PREDICTING AND IMPROVING THERMAL TOLERANCE WITHIN A COMMERCIAL ATLANTIC SALMON Salmo salar BREEDING PROGRAM

The marine (post-smolt) phase of the salmon production cycle typically exposes fish to seasonal temperature fluctuations that are potentially harmful, a concern that is exacerbated by anthropogenic climate change (i.e., global warming). Selection for thermal tolerance could present a successful long-term mitigation strategy. To this end, we examined individuals from 105 families from a single year class from an Atlantic salmon breeding program collaboration (Mowi Canada East and Huntsman Marine Science Centre) to: (1) develop a suitable method for explicitly testing climate change thermal tolerance and (2) determine the level of variability and heritability for relevant traits, and correlations between them, to allow selection.

It is imperative that broodstock programs collect relevant data to measure performance for the specific traits of interest. Our inclination was to test temperature tolerance using the critical thermal maximum challenge (CTmax) given its relatively quick and non-lethal biological endpoint. However, this approach does not use real-world rates of temperature increase as experienced in marine farm operations. Therefore, our initial efforts had to compare individual fish CTmax performance with that from the long-term and lethal incremental thermal maximum challenge (ITmax).

We tested 1,503 individually tagged Atlantic salmon over 126 CTmax trials using an increase of 0.4 °C/min to loss of equilibrium. Following recovery, 765 of these fish and 127 naïve fish were entered into a single ITmax challenge with a more gradual temperature increase of 1.0 °C/day to 16 °C and then 0.3 °C/day to reach morbidity/mortality of the entire population. Individual fish performance from the CTmax challenges had no predictive value for individual performance in the more realistic ITmax challenge (linear regression:  p = 0.22, r² = 0.002). These results were consistent when assessed using pedigree information, with genetic and phenotypic correlations of -0.01 ± 0.16 and 0.07 ± 0.05, respectively (± SE). Heritability estimates were high for both challenges: 0.47 ± 0.08 and 0.40 ± 0.08, respectively (h² ± SE).

Our climate change tolerance research conclusions to date are that:

1. Completing ITmax challenges are required in favour of CTmax trials to measure the effects of climate change on farmed Atlantic salmon populations;
2. Estimated heritabilities are quite high thereby allowing effective selection to increase seawater temperature tolerance due to climate change; and,
3. Genetic and phenotypic correlations between several measured traits indicate that both increases in seawater weight (growth) and increasing seawater temperature tolerance can be selected and improved simultaneously.

Aspects of this presentation were recently published https://doi.org/10.1016/j.cbpa.2022.111143.