Sockeye Salmon Oncorhynchus nerka Return After an Absence of Nearly 90 Years: a Case of Reversion to Anadromy

            We document the recent re-appearance of anadromous sockeye salmon (Oncorhynhus nerka) that were thought to have been extirpated by the construction of hydroelectric dams on the Coquitlam and Alouette Rivers, British Columbia, in 1914 and 1927 respectively. Unexpected downstream migrations of juveniles during experimental water releases into both rivers in 2005 and 2006 preceded upstream return migrations of adults in 2007 and 2008. Genetic (microsatellite and mitochondrial DNA) markers and stable isotope (δ³⁴S and ⁸⁷Sr/⁸⁶Sr) patterns in otoliths confirm that both the juvenile downstream migrants (JDM) and adult upstream migrants (AUM) were progeny of non-anadromous sockeye salmon (kokanee) that inhabit Coquitlam and Alouette reservoirs. Low genetic diversity and evidence of genetic bottlenecks suggest that the kokanee populations in both rivers originated from relatively few anadromous individuals that residualized in the reservoirs after the option of anadromy was eliminated by the construction of dams. If given an opportunity for upstream and downstream migration, both populations appear capable of reverting to a successful anadromous form, even after twenty five generations.