Action: Bathe in freshwater
Key messagesRead our guidance on Key messages before continuing
- Two studies in Australia provide mixed evidence of the effect of freshwater bathing to reducing amoebic gill disease. One study found a reduction in numbers of amoebae on salmon gills persisting for up to 10 days after bathing. One replicated, controlled study found similar levels of amoebae in the gills of treated and untreated salmon.
Atlantic salmon cultured in commercial farms are at risk of contracting and transferring infectious bacterial diseases and other pathogens such as amoebic gill disease (AGD). Freshwater bathing aims to mitigate the risk of AGD by removing live ameobae from the gills.
Supporting evidence from individual studies
In 2002, a study on a salmon farm in Tasmania, Australia (Clark et al., 2003) found an 86% reduction in the number of live amoebae found on the gills of salmon, Salmo salar, after freshwater bathing. The presence of amoebae increases the risk of amoebic gill disease. The lower levels of amoebae persisted for 3 days before gradually increasing to pre-bathing levels on day 10. Salmon were given freshwater baths for two hours then monitored for 10 days. Samples were removed from fish before bathing and at 1, 3, 5 and 10 day intervals after bathing to determine the number of amoebae present on the gills.
Between 2000 and 2002, a replicated, controlled study in Tasmania, Australia (Douglas- Helders et al., 2004) found similar levels of amoebic gill disease in groups of salmon, Salmo salar, that had undergone freshwater bathing compared to those which had not. By the end of the study, amoebic gill disease occurred within 35% of fish within both groups. Monthly samples were removed from salmon in three groups receiving freshwater baths and three un-bathed groups. Average biomass per pen was 11, 663 kg for bathed groups and 20, 929 kg for un- bathed groups. Signs of clinical disease were assessed monthly using the routine Tasmanian salmon farmers gill assessment method
- Clark G., Powell M. & Nowak B.F. (2003) Effects of commercial freshwater bathing on reinfection of Atlantic salmon, Salmo salar, with Amoebic Gill Disease. Aquaculture, 219, 135-142
- Douglas-Helders G.M., Weir I.J., O'Brien D.P., Carson J. & Nowak B.F. (2004) Effects of husbandry on prevalence of amoebic gill disease and performance of reared Atlantic salmon (Salmo salar L.). Aquaculture, 241, 21-30