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Is tolerance to rising seawater temperature heritable in Atlantic salmon?

St. Andrews, New Brunswick, Canada – September 17, 2024

Rising temperature due to climate change poses a threat to both wild Atlantic salmon (Salmo salar) populations in their natural habitat and farmed populations during their major growth phase in coastal (seawater) net pens. Recent research completed in the wet laboratory facilities at The Huntsman Marine Science Centre examined variation in post-smolt seawater thermal tolerance amongst 105 Atlantic salmon families to determine the capacity for adaptation in an increasing temperature future. 

“We must determine whether a specific trait of interest is passed down genetically from parents to their offspring before incorporating any new trait into a commercial broodstock program. To do this, we estimate heritabilities of the traits which, in this case, are measures of thermal tolerance,” explains Dr. Amber Garber, a Huntsman Marine Research Scientist and anchor co-author on the published study in the peer-reviewed journal Aquaculture Reports. 

There are two established thermal challenge protocols that are used with fish species. The critical thermal maximum (or CTMax) raises the water temperature rapidly until the challenged fish lose their equilibrium as the scientific endpoint. This challenge does not reflect real-world scenarios but are favored given its short duration and non-lethal endpoint. The incremental thermal maximum (or ITMax) provides a more natural increase in water temperature over an extended period of time but uses morbidity or mortality as the scientific endpoint. “Our group previously compared the CTMax and ITMax values for the same fish in an earlier publication and unfortunately the CTMax value of a specific fish does not reliably predict performance of the same fish when presented with environmentally relevant ITMax conditions,” adds Garber. 

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Past research has demonstrated significant CTMax and ITMax variation within Atlantic salmon populations but none have reported heritability of CTMax performance and only previous research from the Huntsman Marine group have published ITMax heritability from a single year class of Atlantic salmon. This new research offered the first ever heritability estimates for CTMax and ITMax from within the same year class of Atlantic salmon while also exploring the consistency of ITMax heritability across multiple year classes. 

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The research team established challenges as follows: 1,506 Atlantic salmon experienced a CTMax challenge with rapid temperature increase of 0.4◦C per minute while 936 fish were enrolled in a slower ITMax increase of 1.0◦C per day to 16◦C and then 0.3◦C per day to the end of the experiment. All fish were weighed after smoltification and again just after reaching their CTmax loss of equilibrium temperature and upon completion of the ITmax challenge. Blood samples and heart measurements were also taken from a subset of challenged fish to explore the potential to identify proxies that might predict high temperature tolerance without having to complete the actual challenges.

There was considerable within- and among-family variability measured in both CTMax and ITMax values. Having variability present within a population is essential for selective improvement to occur. The study reported both CTmax and ITmax to be strongly heritable traits with estimated heritability of 0.47 and 0.40, respectively. Heritabilities were also quite high for the other measured traits, such as body mass at 0.30, fish condition factor at 0.43 and heart ventricle mass at 0.36 for ITMax fish but none of these additional measured or calculated traits were correlated with thermal tolerance and so cannot be used as a reliable predictor of temperature tolerance performance. 

“Our results suggest that improvements for acute and chronic high-temperature tolerance may be possible for farmed Atlantic salmon through directed breeding programs,” offers Garber while wild salmon populations could have some capacity to adapt to increasing water temperature brought about by climate change but intergenerational gains may be too slow to meet the rate of warming experiences. Unfortunately, no reliable, easy to measure, proxy trait was identified to avoid completing challenges with fish and families. The large within-family variability amongst siblings can also make it difficult to select exceptional performing individuals within the same families. “We are now placing more effort into genomic studies to discover specific genetic markers important for thermal tolerance and that information can help us distinguish high performing individuals in the breeding groups,” concludes Garber. 

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The research publication was co-authored by researchers at the University of New Brunswick (Fredericton, NB) and Mowi Genetics AS (Bergen, Norway). The study was funded by the Atlantic Canada Opportunities Agency (ACOA) Atlantic Innovation Fund, New Brunswick Innovation Foundation (NBIF) Research Innovation Fund, and industry partners.

Research Publication: 

Benfey, T., S. Gonen, C.B. Bartlett and A.F. Garber. 2024. Thermal tolerance has high heritability in Atlantic salmon, Salmo salar. Aquaculture Reports 37: 102249. https://doi.org/10.1016/j.aqrep.2024.102249


Image 1: Atlantic salmon fry assessment and PIT tagging pre-challenge on the Huntsman Marine campus to later identify individuals to family.

Image 2: Study wet laboratory on the Huntsman Marine campus where CTMax and ITMax challenges occurred.

Image 3: Heart sampling from challenged Atlantic salmon in the Huntsman Marine processing lab to assess possible correlations with increasing temperature tolerance. Story by Bud Adams, The Huntsman Marine Science Centre. 

Story by Bud Adams, The Huntsman Marine Science Centre.  

For additional information or images relating to this article, please email huntsman@huntsmanmarine.ca. 

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