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Background
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. Operations by CFS have occurred annually at this site to estimate
abundance of out-migrating juvenile fall-run Chinook salmon Oncorhynchus tshawytscha and
steelhead/rainbow trout O. mykiss in order to understand the impacts of instream flow
schedules and management actions.
Methods
From 1996 through 2008, we utilized two rotary screw traps (RST) in a tandem, side-by-side
configuration (Figure 2). In 2009, due to persistent low flow conditions and morphological
changes to the channel at the pre-existing trap location, we are using a single 8-ft rotary screw
trap to capture out-migrants between January and June. We develop abundance estimates
by measuring trap efficiency, whereby a known number of marked fish are released
upstream of the trap and compared to the number of recaptured marked fish. A
predictive model that uses efficiency data from previous years and the efficiency tests
from the current year will determine daily trap efficiency.
These daily efficiency values
and associated error are applied to expand daily catch numbers to develop an estimatof passage with 95% confidence intervals.
Please see our Operational
Protocol for details regarding daily operations, procedural
information, and data analysis methods.
Findings
Thirteen seasons of trapping yielded robust information and a trend dataset which can be
compared to other operations in the basin (i.e., Oakdale RST monitoring). Adult escapement has
declined to less than 1,000 fish annually in the watershed during 2007 and 2008; concurrent
juvenile out-migration monitoring shows corresponding abundance declines (Figure 3).
Out-migration timing and trends are evident (Figure 4) whereby fry emigrants appear in January
and February, the parr life stage is generally detectable during March and April, and
sub-yearling smolt migrants predominate in April and May with the migration extending
into July in some years. Yearling smolt migrants are captured in most years; however,
they typically make up a very small proportion of the overall out-migration and do not
display discernable trends.
The relative contribution of different life stages to the overall out-migration (Figure 5)
is variable and not well understood at this time. Preliminary results from 2009 indicate
fry abundances were again very low for the third straight year; however, parr accounted
for approximately 33% and smolts 60% of total abundance.
 O. mykiss
are usually captured each year, but in low numbers. Typical O. mykiss caught at
Caswell are >200 mm on average (Figure 6), with only a few representative fish <100
mm for all years.
Fish Health
In 2007, outbreaks of poor fish health were an important factor affecting operations
and procedures. Subsequent fish health assessments indicated columnaris (Flavobacterium
columnare) infections were the cause.
As a result of these observations, CFS adapted our RST operational protocol to incorporate
investigations of fish health. No episodes were observed in 2008; however, in 2009 we
again observed episodic occurrences of poor fish condition. Subsequent histological
analysis revealed no causative factors or diagnosis for morbidity. We will continue
to employ preliminary fish health assessments with our regular operations.
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