Genetic Comparison of Naturalized Rainbow Trout Populations among Lake Superior Tributaries: Differentiation Based on Allozyme Data

Abstract

Allelic frequencies at 27 polymorphic allozyme loci were compared among 15 col- lections of naturalized migratory rainbow trout Salmo gairdneri sampled from 10 stream drainages leading to Lake Superior. Multiple collections were made within the Brule River drainage in Wisconsin: one of spring-run spawning adults, one of fall-run adults, and four from Brule River tributaries. Significant allele frequency differences were observed among the 15 collections for 13 out of 21 loci examined (P < 0.05). Statistically significant differences occurred among rainbow trout from different drainages and among the four site-specific samples from Brule River tributaries. No significant differences in allelic frequencies occurred between fall-run and spring-run adult fish. Genetic fixation values Ft indicated that about 2.6% of the total genetic variation occurred among the 10 drainages and that about 2.1% occurred among the six Brule River samples. Groups of collections identified by cluster analysis of genetic distances usually contained samples that were geographically proximate, except where the number of adult spawners was believed to be less than 50. The level of population differentiation reported here for Lake Superior was less than that previously reported for native rainbow trout populations from Pacific coastal drainages or that observed for introduced brown trout resident along the south shore of Lake Superior. The spatial units of focus for management of rainbow trout in Lake Superior should be groups of geographically proximate small streams, and possibly individual large drainage systems.