Post-Emergence Behavior of Subyearling Summer/Fall Chinook in Wells Reservoir and Implications for the Measurement of Passage Survival through the Wells Hydroelectric Project

The Habitat Conservation Plan for the Wells Hydroelectric Project specifies measurement of the survival of emigrating salmonids through the project (reservoir, dam, and tailrace).  Studies of passage survival of age-0 (subyearling) ocean-type Chinook at federal dams on the Columbia River rely on collection and tagging of emigrating individuals.  Factors complicating implementation of such studies for the Wells Project include lack of structures for collection of emigrants at Wells Dam, and the life-history and behavior of subyearling summer/fall Chinook in the reservoir.  We present results from an ongoing study of the susceptibility to capture, distribution, life-history strategies, and migration timing of subyearling Chinook in Wells Reservoir within the context of the assumptions for the Cormack-Jolly-Seber (CJS) paired-release survival model.  Our study reveals several violations of the CJS-model assumptions related to fish behavior, some of which advancements in tag technology cannot rectify.  Juvenile ocean-type Chinook enter Wells reservoir and pass Wells Dam over a range of developmental stages, (swim-up fry to yearling emigrant), and their capture susceptibility varies with developmental stage such that individuals most likely to actively emigrate are least susceptible to capture, while individuals most susceptible to capture are too small to tag and most likely to manifest extended rearing.