Physical constraints of bath treatments of Atlantic salmon (Salmo salar) with a sea lice burden (Copepoda : Caligidae)

Licensed medicines available in the U.K. for treating Atlantic salmon infes ted with sea lice, dichlorvos, azamethiphos, and hydrogen peroxide, can onl y be administered by bath application. Adverse reactions have been reported to bath treatments including mortalities, inappetance, reduction in growth and reduced louse sensitivity to dichlorvos. The physical constraints of b ath treatments are examined and improvements recommended. Oxygen saturation was adequate during treatments but declined rapidly when the tarpaulin was removed. A chemical marker dispersed uniformally both horizontally and ver tically in a cage within 5 mins of dispensing indicating dispersal of a med icine is rapid and adequate during treatment. The range in enclosed volumes in 86 treatments was 46 to 146% of theoretical, suggesting potential toxic ity due to high concentrations at low volumes and the risk of reduced sensi tivity at high volumes. Residual concentrations of hydrogen peroxide varied from 50 to 400 ppm from 5 to 15 mins after the tarpaulin was removed. Wate r exchange should be encouraged by aerating the cage and flushing at the en d of treatment.