Evidence for extirpation of native kokanee in a large impounded watershed following introduction of a conspecific
Paige W. Breault, Mike Wetklo, J. Mark ShrimptonAbstract
Objective
Williston Reservoir in north‐central British Columbia was stocked with kokanee Oncorhynchus nerka from the Columbia River during the 1990s. A native population of kokanee already existed in the reservoir, but that population has not been found in sampling efforts since 2000. Most of the recent sampling, however, has targeted tributaries, and the stocked Columbia River‐type kokanee are stream spawners, whereas Thutade Lake kokanee, from which the native Williston Reservoir kokanee originated, are shore spawners.
Methods
In August 2021, pelagic surveys were conducted in Williston Reservoir by gillnetting and trawling to capture kokanee for subsequent genotyping to assess whether the native Williston Reservoir kokanee have persisted. We used 14 microsatellite loci to genotype 165 samples from the pelagic surveys and compared them to 623 previously genotyped kokanee from four reference populations: native reservoir fish; fish from the source population (Thutade Lake) that naturally colonized the reservoir; fish from isolated lakes in the Williston Reservoir watershed; and Columbia River‐type fish that now spawn in tributaries to the reservoir.
Result
Kokanee collected from the pelagic survey conducted in Williston Reservoir were entirely assigned to the Columbia River type by using the Bayesian clustering program STRUCTURE and a discriminant analysis of principal components. We found no evidence of any native Williston Reservoir genotypes.
Conclusion
Formation of Williston Reservoir favored pelagic species, such as kokanee, which have gradually increased in proportion over time—particularly since the introduction of kokanee from the Columbia River. Unfortunately, native Williston Reservoir kokanee appear to have been extirpated from the reservoir and were likely outcompeted by the introduced Columbia River‐origin kokanee.