Potential risk factors of amoebic gill disease in Tasmanian Atlantic salmon
Creators: name: Bagley, Carley Anderson
Publisher: name: University Of Tasmania
Year: 2023

description: Amoebic gill disease (AGD) is the most significant health problem affecting the production of Atlantic salmon (Salmo salar L.) in Tasmania, Australia. AGD affects a number of cultured fish species worldwide, however its impact is insignificant when compared to that in Tasmania. The disease was first identified in southern Tasmania in 1986 shortly after the initiation of Atlantic salmon farming. AGD is a result of an endemic parasite Neoparamoeba sp. attaching to the salmon's gill tissue. Research conducted over the years has resulted in a large reduction of AGD associated mortalities. However, the disease continues to place a significant financial burden on the industry with the only effective form of treatment being freshwater baths, a strategy implemented in the late 1980's. Epidemiological studies are essential as they facilitate in identifying causal factors that may be associated with disease outbreaks. An understanding of these complex interactions is required in order to implement effective control and prevention strategies. This thesis examined a number of environmental conditions and husbandry protocols currently utilised in the Atlantic salmon industry in Tasmania. Fallowing of lease sites as a disease management strategy for AGD was found to be unsuccessful. The mean AGD prevalence of Atlantic salmon cultured at a fallowed site (57.5% ¬¨¬± 5.32) was similar to fish at the control site (52.3% ¬¨¬± 5.35). Neoparamoeba sp. was isolated from environmental samples at the fallowed site despite the absence of salmon. Atlantic salmon maintained in copper based antifouling paint treated nets had a higher mean prevalence of AGD (29 .2% ¬¨¬± 6.74) compared to salmon maintained in untreated nets (21.3% ¬¨¬± 6.43). The belief that copper acts as an attractant for Neoparamoeba sp. was not ascertained. Atlantic salmon maintained under continuos artificial lights had a similar prevalence of AGD as salmon maintained under natural light conditions for the majority of a 12 month trial. However when subjected to stressful conditions the salmon maintained under artificial lights had a significantly higher percentage of gill lesions (43.1 % ¬¨¬± 4.38) compared to salmon held under natural light conditions (14.11%¬¨¬± 1.96). An Atlantic salmon farm located in the Tamar River in the north of Tasmania was studied as a control site, as the farm had been operating for approximately 5 years with no history of AGD. Neoparamoeba sp. was isolated from the benthic sediment and nylon nets, but not detected on steel nets or the salmon's gills. However approximately 12 months after the trial concluded the farm experienced its first outbreak of AGD. The work contained in this thesis has identified a number of environmental and husbandry practices that warrant further investigation to accurately understand their influence on the occurrence of AGD in Tasmanian cultured Atlantic salmon. Future research must also concentrate on identifying and understanding the causative agent/s involved in this disease in order to develop effective treatments.