Genetic structure and relationships among steelhead trout (Oncorhynchus mykiss) populations in British Columbia

Steelhead trout. Oncorhynchus mykiss (the anadromous form of rainbow trout) , are declining over much of their range around the Pacific rim. We nondest ructively collected tissue samples from 494 adult steelhead from eight trib utaries and two mainstem river sites within three watersheds in northern Br itish Columbia. Canada. We scored allele size for six highly polymorphic mi crosatellite DNA loci and provide primer sequences and polymerase chain rea ction conditions for five of these loci for the first time. The populations were significantly genetically differentiated (theta = 0.039, 95% confiden ce = 0.030-0.053). AMOVA showed that most of the genetic variation was at t he individual level (95.6%), although significant genetic variation existed at the tributary level (3.09%) and watershed level (1.31%). The calculated unbiased genetic distances were positively correlated with geographical di stance within watersheds (P < 0.01: r(2) = 0.35) indicating probable geneti c equilibrium. Tributary populations in two of the watersheds were not as g enetically divergent as would be expected given their large geographical se paration. Cross-headwater transfers of fish within relatively recent histor y are the most likely explanation of this anomaly. Seven of the eight tribu tary populations lit a regression line of mean heterozygosity vs. rearing h abitat area. The one anomalous population had a much lower heterozygosity t han expected based on the linear regression, and may thus be the population of greatest conservation concern.