Effect of inbreeding on body weight at harvest in rainbow trout, Oncorhynchus mykiss

Three nucleus breeding populations of rainbow trout (Oncorhynchus mykiss) u nder selection for six generations were investigated for the effect of inbr eeding on body weight at harvest. The coefficients of inbreeding for each i ndividual were estimated from complete pedigree information starting at bas e generation zero. For each population within generation, inbreeding effect was estimated using a fixed linear model and a sire-dam mixed additive gen etic model. Estimates of inbreeding effect per generation vary due to the d ifferent distributions of inbreeding coefficients in each generation. The m agnitude of inbreeding effect for each population across generations were e stimated using a fixed linear model and three sire-dam mixed models: additi ve (A), additive plus dominance (A + D), and additive plus dominance plus a dditive by additive epistasis (A + D + A x A) with the inbreeding coefficie nt, F as covariate. The estimates of inbreeding effect from the fixed linea r model were substantially lower than those from the sire-dam mixed models for all populations. The models (A + D) and (A + D + A x A) exhibited quite similar estimates of inbreeding effect, which were substantially higher th an the (A) model. The results suggest that addition of the additive by addi tive genetic effect in the (A + D) model does not have a significant outcom e on the estimation of inbreeding effect because additive by additive genet ic interaction may not be important for growth. For unbiased estimation of inbreeding effect, the (A + D) model with F as a covariate may be sufficien t. The inbreeding effect on body weight at harvest calculated as the averag e of the estimates obtained from models A + D and A + D + A x A, were -1.6, -5.0, and -4.5% per 10% unit increase in inbreeding coefficients for popul ations 1, 2, and 3, respectively. These values meant body weight at harvest was moderately affected by inbreeding but were not high enough to cause an y serious impact in the selective breeding program for rainbow trout in Nor way. This implies that maintenance of moderate effective population size pe r generation and following a strict mating policy for breeders selected as parents for future generations could delay the accumulation of inbreeding a nd minimize its effect on growth. (C) 2001 Elsevier Science B.V. All rights reserved.