Genetic variation of resistance to amoebic gill disease in Atlantic salmon (Salmo salar) assessed in a challenge system

Amoebic gill disease (AGD) is the most serious health problem affecting the culturing of Atlantic salmon in Tasmania. A selective breeding project is selecting for AGD resistance amongst other important commercial traits. This paper presents the first assessment of genetic variation associated with resistance or susceptibility to AGD, and reports on a comparison of three measures of AGD infection leading to a recommended breeding trait measure. Naïve smolts from 30 full sibling families were communally challenged with Neoparamoeba spp., the agent of AGD, in a 19 day challenge in three replicate tanks. At termination of the challenge all fish were euthanized and the level of infection on the gills assessed through a gross gill score, histopathology and image gill score. Broad sense heritability estimates varied between 0.16 ± 0.07 for gross gill score and 0.35 ± 0.10 for image gill score. A high genetic correlation between image gill score and histopathology score was observed (rg = 0.90) suggesting these are measurements of the same trait. A lower genetic correlation was observed for gross gill score with image gill score/histopathology (rg = 0.65). This study suggests a moderate proportion of genetic variance in AGD resistance exists within the Tasmanian Atlantic salmon population and provides good scope for selective breeding for AGD resistance.