Status of Fall Chinook Salmon Populations in the Mid-Columbia River, 1948–1992

Abstract

Historically, fall Chinook salmon Oncorhynchus tshawytscha spawned in the main-stem Columbia River over a distance of approximately 900 km. However, because of hydroelectric development and other human activities, most natural production of upriver stocks of fall Chinook salmon (upriver bright, URB) is now restricted to a 90-km-long section of river known as the Hanford Reach. The adjacent Hanford Nuclear Reservation was managed for production of nuclear materials from 1943 to 1971, which restricted many development activities, including additional hydroelectric projects. Aerial counts of fall Chinook salmon redds since 1948 at the Hanford Reach provide an index of relative abundance among spawning areas and years. Redd counts during peak spawning were less than 1,000 annually from 1948 to 1961, but they increased to as high as 8,800 in 1989 following construction of several main-stem dams on both the Columbia and Snake rivers. Escapement of adult fall Chinook salmon to the Hanford Reach averaged about 25,000 fish annually from 1964 to 1982 and then increased to a peak estimate of 89,000 spawning adults in 1987. Returns of URB fall Chinook salmon declined to about 48,000 adults per year from 1988 to 1992. The Snake River stock of URB fall Chinook salmon has declined steadily since 1960; consequently, the stock was listed as endangered under the U.S. Endangered Species Act in 1994. The relative contribution of URB stocks to all fall Chinook salmon runs in the Columbia River increased from about 2490 of the total in the early 1980s to 50–60% of the total by 1988. The relative contribution of URB stocks to the commercial, tribal, and sport fisheries also has shown a major increase since 1980. A number of factors affect the production potential of fall Chinook salmon in the Hanford Reach. Over the last 40 years, water releases at upstream storage dams have increased discharge variability during spawning, incubation, and hatching. Hatchery production has supplemented natural production since the early 1960s and may have been partially responsible for increased numbers of returning adults in recent years. Juvenile and adult passage at hydroelectric dams, harvest management practices, and ocean conditions also affect the number of fish returning to upriver production areas, including the Hanford Reach.