Rainbow trout (Salmo gairdneri) populations of the Great Lakes showed a great variability in timing of spawning migrations and life histories. This variability was examined to determine if rainbow trout populations of the Great Lakes are comprised of discrete stocks. Differences in timing of migration and spawning indicated that at least one spawning population may be distinctive and that others may be in progressive stages towards emergence of discrete stocks. The innate ability of this species to adapt to different environmental conditions, together with its ability to 'home,' provide rainbow trout in the Great Lakes with the potential to develop discrete stocks. The management of this species that allows for the development of this potential is emphasized.Key words: rainbow trout. Great Lakes, stocks, migrations, spawning, life histories, homing

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Variations in discharge and water temperature influenced movements of spawning rainbow trout between Lake Huron and Bothwell's Creek.

Adult fish moved into Bothwell's Creek, Lake Huron, between October 29 and May 3. Run A (October 29–February 15) was composed of 640 fish, run B (February 16–May 3) of 610 fish. Run A spawned between December 29 and February 14 and left the river. Run B spawned between February 20 and April 30. Total lengths of males ranged from 31 to 80 cm, females from 21 to 90 cm. Fourteen categories of life history were identified within the population of which 99% had become smolts after 2 years of stream life. Fish of age 5 were dominant. Differences in age class structure and size of fish occurred between runs A and B. Sea lamprey scars were evident on 8.6% of fish, all over 46 cm in total length.