Genetic Comparison of Naturalized Rainbow Trout Populations among Lake Superior Tributaries :

--Allelic frequencies at 27 polymorphic allozyme loci were compared among 15 collections 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 F•t 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. The rainbow trout Salmo gairdneri was the first exotic salmonid to establish naturalized populations in the Lake Superior basin. The first introduction of this species occurred near Sault Ste. Made, Ontario, in 1883 and consisted of fish derived originally from rainbow trout present in the McCloud River, California (MacCdmmon and Gots 1972). The fish from this source probably represented a mixture of resident and anadromous (steelhead) types, as both types co-occurred in the McCloud River at that time (Needham and Behnke 1962). The next introduction occurred in 1889 into coastal Wisconsin waters and into the Iron River, a Michigan tributary (MacCdmmon and Gots 1972). In 1892, the first rainbow trout were stocked into the Brute River, Wisconsin (O'Donnell 1944). "Steelhead" were recorded as having been stocked into Lake Superior in 1895 and these may have originated from eggs collected from Redwood Creek, California, and the Willamette and Rogue dyers, Oregon. Potential sources of steelhead that may have been stocked later include the Puget Sound area, the Columbia River, and Baker Lake of Washington. In 1941, rainbow trout of the Kamloops, British Columbia, strain were stocked into the Montreal River, Ontario. Due to the past stocking history, the rainbow trout that now occur in Lake Superior and its tributaries are probably derived from a variety of sources. Migratory spawning populations and fishedes for rainbow trout developed rapidly after the introductions of the late 1800s. By 1905, spawning populations were established along both the north and south shores of the lake (Bidgood and Berst 1967). By 1910, notable sport fishedes were present for rainbow trout in the Brute and St. Mary's rivers (O'Donnell 1944; MacCdmmon and Gots 1972). Rainbow trout are now present throughout the Lake Superior watershed and exhibit characteristics similar to those of anadromous Pacific coast steelhead. Tributary streams are used as nursery areas where young fish eventually smolt and migrate down to Lake Superior for feeding and growth. After 1-3 years, the fish mature and return to tributaries for spawning. Important sport fisheries now exist throughout the watershed for