Estimates of phenotypic and genetic parameters for within-season date and age at spawning of female rainbow trout

The phenotypic and genetic characteristics of female spawning date and age at spawning within season were studied by analysing data from rainbow trout populations in which females spawned completely within the second year. Sp awning date was measured as the number of days from December 31 of the year preceding spawning. The estimates of regression of daughter on dam and ful l-sib intraclass correlation for this trait were positive and large. Based on an animal model, the estimate of heritability was high and full-sib fami ly effect was very low. Age at spawning was calculated as days from birth d ate to spawning date, using the date the eggs are fertilized (i.e., spawnin g date of dam) as the birth date of the daughters. The results showed a neg ative regression of daughter age on dam age but a positive and high full-si b intraclass correlation for age at spawning. Consequently, the estimated a dditive genetic variance was very low along with a high estimate of full-si b family effects. Further examination on the features of covariance between daughter and dam confirmed that the regression of daughter on dam for obse rved age at spawning is expected to be negative. The cyclic nature of the s pawning season over generations could be the biological basis of this resul t. The results indicated that age at spawning measured as the days from dam spawning date to daughter spawning date is not an effective measurement to assess the genetic effects, and selection based on such a phenotype is not expected to yield a response. Therefore, selection for late sexual maturit y, in order to avoid the unfavorable influence of the sexual maturation pro cess on growth, is not applicable for those stocks in which females spawn c ompletely in a single spawning season. Instead, the approach to breed impro vement should be designed to select for high growth rate with culling of ea rly sexual maturing fish, so that fish can reach optimal body weight before sexual maturity. (C) 1999 Elsevier Science B.V. All rights reserved.