GENETIC-VARIATION IN NATURALIZED RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) FROM MINNESOTA TRIBUTARIES TO LAKE-SUPERIOR

Genetic variation at 27 polymorphic loci that code allozymes was compared among collections of naturalized, anadromous-form rainbow trout (Oncorhynchus mykiss) from Lake Superior tributaries in Minnesota and among four hatchery strains. Genetic differences were observed among collections from naturalized populations from tributary streams and among hatchery strains (P < 0.01). In naturalized populations, most genetic variation observed occurred among individuals within samples (95.5%) and was comparable to values reported for other naturalized Lake Superior populations of rainbow trout. Cluster analysis of genetic distance coefficients revealed two main groups. One group included a subgroup of samples from heavily stocked streams (Devil Track, Flute Reed, and Baptism rivers) and the Michigan hatchery strain which had been used to stock these streams. The second group included collections from the Knife River drainage and Knife River hatchery fish. Collections from Indian Camp Creek and of the ''kamloops'' hatchery strain were the most differentiated of the samples and showed little genetic affinity to each other or the two main groups. The ''kamloops'' strain did not exhibit allelic frequencies expected for rainbow trout derived from the Kamloops region of the upper Fraser River of British Columbia. No evidence was found to support the contention that the ''kamloops'' hatchery strain had interbred with wild populations. Widespread stocking of Michigan strain fish may have reduced genetic differences among naturalized, wild populations along Minnesota's Lake Superior shoreline. Population-specific management of rainbow trout by tributary is a justifiable approach to consider. If maintenance of the genetic differences among the wild, naturalized populations is desired, ''kamloops'' and the Michigan hatchery strains should not be stocked. Stocking programs instead should use gametes from returning adults of a river for subsequent juvenile propagation and stocking back into the same river.