Phenotypic integration of behavioural and physiological traits is related to variation in growth among stocks of Chinook salmon

The selection for a single organismal trait like growth in breeding programs of farmed aquaculture species can counter-intuitively lead to lowered harvestable biomass. We outbred a domesticated aquaculture stock of Chinook salmon (Oncorhynchus tshawytscha) with 7 wild stocks from British Columbia, Canada. We then examined how functionally related traits underlying energy management - diel variation in cortisol; foraging, social, and movement behaviours - predicted stock-level variation in growth during the freshwater life history stage, a performance metric under aquaculture selection. Outbreeding generated significant variation in diel cortisol secretion and behaviours across stocks, and these traits covaried, suggesting tight integration despite hybridization. The coupling of nighttime cortisol exposure with daytime behavioural phenotype was the strongest predictor of stock-level variation in body mass. Our results suggest selecting for an integrated phenotype rather than on single mechanistic traits alone can generate the greatest effect on aquaculture fish growth under outbreeding practices. Furthermore, selecting for these traits at the stock level may increase efficiency of farming methods designed to consistently maximize fish performance on a large scale