The Grand Coulee Fish Maintenance Project: a Genetic Blender or Evidence for Local Adaptation in Columbia River Sockeye Salmon

Although contemporary sockeye salmon populations in Lake Wenatchee and the Okanogan River are genetically distinct, it was unknown whether these differences are representative of their ancestral population structure or are a reflection of the extensive transplant and hatchery supplementation that occurred during the Grand Coulee Fish-Maintenance Project (GCFMP). From 1939 through 1943, all sockeye salmon adults returning to the Upper Columbia River were intercepted at Rock Island Dam and transported either to Lake Wenatchee or Lake Osoyoos (the rearing lake for the Okanogan River population) to spawn naturally, or to hatcheries for artificial propagation and distribution of the progeny into one of the two lakes.  The practice mixed the major stocks of the Upper Columbia River system and the genetics of the original populations were assumed to have been homogenized. We examined archival scales from 603 fish, sampled from May to July 1924 in a Lower Columbia River commercial fishery, for genetic variability at 12 microsatellite loci and found two putative historical populations in this mixture that were genetically similar to extant populations in the Okanogan and Wenatchee rivers (pairwise F_ST = 0.0036 and 0.0030, respectively).  Analysis of 185 scale samples from the first generation progeny of the GCFMP returns to Lake Wenatchee (W) revealed that Okanogan (O) and hybrid (WxO) sockeye also returned to Lake Wenatchee 1943. Despite the return of a population mixture, the genetic differences between the Okanogan River and Lake Wenatchee sockeye salmon populations have remained strikingly similar to their ancestors over the past 87 years. These differences have been maintained in spite of large fluctuations in abundance and an history of transplantation and artificial propagation associated with the GCFMP.  These results illustrate the potential of local adaptation and natural selection in maintaining the genetic population structure of lake-rearing sockeye salmon populations.