Estimating in-river survival of migrating salmonid smolts using radiotelemetry

A field study to estimate the survival of outmigrating steelhead (Oncorhynchus mykiss) smolts using radiotelemetry methods is illustrated. A paired release–recapture design was used to estimate pool (i.e., reservoir), dam, and project (i.e., reservoir plus dam) survival at two mid-Columbia River hydroprojects based on maximum likelihood estimation. The release and detection scheme was designed to minimize the possibility of detecting false-positive radio signals from smolts that might have died upstream during the hydroproject passage. Model assumptions and possible violations are discussed. Releases of radio-tagged and passive integrated transponder (PIT) tagged steelhead smolts were also compared to assess the possible effects of tag type on migration behavior. Survival through the Rocky Reach project (P = 0.86) and Rock Island project (P = 0.41) and bypass diversion probabilities at Rocky Reach Dam (P = 0.39) were found to be similar between tag types. Small but significant differences in arrival patterns (P = 0.02) and travel times (P = 0.01) were observed between radio-tagged and PIT-tagged smolts.