Figure 1. Location of Caswell Rotary Screw Trap (RST)
and other relevant features in the Stanislaus River watershed.
In 1996, Cramer Fish Sciences (CFS) implemented a juvenile salmonid out-migration monitoring
program at Caswell Memorial State Park (Caswell) on the lower Stanislaus River (Figure 1).
This project was initiated and funded by the U.S.
Fish & Wildlife Service-Anadromous Fish Restoration Program, which also funds RST operations throughout California. Because RST's are
used as sampling devices on most major Central Valley Rivers, temporal trends and production
estimates within and between years can potentially be compared across rivers, producing a powerful
dataset to inform management decisions throughout California.
CFS operates the Caswell RST annually throughout the rearing and outmigration season (December to
June) to estimate abundance of outmigrating juvenile fall-run Chinook salmon Oncorhynchus tshawytscha
and steelhead/rainbow trout O. mykiss. These data are used to understand the impacts of
flow regimes and other management actions on population dynamics. Annual or bi-annual reports that
summarize salmonid production estimates, temporal patterns of catch, fish size, by-catch, and
environmental data are submitted to AFRP (see reports listed above).
Figure 2. Rotary screw traps operating in a tandem,
side-by-side configuration at Caswell, 2008.
Most years, CFS has deployed two rotary screw traps (RST) in a tandem, side-by-side configuration.
We develop outmigrating population abundance estimates by measuring trap efficiency under a range
of flows, whereby a known number of marked fish are released upstream of the trap and compared to
the number of recaptured marked fish. Trap efficiency data from previous years, the efficiency tests
from the current year, and covariates (flow, fish size) are combined in a statistical model to predict
daily trap efficiency. These daily efficiency values and associated error are applied to expand daily
catch numbers to develop an estimate of passage with 95% confidence intervals.
Please see our Operational
Protocol for details regarding daily operations, procedural information, and data analysis methods.
Figure 3. Passage estimates for Oakdale and Caswell rotary screw traps and survival index from 1996-2008.
Almost two decades of trapping have yielded a robust dataset which can be compared across years to
detect relationships between biological and physical variables and outmigration success. Most
recently, CFS has used these data in concert with data collected from another trap (Oakdale RST)
located approximately 30 river miles upstream from the Caswell trap to estimate a suite of demographic
parameters for the Chinook salmon population. The Caswell RST has consistently lower abundances of
juvenile Chinook salmon compared with the Oakdale RST and an overall reduction in juveniles was
observed for both traps over a period of several years (Figure 3). However, the causes of this
decline are not well understood. Therefore, CFS conducted an analysis using RST data to evaluate
how survival, migration strategy, and fish size respond to flow regime, temperature, and spawner
density (Zeug et al. 2014).
A survival index was calculated to estimate outmigration survival through
the 30RM reach between Oakdale and Caswell. Greater cumulative flow and higher variance in flow
during the migration period resulted in higher survival indices and a larger proportion of juveniles
migrating at smaller size classes (Figure 4). Streamflow, spawner density, and
temperature also affected the size and abundance of different Chinook salmon age classes, and these
effects varied between age classes. See Zeug
et al. 2014 for more information.
Figure 4. Relationship between the Chinook salmon survival
index and the four predictor variables.
The data suggest that periods of high discharge in combination with high discharge variance are
important for successful emigration as well as migrant size and the maintenance of diverse
migration strategies. The information in this study can be used to assist in the development
of flow schedules to support the persistence of salmon in the Stanislaus River and has implications
for populations in other regulated rivers with limited and variable water supply.