Stress and immune response to centrifugal pumping of Salmo salar in an industrial recirculating aquaculture system: A case study

Salmo salar, the most economically important salmonid, is commonly reared in recirculating aquaculture systems (RAS) during its land-based production phases. Although pumps are frequently employed to transfer fish in RAS, their impact on fish is poorly understood. This study is the first to follow a transfer performed through centrifugal pumping in a commercial RAS, investigating its effect on the stress and immune status of fish, and their ability to recover over a week. This was assessed through the analysis of physiological, biochemical and molecular indicators of stress responses and inflammation, in plasma and relevant organs. The primary and secondary stress responses were triggered, as indicated by the alterations in biochemical parameters. Cortisol levels were lower than expected throughout the case study, indicating a possible exhaustion, or decreased sensitivity, of the hypothalamus-pituitary-interrenal (HPI) axis. In the gills and head kidney, all genes displayed significant alterations. The results indicate that, in this specific industry-based case, centrifugal pumping elicited a stress response in S. salar smolt, but this stress did not appear to be maladaptive, as the fish were able to maintain homeostasis. This study, having been performed in industrial settings, provides results of greater representativeness of real-life scenarios compared to laboratory experiments or simulations. Further research following the stress and immune status of S. salar in industrial settings throughout the production cycle is required to investigate whether rearing conditions in commercial RAS do indeed alter the HPI axis responsiveness and the immune-endocrine interactions.