Ion-regulation, acid/base-balance, kidney function, and effects of hypoxia in coho salmon, Oncorhynchus kisutch, after long-term acclimation to different salinities

Land-based salmon farming in closed containment recirculation aquaculture systems (RAS) is an emerging industry with the potential for reducing disease prevalence, improving production efficiency, and reducing environmental impacts compared with open net-pen aquaculture. Salinity and oxygen levels are environmental factors that have significant impacts on the physiology of fishes and possibly influence the production efficiency of fishes in aquaculture. These parameters are particularly relevant for fish production in RAS, where most water parameters can be manipulated and tightly controlled. While much is known about the specific mechanisms associated with the acute transfer between FW and SW and the reverse, little is known about the effects of long-term acclimation on ion- and acid/base-regulation, especially at intermediate salinities. We measured gas-exchange, ion-regulation, acid/base balance, and renal function in coho salmon (Oncorhynchus kisutch Walbaum, 1792) acclimated to four salinities (2.5, 5.0, 10, and 30 ppt) for over a year in RAS and investigated how animals prioritize these functions when faced with 24 h exposure to hypoxia (PO2 = 63 mmHg = 8.4 kPa = 3.76–4.48 mg l−1). We show that fish that were long-term acclimated to 30 ppt had substantially higher plasma osmolality and [Na+] than fish acclimated to lower salinities. These changes were associated with a marked reduction in blood pH at 30 ppt relative to 2.5 ppt, and we discuss a possible thermodynamic link between salinity acclimation and acid/base regulation. Further, we show that hypoxia exposure results in changes in plasma osmolality by over 80 mOsm kg−1, but only in 30 ppt water, demonstrating a salinity-dependent trade-off between gas exchange and osmoregulation. This study provides insight into the physiological state and hypoxia sensitivity of market sized salmon reared under industry-relevant conditions in RAS.