Intact genetic structure and high levels of genetic diversity in bottlenecked sockeye salmon (Oncorhynchus nerka) populations of the Fraser River, British Columbia, Canada

Analysis of six microsatellite loci in 5800 sockeye salmon (Oncorhynchus ne rka) from 29 Fraser River populations provided little evidence of genetic b ottlenecks or mass straying in upper Fraser sockeye salmon resulting from r educed abundances following 1913-1914 rockslides in the Fraser canyon and s uccessive decades of high exploitation. Upper Fraser populations were not c haracterized by a paucity of rare alleles, a sensitive indicator of populat ions in which effective size has been recently reduced. Heterozygosity and allelic diversity did not differ consistently between lower and upper Frase r populations. Throughout the watershed, early-migrating populations had lo wer allelic diversity and a lower proportion of rare alleles than did late- migrating ones. Genetic differentiation between upper and lower Fraser popu lations and heterogeneity among lower Fraser populations supported the sugg estion that Fraser sockeye salmon are descendants of at least two postglaci al "races." Variation among lakes within regions was the strongest componen t of genetic structure, accounting for five times the variation among popul ations within lakes and more than two times the variation among regions. Ex tensive historical transplants of eggs and juveniles apparently resulted in lit tle gene flow among regions, but three populations were reestablished or rebuilt as the result of more recent transplants.