Abstract
The relationship between specific growth rate and fish size (weight) was influenced by temperature and genotype in two strains of rainbow trout Oncorhyncus mykiss (formerly Salmo gairdneri) and their hybrid. Heterogeneity among the regression slopes for strains (genotype) as well as interaction with temperature invalidated the use of analysis of covariance to adjust for initial fish size differences. Regression slopes ranged from –0.74 ± 0.21 (SE) to –0.01 ± 0.16 among fish families reared at 7°C and from –0.41 ± 0.03 to –0.24 ± 0.09 among fish families reared at 15°C. The use of a particular slope value as a universal constant to compare specific growth rates offish is questionable.

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