Estimation of Kenai River Sockeye Salmon Smolt Abundance Using a Genetic Mark-Recapture Method

In 2008-2010, a genetic mark-recapture method was used to estimate the abundance of sockeye salmon smolts emigrating from the Kenai River watershed using the Russian River subpopulation as a genetic mark.  The number of marked smolts (i.e. Russian River subpopulation) in the experiment was estimated using a dye-based mark-recapture method and inclined-plane traps deployed in the Russian River.  The marked fraction in the total population emigrating from the Kenai River was estimated from tissue samples taken from smolts captured below Skilak Lake using inclined-plane traps and fyke nets.  Genetic classification accuracy for the Russian River subpopulation was very high. Estimates of Kenai River smolt abundance appeared to be biased due to the relatively large size of Russian River smolt and size-selective sampling in the recapture event related to size-dependent migration timing.  In 2008, recapture probabilities were lower than expected for small smolt (45-88 mm), possibly because smaller Russian River smolt migrated later in the season near the end of the project.  In 2009, recapture probabilities were lower than expected for large smolt (108-144 mm), possibly because larger Russian River smolt migrated during the day while most tissue samples were collected at night when catches were typically highest.  In 2010, the sampling design was modified to collect tissue samples in proportion to the catch throughout the night and into the morning. Results from genetic analyses of tissue samples collected in 2010 are not yet available.  We will discuss sources of error in genetic mark-recapture experiments and methods for improving precision and accuracy of abundance estimates.