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 and environmental components of phenotypic variation for body weight was explored. It was hypothesized that high stocking density may suppress environmental variation in body weight arising from hierarchical interactions, resulting in a larger ratio of genetic to environmental variation. 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 mean body weight were encountered among stocking densities at older ages. A family by environment interaction was observed at 130 days; subsequent analyses suggested that both genotype by rearing container and genotype by density effects may have contributed to the interaction. Genetic 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 expression of social behaviors or differences in genes controlling growth were primarily responsible for the observed differences in variation across densities.