Population structure and history 
of steelhead trout in California

John Carlos Garza, Ph.D.
Southwest Fisheries Center
Santa Cruz Laboratory

Wednesday, February 5, 2003
3:00 p.m. Pacific Forum

Steelhead trout (Oncorhynchus mykiss) are the most wide-spread of the anadromous salmonids, ranging naturally from Southern California to Russia. Genetic population structure and demographic history of coastal steelhead trout in California are investigated using data from 18 microsatellite loci and samples collected from approximately 60 sites covering almost the entire range of the species in coastal California. The importance of such data in Endangered Species Act recovery efforts will be outlined.

A new method for detecting recent reductions in effective population size using population genetic data will also be presented, and the performance of three methods will be com-pared. Error rates, recovery times, and effects of migration are examined. A method for estimating the loss of genetic variation due to reduction in population size is also presented. The results emphasize the importance of using multiple methods and suggest a way to detect migration into the focal population.

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