Historically, the Central Valley produced strong runs of Chinook salmon, with up to 35,000 fall-run Chinook returning to the Stanislaus River. Numbers began to decline at the turn of the century as commercial fisheries expanded. Further, Stanislaus River Chinook were affected by stream blockage and degradation from mining practices and by the reduction of salmon habitat and stream flows by dams and water diversions.
In the last decade, between 3,000 and 8,500 Chinook salmon have returned to the Stanislaus 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.
Rainbow/steelhead trout have also declined over the years due to loss of habitat and are now listed under the Endangered Species Act (ESA) as a threatened species. Resident rainbow trout rear in the upper reaches below Goodwin Dam, and adult steelhead are occasionally observed in upstream and downstream fish sampling, but it is unclear whether a distinct run of steelhead is present.
Several research projects have been conducted on the Stanislaus River since the early 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 Stanislaus River.
Outmigration monitoring has been conducted in the Stanislaus River since 1993 to estimate the number of juvenile Chinook produced and migrating out of the Stanislaus River each year.
Currently, juvenile outmigration abundance is monitored at two sites in the Stanislaus River (i.e., Oakdale Recreation Park and Caswell State Park). 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.
Adult Migration Monitoring
Adult Chinook escapement surveys have been conducted by the California Department of Fish and Game (CDFG) using various methods since the early 1950's. The CDFG have used carcass surveys to estimate escapement for the past several decades on the Stanislaus 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.
The escapement surveys cover a 25-mile reach extending from Goodwin Dam to Riverbank and are divided into four sections: Goodwin Dam to Knight's Ferry; Knight's Ferry to Horseshoe Road Recreation Area; Horseshoe Road Recreation Area to Oakdale Recreation Area; and Oakdale Recreation Area to Jacob Meyers Park.
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.
Stanislaus River Weir
The Stanislaus River Weir Project was initiated in 2002 with a grant awarded through the USFWS Anadromous Fish Restoration Program (AFRP). The primary goal of the initial grant term was to demonstrate the feasibility of using a portable resistance board weir for monitoring and enumerating adult salmonid passage in a Central Valley stream. The weir effectively demonstrated that operation is feasible and that it also provides valuable information to complement and validate data obtained by other sampling techniques traditionally employed in the Central Valley to monitor adult Chinook salmon escapement. For example, weir monitoring provides hourly passage data that can be used to evaluate environmental influences on migration timing, whereas carcass surveys provide weekly time step data which is inadequate for such analyses. Several environmental factors have the potential to adversely affect migration and spawning success of salmonids including high water temperatures and low dissolved oxygen concentrations, and more monitoring is needed to obtain a longer time series of data to account for the range of variability in response to these parameters.
Additionally, trends in adult escapement are the primary method by which the influence of restoration actions on Chinook salmon production are evaluated. However, the accuracy of estimates generated by traditional carcass survey techniques is largely unknown. Direct counts at the weir provide an opportunity to validate carcass survey estimates on the Stanislaus River. Further, the weir is the only method capable of enumerating upstream migrating steelhead, which do not die after spawning and are not counted in traditional carcass surveys. Unlike carcass surveys, the weir is also able to provide detailed migration timing information upon which correlations to environmental conditions can be based.
The Stanislaus River Weir Project has three main objectives which include:
- Determine the abundance of adult Chinook salmon and steelhead entering the Stanislaus River.
- Characterize the population demographics of adult Chinook salmon and steelhead entering the Stanislaus River.
- Compare the effectiveness of weir counts to carcass survey estimates for estimating abundance and characteristics of Chinook salmon and steelhead runs.
A resistance board weir consists primarily of an array of rectangular panels made of evenly spaced PVC pickets aligned parallel to the direction of flow. The upstream end of each panel is hinged to a rail that is anchored to the substrate. The downstream end of the panel is lifted above the surface by a resistance board that planes upward in flowing water. When all components are installed, the resulting barrier inhibits fish from migrating upstream, yet allows water to pass. A passing chute on one of the panels guides fish into a livebox where they will be counted as they migrate upstream.
The Stanislaus River weir is installed annually in September and operates until at least the end of the fall-run Chinook migration period (i.e., December/January), but has operated as late as June.
To view recently summarized data regarding the Stanislaus River Weir click here. To view older summarized data, click on the archives link on the top of the page.