Potential Factors Responsible for Trends in Wenatchee and Okanagan Sockeye Salmon Abundance

In recent years, the proportion of the sockeye run passing Priest Rapids Dam bound for the Wenatchee Basin has declined to less than 25% of the run compared to approximately 50% of the run between 1970 and 1999.  This has been primarily due to an increase in escapement to the Okanagan Basin.  Since Snake River sockeye salmon returns have also increased dramatically in the last few years (although numbers are still extremely low), this raises the question as to why Wenatchee sockeye salmon abundance has not also increased.  Two of the most likely factors in the increasing abundance of the Okanagan and Snake basin sockeye stocks, improved ocean productivity and improved juvenile passage through the hydrosystem, would be expected to also benefit Wenatchee sockeye salmon.  In addition, a Wenatchee sockeye salmon hatchery program has apparently had little effect on escapement since it began in the early 1990s.

Data from PIT tagging programs shows little difference in upstream survival between Wenatchee and Okanagan stocks within the Columbia Basin.  Recent installation of PIT tag detection arrays as well as an Okanagan acoustic tagging program in the natal basins suggests 25-50% mortality from the Columbia River to the spawning grounds in the Okanagan Basin but minimal mortality to Tumwater Dam in the Wenatchee Basin in 2009-2011.  However, passage delays at Tumwater Dam may be decreasing escapement.

Low egg-to-smolt survival from rain-on-snow events scouring eggs from redds, high rates of predation from abundant bull trout, and/or low lake productivity are the most likely reasons for recent low Wenatchee sockeye abundance relative to the Okanagan stock.  Conversely, the Okanagan stock has benefited from innovative water management, habitat restoration, and reintroduction of sockeye into Skaha Lake.