THE GENETICS OF BODY-WEIGHT AND ITS EFFECT ON EARLY MATURITY BASED ON INDIVIDUALLY TAGGED RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

The effect of sex and temperature on body weight, the effect of body weight on age of maturity, and genetic parameters for body weight at ten ages were investigated for progeny from 18 two-year females and 54 yearling males through 740 days post-fertilization. Sexes were classified into three categories: females, early maturing males (E-males), and other males (T-males). E-males were significantly heavier than T-males and formed a disproportionate percentage of the heaviest males at ages prior to maturity the first year. However, T-males were significantly heavier than E-males after the E-males passed first maturity, and they were heavier than females between 362 and 628 days. Members of each full-sib family were placed into low and high temperature treatments. Temperature significantly influenced body weight at all ages except 278, 362, and 740 days. By increasing body weight, the high temperature treatment produced almost three times the proportion of E-males as the low temperature treatment. Genetic parameter estimates were obtained using derivative-free restricted maximum likelihood techniques to analyze a mixed, additive genetic and maternal model. Additive genetic, maternal, and residual variances were estimated as well as the additive genetic-maternal covariance. Heritability estimates ranged from 0.21 to 0.53 and were highest at young ages and lowest at later ages. The ratio of maternal to phenotypic variance was consistent and low, ranging between 0.02 and 0.03. However, additive genetic-maternal correlations varied considerably with age. They were all positive and ranged from 0.28 to 1.00; only three estimates, at 278, 398 and 740 days, were above 0.53. Genetic correlations between body weights at seven ages ranged from 0.19 to 0.95 and phenotypic correlations ranged from 0.29 to 0.94. Genetic and phenotypic correlations were consistently high between contiguous ages (0.74 to 0.95) and generally decreased as the difference between ages increased. The evidence supported the hypothesis of a strong positive relationship between body weight and early maturity based on numerous genetic parameters estimated for body weight. The availability of such information will facilitate genetic decision making.