Colonization of Wild and Hatchery Steelhead in a Small Tributary After Barrier Removal

Access into Beaver Creek, a tributary in the Methow River Washington, has been limited by 7 small irrigation diversions for more than 100 years.  In 2002-2004, state and federal agencies began re-designing these irrigation diversions in cooperation with private landowners to allow passage for steelhead (Oncorhynchus mykiss) and other ESA-listed species.  Fisheries managers were concerned that the high proportion of hatchery steelhead returning to the Methow and a higher probability for these fish to stray would influence the colonization of the newly opened habitat.  Therefore, we measured the relative reproductive success and the source population of all adult steelhead spawning in Beaver Creek.  Adult steelhead entering this stream and parr and smolts emigrating from the stream were monitored using a picket weir from 2005 through 2008.  Microsatellite markers were used to determine the reproductive success and potential source population for colonizers.  Most of the returning adults were identified as wild-origin (>90%).  Total adult escapement of steelhead trout was stable (no increase or decrease) during the first 4 years after barrier removal; however, the proportion of fluvial rainbow trout using Beaver Creek for spawning varied across years.  Individual steelhead had a wide range of reproductive success and only a few individuals accounted for more than 80% of the outmigrants from the stream.  Most individual steelhead had multiple spawning partners.  Hatchery steelhead made up a small proportion of the colonizers and had poor reproductive success relative to wild fish indicating they likely had little genetic influence on the initial colonization process in our study.