Summer survival has become an increasingly critical focus for salmonid producers, as climate change has heightened thermal stress, leading to reduced growth and increased mortality. Improving thermotolerance through selective breeding offers a promising strategy to protect animal welfare and commercial returns. However, selecting for thermotolerance remains challenging due to the variability of environmental conditions and the invasive nature of heat challenge tests.
This study reports genetic parameters and improvement strategies for thermotolerance in a multi-generational Chinook salmon (Oncorhynchus tshawytscha) breeding program, based on both laboratory challenge tests and summer cage survival data. A laboratory-based thermotolerance challenge was conducted across three year-classes. Individuals were exposed to progressively increasing temperatures (15°C to 23.5°C) and maintained at elevated temperatures for over two weeks in recirculating systems. Traits recorded included feeding behaviour, weight and length (pre- and post-challenge), and time to death (TTD).
TTD displayed moderate heritability (h² ~0.4) across all year-classes, supporting its utility for selection. Incorporating genomic predictions via a medium-density, sequencing-based platform increased Estimated Breeding Value (EBV) accuracy by ~10% compared to pedigree-based methods. Importantly, thermotolerance traits measured in the lab were moderately and consistently correlated with summer cage survival within the same year-class (r ≈ 0.3–0.5), validating the relevance of the challenge test to real-world performance.
These results highlight the value of combining validated laboratory phenotypes with genomic selection to address complex traits like thermotolerance. Integrating challenge-test performance with field-based survival data enables cumulative genetic gains, enhances fish resilience under warming conditions, and strengthens the sustainability of salmonid aquaculture.