Natural Production and Genetic Introgression from Hatchery Summer Steelhead in the Upper Willamette River

Since 1966, the Oregon Department of Fish and Wildlife has managed a summer Steelhead Oncorhynchus mykiss hatchery program in the upper Willamette River (UWR) to mitigate for impacts of Willamette Valley Project dams.  Summer Steelhead are not native to the UWR, and may pose genetic and ecological risks to native, ESA-listed winter Steelhead.  To evaluate natural production by and introgression from summer Steelhead, we collected tissue samples from unmarked smolts at Willamette Falls in 2009-2011.  We then used microsatellite data to assign these individuals as summer Steelhead, winter Steelhead, resident Rainbow Trout or hybrids.  We found that about 10% of sampled fish were naturally-produced summer Steelhead and an additional 10% were hybrids with >20% summer Steelhead ancestry.  To investigate spatial patterns of natural production by summer Steelhead, we electrofished 196 randomly selected UWR sites in summer 2014.  We collected 464 juvenile O. mykiss samples from 65 of these sites.  These samples, baseline samples, and those collected in 2009-2011 were genotyped at a suite of single nucleotide polymorphisms, identified through restriction site-associated DNA sequencing and alignment.  We analyzed these data to map summer steelhead natural production, refine previous hybridization estimates, and assess implications for the recovery of UWR winter steelhead.