Abstract
The decline of Sacramento River winter-run Chinook salmon (Oncorhynchus tshawytscha) remains one of the major water management issues in the Sacramento River. Few field studies have been published on winter-run, leaving gaps in our knowledge about their life history. This is especially true in the Sacramento–San Joaquin Delta, which provides essential rearing and migratory habitats for winter-run, and serves as the center of water operations for California. Using long-term monitoring data that identified winter-run-sized fish (“winter-run”) using length-at-date criteria, we examined patterns of juvenile migration in terms of geographic distribution, timing, numbers, and residence times. We analyzed the role of flow, turbidity, temperature, and adult escapement on the downstream movement (“migration”) of winter-run. Winter-run passed Knights Landing (rkm 144 or 51 rkm upstream of the Delta) between October and April, with substantial variation in peak time of entry that was strongly associated with the first high flows of the migration season. Specifically, the first day of flows of at least 400 m3 s-1 at Wilkins Slough (rkm 190) coincided with the first day that at least 5% of the annual total catch was observed at Knights Landing. While the period during which winter-run left the Delta spanned several months based on Chipps Island (rkm 29) catch data, the median catch typically occurred over a narrow window in March. Differences in timing of cumulative catch at Knights Landing and Chipps Island indicate that apparent residence time in the Delta ranges from 41 to 117 days, with longer apparent residence times for juveniles arriving earlier at Knights Landing. We discuss the potential importance of the Yolo Bypass floodplain as an alternative rearing and migratory corridor, contingent on the timing, duration, and magnitude of floodplain inundation. These results carry implications for habitat restoration and management of Sacramento River flows.
Highlights
California is unusual in having four different runs of Chinook salmon (Oncorhynchus tshawytscha): fallrun, late fall-run, spring-run, and winter-run (Fisher 1994; Yoshiyama and others 1998)
We obtained data on winter-run Chinook salmon from long-term field monitoring studies conducted by the California Department of Fish and Game (DFG) and U.S Fish and Wildlife Service (USFWS)
Because winter-run Chinook salmon are an important species influencing regional water management (USFWS 2007; NOAA Fisheries 2009), a critical first step in management is to describe the timing of juvenile winter-run as they migrate through the Delta
Summary
California is unusual in having four different runs of Chinook salmon (Oncorhynchus tshawytscha): fallrun, late fall-run, spring-run, and winter-run (Fisher 1994; Yoshiyama and others 1998). The Sacramento River winter-run is unique in terms of migration timing. Winter-run is endemic to California’s Central Valley, where only one population remains. Winter-run spawned in the headwaters of the upper Sacramento River and its tributaries. Since the construction of Shasta Dam in 1945, winter-run spawning has been limited to a relatively small cold-water reach just downstream of Keswick Dam. The population’s decline in the 1970 to 1980s, and restricted habitat range, are primary reasons for their endangered status under the federal Endangered Species Act (Federal Register 1994) and California Endangered Species Act (Title 14, Section 670.5). The migratory route between their upstream freshwater habitat and the ocean has been legally designated as critical habitat (Federal Register 1993), portions of which include rearing and migratory habitat through the lower Sacramento River and western Sacramento–San Joaquin Delta (Delta; Figure 1)
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