Beacham et aL; Geographic basis for population structure in Oncorhynchus tshawytscha 



235 



and north Thompson regions were both closely related to 

 mid Fraser populations, but less similar to each other. The 

 lower and south Thompson regions were distinctive from 

 other regions (with the exception of the Louis Creek popu- 

 lation from the north Thompson which resembled lower 

 Thompson populations), and from each other Removal of 

 the Louis Creek population from the analysis resulted in 

 the Fgj. value among the North Thompson populations de- 

 creasing from 0.043 to 0.026 and the Fg^. value between the 

 north and lower Thompson regions increasing from 0.047 

 to 0.058 (Table 3). 



Heterozygosity was highest in the lower Fraser region 

 but similar among all interior Fraser regions (Table 4). Al- 

 lelic diversity was also high in the lower Fraser samples, 

 but on a regional basis, allelic diversity was highest in the 

 mid Fraser region followed by the south Thompson and 

 lower Fraser regions. More unique alleles were observed 

 per hundred fish sampled in the Birkenhead and Lower 

 Fraser region than elsewhere in the drainage (Table 4). 

 Among the interior regions, the mid Fraser and south 

 Thompson regions each had more than one unique allele 

 recorded per hundred fish sampled, whereas the remaining 

 regions had less than one. 



Distribution of genetic variation 



Gene diversity analysis of the 12 microsatellite loci in 

 HWE was used to determine the magnitudes of variation 

 among annual samples within populations and of varia- 

 tion among chinook salmon populations of seven regions 

 (Birkenhead treated as a region) or six regions (the Birken- 

 head River population excluded). For seven regions, the 

 hierarchical analysis indicated that 95.0% of the variation 

 occurred within populations (Table 5). Variation among 

 regions was the greatest source of the remaining variation 

 (3.1%), followed by variation among populations within 

 regions (1.3%), and variation among years within popula- 

 tions (0.5%). With the Birkenhead population excluded, 

 regional variation accounted for 2.6% of the total, whereas 

 the variation attributed to populations and sampling year 

 was unchanged. The differences observed among regions. 



among populations within regions, and among years within 

 populations were significant (all P<0.01). On average, dif- 

 ferences among populations (includes regional and popula- 

 tion components) were about 8.5 times greater than annual 

 variation within populations. 



Structure within major tributaries 



In many cases, samples from separate locations within trib- 

 utary systems clustered together (Fig. 2), indicating that a 

 higher rate of gene flow may occur among chinook salmon 

 spawning aggregates within tributaries. Gene diversity 

 analysis confirmed that chinook salmon sampled from 

 separate spawning locations within tributaries tended 



