Assessment of acute effects of in situ net cleaning on gill health of farmed Atlantic salmon (Salmo salar L)

Regular in situ cleaning of net pens is performed to prevent overgrowth and negative effects of biofouling during the sea phase of salmon production in Norway. Possible negative health effects of in situ net cleaning include a reduction in appetite, increased stress, and gill damage. Gill lesions have been reported after exposure to Ectopleura larynx in laboratory trials, but there is currently little information available on whether exposure to biofouling debris from in situ net cleaning significantly contributes to development of gill disease and reduced gill health of farmed Atlantic salmon (Salmo salar) in sea cages. To assess this, gill tissue from healthy fish (n = 30 per pen/time point) from 3 net pens were collected before, as well as one and eight days after the first net cleaning event after sea transfer. The tissue was examined using a standardized histology scoring system. All fish originated from the same commercial sea site located in North-western Norway and net cleaning was performed as per routine for this site. The level of net fouling was scored using a semiquantitative scoring system ranging from 0 to 6, i.e., from totally clean to heavy fouling. Two pens were moderately fouled, and one had a low degree of fouling. The probability (odds) of fish from moderately fouled net pens having subacute vascular damage (thrombi) in the gills at one day after net cleaning was 2.36 (95% CI 1.21-5.71) times higher compared to fish sampled before net cleaning. When all fish were included in the statistical analysis no difference in the number of fish with the recorded lesions were found across the different sampling points and there was even a small decrease in the probability (odds) of fish having acute vascular lesions at eight days after net cleaning. Our results suggest that exposure to biofouling debris during net cleaning might contribute to development of thrombi/subacute vascular lesions in the gills. However, the proportion of gill tissue affected was low, generally estimated to be less than 1%, and no significant difference in the number of fish with these lesions was found at eight days after net cleaning. Thus, the negative impact of in situ net cleaning on gill health in this study is small and short lived, and the clinical implication of these results remains to be established.