Effects of copper-based antifouling treatment on the presence of Neoparamoeba pemaquidensis Page, 1987 on nets and gills of reared Atlantic salmon (Salmo salar)

Amoebic gill disease (AGD) is the main disease affecting the salmon industry in Australia. Little information is available on the epidemiology of AGD and the biology of Neoparamoeba pemaquidensis [Page, 1987], the disease-causing organism of AGD. In previous studies, N. pemaquidensis was found on biofouled netting of sea cages, and a reduction in AGD prevalence was achieved with increasing number of net changes. Presently, it is not known if N. pemaquidensis on netting is able to induce AGD. To reduce biofouling on nets, antifouling paints are commonly used on Tasmanian salmon farms. This study investigated the effects of a copper-based antifouling paint on the N. pemaquidensis densities on nets and the AGD prevalence of Atlantic salmon reared in these nets. Four sea cages stocked with 5-9 kg/m3 Atlantic salmon were used in this study. Two nets were coated with a copper-based antifouling paint and two nets were not treated and used as a control. Fish were sampled every 2 weeks for 10 weeks. A gross gill score was determined and gill mucus samples were taken for dot blot analysis to determine the presence of N. pemaquidensis for each fish. Biofouling samples from netting were inoculated onto 75% malt yeast agar culture plates, and presence of N. pemaquidensis assessed using conventional culture techniques, followed by indirect immunofluorescent antibody test (IFAT). The presence of N. pemaquidensis was confirmed from culture-enriched biofouling samples from weeks 2 and 8 were tested using nested PCR. Results suggest that copper paint treated cages had significantly higher paramoebae (P=0.002) and AGD (P=0.014) prevalence compared to the control cages. No treatment effect was found on the intensity of infection, determined by gross gill scores (P=0.243). At the end of the study, the paramoebae prevalence of net samples was 58.5% (S.E. 1.5) and AGD prevalence was 42.5% (S.E. 2.5) for copper treated nets, while no paramoebae were found on control nets and AGD prevalence was 35.0% (S.E. 5.0). Nets could be the source of N. pemaquidensis infection of fish with AGD, and therefore copper paint treated nets could be a risk factor for AGD.