Historically, the Central Valley produced strong runs of Chinook salmon, with up to 130,000 fall-run Chinook returning to the Tuolumne River. Population levels in the Tuolumne River have declined in the latter half of the 20th century to a low of only 77 Chinook reported in 1991.
In the last decade, between 500 and 18,000 Chinook salmon have returned to the Tuolumne River each year to spawn. The run is currently decreasing and remains far below its historical abundance and therefore, are considered a species of concern by state and federal governing agencies.
Several research projects have been conducted on the Tuolumne River since the mid-1990's. The purpose of the research is to estimate fish populations and better understand the relationship of salmonid population in response to both physical habitat restoration measures and flow management actions currently underway in the Tuolumne River.
Outmigration monitoring has been conducted in the Tuolumne River since 1995 to estimate the number of juvenile Chinook migrating out of the Stanislaus River each year.
Currently, juvenile outmigration abundance is monitored at two sites in the Tuolumne River (i.e., Waterford and Grayson). Both sites are monitored using a rotary screw trap to catch the fish as they migrate downstream. The fish are identified, measured, counted, weighed and released back into the river each day the trap is sampling.
A rotary screw trap consists of a funnel shaped core suspended between two pontoons. Each trap is positioned in the current so that water can enter the 8 ft wide funnel mouth. Water enters the funnel and strikes the internal screw core, causing the funnel to rotate. As the funnel rotates, fish are trapped in pockets of water and forced rearward into a livebox, where captured fish cannot escape.
The rotary screw traps are installed in early January and remain in the river until late June to mid-July.
To view recently summarized data regarding juvenile outmigration click here. To view older summarized data, click on the archives link on the top of the page.
Seining and Snorkel Surveys
Seining surveys have been conducted during the juvenile outmigration period in the Tuolumne and San Joaquin Rivers since 1986 by the Turlock and Modesto Irrigation Districts. The purpose of the surveys is to document juvenile salmonid size, distribution, and relative abundance in nearshore rearing habitats, as well as gather similar information on other fish species. Snorkel surveys are also conducted annually in the Tuolumne River in June and September (flow permitting) in conjunction with seining surveys to evaluate salmonid and other species populations.
To view data on seining and snorkel surveys, click on the archive link on the top of the page.
Adult Chinook escapement surveys have been conducted on the Tuolumne River by the California Department of Fish and Game (CDFG) using various methods since the early 1940's. The CDFG have used carcass surveys to estimate escapement for the past several decades on the Tuolumne River. Data collected from the carcass surveys allow CDFG to estimate fall-run Chinook escapement; evaluate the distribution of redds in the study area; collect length and sex data; collect scale and otoliths for age determination and cohort analyses; and collect and analyze coded-wire tag data. Beginning in 1992, CDFG escapement surveys have been utilized as part of the New Don Pedro FERC Project No. 2299 license monitoring program and annual reporting.
The escapement surveys cover a 26.5-mile reach extending from La Grange Dam (RM 52) to Fox Grove (RM 24.1) and are divided into four sections: La Grange Dam to Basso Bridge (RM 47.5); Basso Bridge to the Turlock Lake State Recreation Area (TLSRA; RM 41.9); TLSRA to riffle S1 at river mile 34; and riffle S1 at river mile 34 to Fox Grove.
All riffles in the study area have been identified, mapped, and systematically named in an upstream to downstream direction using sequential letter/number designations for river mile and riffle number, respectively.
Two methods are used to estimate escapement each year, the Jolly-Seber and Schaefer mark-recapture methods. Using these mark-recapture methods, each carcass is tagged with a unique number by attaching an aluminum head tag to the lower jaw. Each tagged carcass is released back into the river near the lower end of the riffle then subsequently recovered during weekly surveys of the spawning areas. The weekly population estimates are obtained by calculating the ratio of recovered carcasses to the number of carcasses counted (i.e., counted fish include total fish tagged, skeletons and fresh carcasses that week).
Carcass surveys typically begin in October and continue through December to early-January depending on abundance and flow conditions.