Beacham et a\: Geographic basis for population structure in Oncorhynchus tshawytscha 



239 



The disequilibrium observed in the Bessette Creek sample 

 may simply reflect small sample size { 17 fish). 



Samples of Fraser River chinook salmon of the Harrison 

 and Chilliwack rivers were little differentiated (Fgj,=0.004), 

 in accordance with their common origin from the Harrison 

 River system. In addition, eggs from early migrating red- 

 fleshed chinook salmon from Bowron River and Slim Creek 

 in the upper Fraser and from the Quesnel and Chilko rivers 

 of the mid Fraser were transplanted to the lower Fraser 

 hatcheries between 1985 and 1988. Fish from these trans- 

 plants returned to the lower Fraser hatcheries, maintaining 

 an early migration time and red flesh coloration that dis- 

 tinguishes them from the native fall-run population. Fish 

 returning from the initial releases were chosen as brood 

 stock for succeeding generations only if they carried a coded 

 wire tag indicating that they were of Slim Creek or Bowron 

 River origin. Transplantation from the Chilliwack to the 

 Chehalis hatchery of the red-fleshed fish occurred during 

 years in which returns to the Chehalis hatchery were low. 

 The observed strong genetic affinity of the Chehalis and 

 Chilliwack red-fleshed fish to each other and to upper Fra- 

 ser chinook salmon reflects this transplantation record. 



Although the lower Fraser was the most genetically dis- 

 tinct of the six geographic regions examined, the genetic 

 distinctiveness of the upper Fraser, lower Thompson, and 

 south Thompson populations from each other was almost 

 as great as their differentiation from the lower Fra- 

 ser populations. Differentiation among regions within the 

 smaller Thompson drainage was greater than that between 

 the mid and upper Fraser regions of the larger interior 

 Fraser drainage. The mean pairwise F^j. value within the 

 Thompson drainage was 0.050, whereas within the interior 



Fraser drainage it was 0.028. Relatively strong differen- 

 tiation among salmonids of the north, south, and lower 

 Thompson regions has been noted previously not only for 

 chinook salmon (Teel et al., 2000), but also coho (Small et 

 al., 1998) and sockeye (Withler et al, 2000) salmon. The 

 consistent population structure of the region across species, 

 with quite distinct patterns of spatial variability (Myers et 

 al., 1998), indicates that colonization of the Thompson and 

 interior Fraser drainages may have been episodic, rather 

 than a single event. If this is the case, the current affinities 

 among the interior Fraser and Thompson regions may rep- 

 resent some combination of related but distinct founding 

 populations and subsequent gene flow. 



Origin of chinook salmon in the Fraser River 



During the Wisconsin glaciation, ice-free refugia existed to 

 the south of British Columbia in the Columbia River drain- 

 age and Pacific coastal regions. Extant chinook salmon pop- 

 ulations in these areas display a genetic dichotomy that is 

 well correlated with juvenile life history type (Myers et al., 

 1998). The presence of genetically distinct chinook salmon 

 in the lower and interior Fraser River drainages has led to 

 the suggestion that the drainage was colonized indepen- 

 dently by stream-type chinook salmon from the Columbia 

 refuge and ocean-type chinook salmon from a Pacific coastal 

 (Teel et al., 2001) or northern Beringial (Utter et al., 1989) 

 refuge. In a large survey of chinook salmon populations, 

 including those of ocean- and stream-type populations from 

 nonglaciated southern regions. Utter et al. ( 1989) found that 

 87% of genetic variation was contained within populations, 

 and the remainder was partitioned among populations. In 



