Assessment of resistance to amoebic gill disease in the Tasmanian Atlantic salmon selective breeding population

• Main Author: Taylor, RS
• Format: Thesis
• Language: English
• Published: 2010
• Subjects: Atlantic salmon; Bacterial gill disease; Atlantic salmon fisheries; Gills; Salmon farming
• Online Access: https://eprints.utas.edu.au/22264/; https://eprints.utas.edu.au/22264/1/whole_TaylorRichardSimon2010_thesis.pdf

The main priority in the breeding goal of the recently established Tasmanian Atlantic salmon selective breeding program is for resistance to amoebic gill disease (AGD), which is caused by the marine ectoparasite Neoparamoeba perurans and is characterised by hyperplastic changes to the gill tissue. A categorical gross 'gill score' is used to schedule freshwater bath treatments. Bathing and direct production losses to AGD are estimated to add over 20% to the cost of production. The purpose of this thesis was to (i) investigate whether genetic variation of resistance to AGD exists (ii) determine whether this variation can be adequately measured using gill score (iii) examine phenotypic trends for evidence of developing resistance (iv) assess whether serum antibodies to Neoparamoebae are related to AGD resistance and (v) establish the relationship between the selection trait (gill score) and the objective trait (time between freshwater baths). Initial evidence of a moderate broad scale genetic variation for AGD resistance was provided from a first infection tank challenge. This was adequately measured by gill score, which was moderately correlated with destructive histopathology and image analysis of affected gills. In a second trial, sea caged fish naturally developed AGD and were bathed over two infection rounds. On the third round, fish were gill scored and left unbathed so AGD related mortality occurred. Narrow sense heritability for AGD resistance, assessed by gill score, varied between 0.23 and 0.48 over the three rounds of infection. Heritability of AGD survival was 0.40 to 0.49. Estimates of genetic gains indicate that up to 82% of the expected gain in survival can be achieved utilising gill score breeding values. Phenotypic results were further examined, gill signs at the early onset of losses accurately predicted the incidence of mortality (r = 0.96). There was no relationship between gill score at first infection and the ultimate survival of each fish, though there was a marginal lower separation of the group that died early on. For later exposures, significant gill score differences (P < 0.001) were observed between fish that eventually survived or died in the challenge. The systemic immune reaction to Neoparamoebae was characterised, the proportion of seropositive fish increased from 46% to 77% with each infection. A measurable immune titre was seen in 13% of the survivors but there was no evidence that serum antibodies provided significant protection against AGD. The aim of the final chapter was to estimate whether gill score at a population average threshold, is under common genetic control with the required period between individual freshwater baths. The results confirm that gill score closely predicts time to bath, a simple estimation of the response to gill score selection predicts that the period between freshwater baths can be increased by up to 9.3% per generation. Together, these data have established that genetic variation for resistance to AGD exists and that this can be effectively measured by gill score. This non-destructive measure can be applied over reiterative rounds of AGD development and is closely linked to the required bathing frequency. Phenotypic trends and genetic parameters indicate that the nature of resistance changes after first infection. However, there is no evidence that the acquisition of resistance to AGD is linked to the development of serum antibody to Neoparamoebae.