Effects of dissolved organic carbon and hardness in freshwater used to treat amoebic gill disease

Amoebic gill disease (AGD) is a significant disease of Atlantic salmon farmed in South East Tasmania. The commercial treatment for the disease is a freshwater bath for up to 4 h. Previous studies have shown that the chemical composition of the freshwater, in particular total water hardness, affects the efficacy of the treatment. The aim of this study was to determine if other water chemistry parameters, such as dissolved organic carbon (DOC), interact with total water hardness to affect treatment success. Firstly, the relative survival of isolated gill amoebae incubated for up to 3 h with hard or soft water (346.0 and 34.6 mg L^-1 CaCO₃ respectively) with low or high concentrations of humic or tannic acid (5 and 50 mgL^-1 respectively) was determined. Secondly, fish with AGD were bathed for 2.5 h in hard or soft water (249.3 and 35.3 mg L^-1 CaCO₃) containing either 5 or 20 mg L^-1 humic acid. The number of viable amoebae surviving on the gills and number of gill lesions were determined. It was found that the concentration of DOC used in this study that represents the levels commonly found around SE Tasmania is unlikely to have any commercial significance in the reduction in amoebae on the gills of Atlantic salmon. However, this study provided further support that freshwater selected for bathing AGD-affected salmonids should be chosen primarily on its total water hardness.