Negligible differences in metabolism and thermal tolerance between diploid and triploid Atlantic salmon (Salmo salar)

The mechanisms that underlie thermal tolerance in aquatic ectotherms remain unresolved. Triploid fish have been reported to exhibit lower thermal tolerance than diploids, offering a potential model organism to better understand the physiological drivers of thermal tolerance. Here, we compared triploid and diploid juvenile Atlantic salmon (Salmo salar) in freshwater to investigate the proposed link between aerobic capacity and thermal tolerance. We measured specific growth rates (SGR) and resting (aerobic) metabolic rates (RMR) in freshwater at 3, 7 and 9 weeks of acclimation to 10, 14 and 18°C. Additionally, maximum metabolic rates (MMR) were measured at 3 and 7 weeks of acclimation, and critical thermal maxima (CT_max) were measured at 9 weeks. Mass, SGR and RMR differed between ploidies across all temperatures at the beginning of the acclimation period, but all three metrics were similar across ploidies by week 7. Aerobic scope (MMR-RMR) remained consistent across ploidies, acclimation temperatures and time. At 9 weeks, CT_max was independent of ploidy, but correlated positively with acclimation temperature despite the similar aerobic scope between acclimation groups. Our findings suggest that acute thermal tolerance is not modulated by aerobic scope, and the altered genome of triploid Atlantic salmon does not translate to reduced thermal tolerance of juvenile fish in freshwater.