Genetic parameters for upper thermal tolerance and growth-related traits in rainbow trout (Oncorhynchus mykiss)

We estimated genetic (co)variance parameters for body weight (BW), (Fulton's) condition factor (Kf) and resistance to acute thermal shock (upper thermal tolerance; UTT) in a three-generational rainbow trout (Oncorhynchus mykiss) pedigree derived from two commercial strains (Spring Valley (SV), Rainbow Springs (RS)). Two grandsires and several granddams in each strain were bred to create 63 second-generation pure strain families which were bred in turn to form 118 mixed third-generation half sib families (n = 5752). Heritability was high for all three traits (h2UTT = 0.41 ± 0.07; h2BW = 0.46 ± 0.04; h2Kf = 0.52 ± 0.04) in an extended animal model including terms for strain origin and hybrid effects. Genetic correlations between UTT and BW were essentially zero (ra = − 0.03 ± 0.18), positive between UTT and Kf (ra = 0.49 ± 0.13) and positive between BW and Kf (ra = 0.11 ± 0.08). RS fish had higher UTT (α = 303 deg min) but lower BW (α = − 0.19 g) and Kf (− 0.032 g (mm3)− 1) than SV fish, suggesting fixed genetic effects of strains. However, only genetic variance for UTT appeared to be biased upwards by the failure to include strain effects in heritability estimation (h2UTT = 0.48 ± 0.08). Failure to account for strain effects also resulted in slight overestimation of genetic correlation between UTT and BW (ra = − 0.17 vs. − 0.03) but partial underestimates between UTT and Kf (0.29 vs. 0.49). Hybrid fish were less thermotolerant (α = − 59 ± 15 deg min) and heavier (0.16 ± 0.049 g) but had lower Kf (− 0.023 g (mm3)− 1) than pure strain individuals, but hybrid effects accounted for relatively little genetic variance overall. Our results indicate substantial selective potential on these traits, but that the efficacy of selection for certain traits might depend on more complex genetic architecture.