Triploid Atlantic salmon are physiologically disadvantaged at larger body sizes.

Triploid Atlantic salmon are sterile and used in aquaculture to prevent escapees from breeding in the wild. Meanwhile, triploids suffer poor animal welfare in the latter marine growth phase. Previous experiments have mainly tested smaller fish, and physiological differences between triploids and diploids tended to be subtle or non-existing. We therefore hypothesized that triploidy first becomes a disadvantage at larger body sizes where scaling constraints become more magnified in triploids owing to them having larger cells with lower surface to volume ratios. We measured metabolic rates, stress responses, hypoxia tolerance, and critical thermal maximum in big (≈3 kg) triploid and diploid Atlantic salmon. Additionally, we assessed gill histology metrics. Big triploids had higher standard metabolic rates, lower aerobic scopes, and reduced tolerances to hypoxia and thermal stress. Oxygen extraction coefficients were overall lower in triploids, suggesting reduced efficiency in gill oxygen uptake. This was further supported by lower lamellar densities which indicate less gill surface area. In conclusion, big triploid Atlantic salmon were more vulnerable to environmental extremes driven by oxygen supply limitation and higher basal maintenance costs. This provides a mechanistic explanation for why triploids become prone to animal welfare issues in the latter growth phase of marine aquaculture.