salmonis) infected fish held in seawater (procedural controls), infected fish treated in freshwater (buffered with 1% seawater), uninfected fish treated in freshwater, and infected fish treated with hyposaline water. The ion-modified, hyposaline water (~5 ppt salinity) was created through nano-filtration technology which desalinated seawater. Salmon were held for a period ranging from 30 minutes to 48 hours in their designated treatment bath, lice counts were made before and after treatment to measure efficacy of delousing, and blood samples were taken to investigate the physiological effects on fish. The freshwater treatment had no apparent delousing effect when compared with the seawater group, but just 5% of lice remained on fish after a 4-hour hyposaline treatment. The osmotic balance of fish in the freshwater and hyposaline treatments shifted towards a lower internal concentration of ions with longer bath durations, but the fish recovered once transferred back to seawater. Indicators of stress reflected a less clear pattern with bath duration which may be due to the additional handling stress associated with moving fish between tanks and an elevated stress response triggered by transition back to a freshwater environment.

Delousing Efficacy and Physiological Impacts on Atlantic Salmon of Freshwater and Hyposaline Bath Treatments