Stillaguamish River Chinook Salmon Abundance Estimated Using Trans-Generational Genetic Mark-Recapture

The Stillaguamish River Chinook salmon in Washington State, USA are listed for protection under the US Endangered Species Act and are a stock of concern under the Pacific Salmon Treaty due to declines from historic levels, current low abundance and limitations this imposes on fisheries management. Using visual surveys (redd counts) to estimate spawner abundance is difficult due to limited visibility and access because of freshets during the spawning period.  We used a genetic application of traditional mark recapture (Genetic Mark-Recapture, GMR) to estimate spawner abundance for five brood years.  The first sampling event (marks) included carcasses collected in the fall, and the second sampling event (captures) included juveniles collected in the spring from a juvenile outmigrant trap.  Recaptures were parental genotypes recaptured in juveniles assigning back to carcasses.  We compared GMR estimates from the Lincoln-Peterson model to redd-based estimates.  In all years, precision of the GMR estimate was high (CV ≤ 15%) and GMR estimate was higher than redd-based estimates.  The GMR is a useful tool for fisheries management where environmental conditions prevent accurate counts and when it is important to assess the uncertainty in spawner abundance estimates.