GENETIC AND ENVIRONMENTAL VARIATION OF BODY-WEIGHT IN RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

Body weight data covering the period from birth year 1978 to 1986 from a selection experiment with rainbow trout conducted at the University of California at Davis were analyzed with regard to the genetic and e nvironmental variation in three lines (Line C reproduced by random sel ection; Line E selected for egg size; Line Y selected for body weight at 364 days of age). Variance components for body weight from 168 to 3 64 days were estimated using a derivative-free restricted maximum like lihood algorithm under a single trait animal model. Heritability estim ates were low, ranging from 0.03 to 0.13. The estimates for Line Y wer e lower than those for Line C and Line E, possibly a result of samplin g error when the three lines were formed from a small common base popu lation. The heritability estimates tended to increase with age; averag ed over the three lines, the estimates were lowest at 168 days (0.05) and highest at 336 and 364 days (0.10). Full-sib family effects caused by factors other than additive genetic effects were relatively large, ranging from 0.01 to 0.17 as a proportion of the phenotypic variance, and also increased with age, being lowest at 196 days (0.03) and high est at 364 days (0.14) averaged over the three lines. Both common envi ronmental and non-additive genetic effects could contribute to this co mponent. Common environmental effect may result from the competition f or space between families owing to the particular rearing procedures u sed for the experiment. In addition, the different inbreeding levels a mong families may have increased the variation between families by red ucing the expression of non-additive genetic effects to different degr ees. The age trend of full-sib family effects on body weight may be a reflection of the accumulation of competition effect and the increase of non-additive genetic effect as the fish grew larger.