Genetic variability in rainbow trout populations

Rainbow trout ( Oncorhynchus mykiss) populations have been distributed by man from a relatively small range on the Pacific coast of the US and Canada to a new range that extends around the world. The magnitude of this range extension and the broad array of situations in which this species has been employed suggest it to be a very adaptable organism. A review of the literature on the distribution of rainbow trout and reports on the genetic structure of artificially and naturally reproducing populations revealed some unique characteristics. Rainbow trout have a rather high level of heterozygosity, as revealed by electrophoretic analyses of constitutive proteins. Within the natural range, this variation is rather uniformly distributed across population units. Most genetic variation (92%) was found within populations, which appeared to be an important characteristic of hatchery stocks. Only in the presence of strong selection pressure or severe bottlenecks was the intrapopulation genetic variability diminished, even after a number of generations under artificial rearing conditions. Maintenance of within-population genetic variation is thus a major feature of rainbow trout that needs to be considered in working with both naturally and artificially reproducing populations.