The limits of oxygen : the effect of salinity and photoperiod on metabolism and hypoxia tolerance and mechanisms of enhanced oxygen unloading associated with hypoxia tolerance in Atlantic salmon

Recirculating aquaculture systems (RAS) are one avenue by which the environmental impact of aquaculture may be reduced, although optimal rearing conditions are not conclusively known. Salinity and photoperiod are two such conditions. Atlantic salmon (Salmo salar) were raised in three different salinities, 2.5 ppt, 10 ppt, and 30 ppt combined with two light regimes, 12 h of light and 12 h of darkness, or 24 h of continuous light. Intermittent flow respirometry was conducted after 180–200 and 360–380 days and oxygen consumption was measured to determine the animals’ metabolic profile across 12 respiratory indices, including maximum oxygen uptake (ṀO₂max), standard metabolic rate (SMR), and absolute aerobic scope (AAS). Fish reared at 2.5 ppt (429±101 mg kg⁻¹ h⁻¹) had an ṀO₂max 16% greater than those raised in 10 ppt (369 ± 78.1 mg kg⁻¹ h⁻¹) and 22% greater than the fish raised in 30 ppt (351 ± 84.2 mg kg⁻¹ h⁻¹). This difference in ṀO₂max was responsible for a 26% larger AAS when fish were reared in 2.5 ppt (336 ± 96.1 mg kg⁻¹ h⁻¹) relative to 30 ppt (265 ± 83.0 mg kg⁻¹ h⁻¹). The critical oxygen tension (O₂crit) and incipient lethal oxygen saturation (ILOS) were measured to assess hypoxia tolerance. Salmon reared in 2.5 ppt (O₂crit: 14.8 ± 4.73%; ILOS: 12.8 ± 5.89%) had a greater tolerance for hypoxia when compared to 10 ppt (O₂crit: 20.8 ± 8.67%; 16.4 ± 3.23%) and 30 ppt (O₂crit: 18.7 ± 1.77%). Recent studies have found that localized plasma-accessible carbonic anhydrase (paCA) at the tissues and β-adrenergic sodium (Na⁺)/hydrogen (H⁺) exchangers (βNHE) on the red blood cells allow teleosts to greatly increase oxygen extraction to metabolically active tissues, without compromising O₂ uptake at the gill. This has been demonstrated during sustained exercise in salmon, but its role has not been investigated during exposure to environmental hypoxia. To address this, O₂ levels were reduced at a rate of 1.79 ± 0.21% O₂ min⁻¹ with and without injection of C18, a membrane impermeable CA inhibitor. In measurements of % air saturation at ILOS, fish injected with C18 (19.50 ± 1.74%) demonstrated reduced hypoxia tolerance compared to those receiving a saline injection (13.17 ± 1.90%), indicating that functioning paCA enabled salmon to maintain their orientation in waters with over 30% less environmental O₂