Thermal tolerance has high heritability in Atlantic salmon, Salmo salar

Rising temperatures due to anthropogenic climate change pose a threat to wild Atlantic salmon (Salmo salar) populations in their natural habitat and to farmed populations during their major growth phase in coastal (seawater) net pens. Given that tremendous gains have been made in farmed salmon production through artificial selection programs for traits such as growth rate and disease resistance, we therefore examined variation among 105 families in post-smolt seawater thermal tolerance to assess whether this trait warrants inclusion in a selective breeding program. We used two established thermal challenge protocols for this: a rapid temperature increase using loss of equilibrium as the endpoint (critical thermal maximum; CTmax [1506 fish]) and a slower increase with mortality or morbidity as the endpoint (incremental thermal maximum; ITmax [936 fish]). High estimated heritability values were obtained for both (h² = 0.47 and 0.40, respectively), suggesting that improved acute and/or chronic high-temperature tolerance may be attainable for farmed salmon through artificial selection. Furthermore, given that farmed salmon are not many generations removed from wild, wild populations may also have some capacity to adapt to increasing temperatures brought about by climate change. However, we found no genetic correlation between CTmax and ITmax. Genetic correlations between these indices and other traits that might influence thermal tolerance (body size, condition factor, ventricle size, and hematocrit) were absent or, at most, weak. •Both critical and incremental thermal tolerance (CTmax and ITmax) have high heritability.•There is no significant genetic correlation between CTmax and ITmax.•Thermal tolerance is not genetically correlated with body size, condition factor, ventricle size, or hematocrit.