WDFW Cowlitz Steelhead Report 2010

Natural-origin steelhead were sampled in tributaries of the lower Cowlitz basin and compared genetically to Cowlitz Hatchery steelhead broodstocks (summer, early-winter, and late-winter) to examine hatchery introgression into natural populations. Cowlitz tributary natural-origin adults were also compared to natural-origin steelhead collected in other nearby tributaries in the lower Columbia drainage to characterize existing genetic diversity. Lower Cowlitz tributary natural-origin steelhead had highest ancestry in a native gene pool and were genetically distinct from the three Cowlitz Hatchery stocks. However, we estimated ancestry or introgression from all three hatchery stocks in the natural-origin steelhead. The Cowlitz Hatchery early winter-run stock (non-native, Puget Sound origin) showed the highest introgression level among the three stocks. The eight marked, hatchery-origin adults collected during sampling appeared to originate from all three hatchery stocks, with the early-winter stock being the major contributor. Lower Cowlitz tributary natural-origin steelhead were most similar genetically to natural-origin steelhead in the Coweeman and Elochoman rivers, and were distinct from steelhead in the nearby Toutle River sub-basin. Introgression from hatchery stocks is a significant issue in recovery planning for the ESA-listed Lower Cowlitz population.