Colonization of Coho Salmon Above Natural Barrier Falls in Oregon: A History of Supplementation and the Expansion of Coho Salmon Range Above Willamette Falls

Historically, coho salmon (Oncorhynchus kisutch) were not found in the Willamette River basin above Willamette Falls located in Oregon City at RM 26.  Hydrologic factors and flows occurring in fall are believed to have precluded adult coho upstream migration.  The first fish ladder was constructed at Willamette Falls in 1877 to improve passage for spring Chinook (Oncorhynchus tshawytscha) and winter steelhead (Oncorhynchus mykiss), but also provided coho a means of navigating the falls.  Beginning in the early 1950s, ODFW began to release coho fry and presmolts into almost every basin above the falls to improve commercial and sport fishing in the ocean and Columbia River.  Early stock coho from several hatcheries in the lower Columbia River were typically used.  Stocking of hatchery coho upstream of the falls dropped significantly through the 80's and continued until the last release was made in the Tualatin River in 1997 in response ESA listing of numerous salmon and steelhead populations.  After reducing releases through the late 80's, counts of adult coho salmon at the Falls dropped to less than 1,000 fish annually through 1999.  Counts increased noticeably in 2000 when 2,839 adults moved upstream.  Numbers have continued to grow each year, almost entirely due to natural production.  Based on ad-clip detections, 3% or less of the run is composed of hatchery coho.  Escapement increased significantly in 2009 when 25,298 adults migrated upstream from a parent run of 6,100 fish (4:1 spawner-to-recruit ratio). In 2010, a run of 20,103 adults were counted, produced from a parent run in 2007 of just over 7,000 adults, (3:1 spawner-to-recruit ratio).   Summer surveys for naturally produced presmolts showed varied, but often abundant natural production in many tributaries at and downstream of the Luckiamute River.  Counts of naturally produced adult coho at Willamette Falls show marked increases in abundance over the past ten years and are expected to grow.  Uncertainty remains about factors contributing to the population’s success, but substantial miles of good spawning and low gradient rearing habitat exists in multiple tributaries above Willamette Falls.  Other factors favoring coho survival in the upper Willamette include improved flow management in the basin and reduced harvest impacts in ocean and Columbia basin commercial and sport fisheries.  Monitoring and research are needed to better understand the survival mechanisms that favor natural production success in the upper Willamette, and potential effects on native ESA listed Willamette winter steelhead and spring Chinook salmon.