Effects of repeated anaesthesia on gill and general health of Atlantic salmon, Salmo salar

Fish are the second most widely utilized vertebrate group used for scientific procedures in the United Kingdom, but the development and application of 3Rs (the principles of replacement, reduction, and refinement) in aquaculture disease research lags behind methodologies in place for mammalian studies. With a need for individual monitoring and non‐lethal sampling, the effect of repeat anaesthesia on experimental fish needs to be better understood. This study to analyses the effect of repeat anaesthesia with MS‐222, metomidate and AQUI‐S upon the gill and general health of post‐smolt Atlantic salmon Salmo salar. A single, lethal dose of anaesthetic was compared with seven anaesthetizing time points over 28 days, terminating in a lethal dose. No anaesthetic showed significant differences in accumulation in the muscle tissue, or changes in plasma glucose after repeated or single dosing. Fish repeatedly anaesthetized with MS‐222 or AQUI‐S exhibited upregulation of osmoregulatory genes in the gill and AQUI‐S‐treated individuals showed, histologically, epithelial lifting from the lamellae capillary irrespective of whether they had a single or repeated dose history. No significant changes were seen in inflammatory or stress genes in the head kidney of fish repeatedly anaesthetized with AQUI‐S or metomidate, however MS‐222 treatment resulted in upregulation of tnfα3 . Repeated anaesthesia with MS‐222 and metomidate gave a significant decrease and increase in peripheral blood neutrophils, respectively. This study concludes that no increase in cumulative stress or inflammation is induced by the repeated anaesthetization of S. salar with any of the tested anaesthetics, however gill osmotic regulation and blood parameters may be affected.