Acclimation to moderate hypoxia improves acute hypoxia tolerance of diploid and triploid brook charr, Salvelinus fontinalis

Triploid salmonids are used in aquaculture and stocking programs for recreational fishing because they are reproductively sterile and therefore cannot breed in the wild. However, they appear to be more sensitive than diploids to environmental stressors. The objective of this study was to develop a better understanding of the hypoxia tolerance of triploids. We compared the acute hypoxia tolerance of sibling diploid and triploid brook charr that were acclimated to either moderate hypoxia (65 % of air saturation) or normoxia (98 % air saturation). Fish then underwent acute hypoxia trials, by slowly injecting nitrogen gas to displace oxygen and using loss of equilibrium (LOE) as the endpoint. The fish acclimated to moderate hypoxia had a lower oxygen tension (PO₂) at LOE compared to fish acclimated to normoxia, regardless of ploidy. Ploidy did not affect time to LOE, but triploids had a higher PO₂ at LOE (i.e., were less tolerant of acute hypoxia) compared to diploids. Potential predictors of acute hypoxia tolerance (hematocrit, blood glucose, blood lactate, relative ventricular mass, hepatosomatic index, condition factor, compact myocardium thickness, and interlamellar cell mass) did not provide conclusive results. However, relative interlamellar cell mass size was larger in triploids, suggesting reduced functional gill surface area for respiratory gas exchange as a possible explanation for their reduced acute hypoxia tolerance. This study has shown the capacity for moderate hypoxia acclimation to improve triploid performance, although not to the same extent as for diploids.