A GENETIC EVALUATION OF THE INFLUENCE OF STOCKING DENSITY ON THE EARLY GROWTH OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

The influence of stocking density on mean body weight and on genetic a nd environmental components of phenotypic variation for body weight wa s explored. It was hypothesized that high stocking density may suppres s environmental variation in body weight arising from hierarchical int eractions, resulting in a larger ratio of genetic to environmental var iation. Thirty-two genetically marked full-sib families were reared to 130 days at five stocking densities ranging from 5 to 95 fish l-1 in blocks of eight families. Intermediate densities of 20 and 45 fish l-1 provided the best growth through 81 days, though no differences in me an body weight were encountered among stocking densities at older ages . A family by environment interaction was observed at 130 days; subseq uent analyses suggested that both genotype by rearing container and ge notype by density effects may have contributed to the interaction. Gen etic variation was significantly smaller, and environmental variation significantly larger, at 20 fish l-1 than at stocking densities of 45 fish l-1 or more. It could not be determined whether differences in ex pression of social behaviors or differences in genes controlling growt h were primarily responsible for the observed differences in variation across densities.