Gill observations in Atlantic salmon ( Salmo salar, L.) during repeated amoebic gill disease (AGD) field exposure and survival challenge

Abstract

Amoebic gill disease (AGD) of Atlantic salmon in Tasmania is proactively treated by freshwater bathing when gross gill assessment (‘gill score’) indicates a moderate level of disease in a cage population. This generally ensures that few fish are exposed to severe disease symptoms and that few die, but also means that a proportion of the population shows little gross evidence of AGD. Individuals exhibiting few AGD symptoms at bath may be more resistant, or simply reflect an uneven spread of the disease through the population. This study had three main aims, firstly, to determine whether all fish in a cage population eventually require freshwater treatment after first infection; secondly, to ascertain whether there is any evidence of development of resistance to AGD; and thirdly, to see if there is a relationship between the level of proliferative gill reaction to the parasite, assessed by gill score, and time to mortality when the disease is left untreated. These aims were achieved by following gill score trajectories of individual fish through three rounds of natural AGD infection and relating these to the eventual fate of the fish in a natural AGD survival challenge after the second freshwater bath. There was no evidence of complete innate resistance to AGD as each fish eventually required a first freshwater bath. There was no relationship between the rate of first infection and the ultimate survival of each fish. For the second and third exposures, significant differences ( P < 0.001) were observed between the surviving fish and those that died in the challenge. Individual gill scores at the latter measurements were suggestive of development of resistance to AGD. Mortality during a natural summer AGD challenge in an un-bathed population of fish, that had two previous treated exposures to the disease, was 67.7% and gill symptoms at the onset of losses accurately predicted the rate of mortality.