The Sacramento River produces four distinct races of chinook salmon (Oncorhynchus tshawytscha) : fall, late fall, winter, and spring. All races have declined substantially. The winter run was listed as "endangered" by the State of California in 1989 and by the National Marine Fisheries Service in 1994. The spring run, once the most abundant chinook in the Central Valley (Reynolds et al. 1990), persists at dangerously low numbers in a few tributaries and is the object of a current petition for inclusion on the endangered list. In an effort to reverse the decline of chinook salmon stocks, natural resource managers have focused on the maintenence and restoration of habitat in the Sacramento River and its larger tributaries (Upper Sacramento River Fisheries and Riparian Habitat Advisory Council, 1989). Small, intermittent tributaries have generally been overlooked by fishery resource managers. While few of these tributaries serve as spawning habitat for chinook salmon, our research suggests they provide important rearing habitat , particularly for the imperiled winter and spring runs.
Rearing of juvenile chinook in nonnatal tributaries has been reported in other river systems. Murray and Rosenau (1989) suggest that the dispersal and migratory patterns of young chinook salmon increase the use of available rearing areas, and that movements of young salmonids from spawning areas to rearing areas consist of complex local migrations (upstream, downstream, or both), that are genetically and environmentally controlled. Scrivener et al. (1994), concluded that seasonally high sediment levels and cold temperatures in the Fraser River may induce juvenile chinook to move into small, nonnatal tributaries to feed and clear their gills of sediment.
Researchers from California State University, Chico, have consistently captured wild and hatchery origin chinook salmon juveniles in small, intermittent tributaries of the Sacramento River where there are no records of spawning adults. Juvenile chinook may migrate into the tributaries to exploit food resources (Williams, 1987); and to escape unfavorable environmental conditions which occur periodically in the mainstem, such as high turbidity and cold temperatures (Upper Sacramento River Fisheries and Riparian Habitat Advisory Council, 1989).
The objective of this study was to document various aspects of nonnatal rearing in intermittent tributaries of the Sacramento River. We estimated the spatial and temporal extent of nonnatal rearing. We also calculated the race distribution and growth rate of juvenile chinook rearing in tributaries. Additionally, the condition factors of juvenile chinook caught in tributaries were compared with those caught in the mainstem.
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