The Adult Salmon Passage Program
The University of Idaho
and NOAA Fisheries has monitored adult salmon and steelhead behavior and
survival in the Columbia and Snake River drainages since 1996. During
this period, more than 18,000 fish have been radio-tagged, monitored and
the fates of individual fish have been assessed. Monitored groups have
included spring, summer, and fall Chinook salmon, summer steelhead, and
sockeye salmon. Samples from later years have included adults that were
PIT-tagged as juveniles at known locations, allowing us to assess
behavior and survival for individuals with known homing destinations.
The tagging program has included several ESA-listed stocks. This work
has been funded primarily by U.S. Army Corps of Engineers and Bonneville
Power Administration with support from the Pacific Salmon Commission,
Bureau of Reclamation, NOAA, USFWS, and others. Following is a summary
of some of the major findings;
Fisheries technician, Rob Hoag, selects a Chinook salmon
(Onchorynchus tshawytscha) for health evaluation and tagging. Field
technicians have tagged more than 1000 fish per year during the past
decade of intensive research in declining salmon populations.
Survival, harvest, and pre-spawn mortality
● Survival. Survival through
the Columbia River Hydrosystem (Bonneville Dam to Lower Granite or
Priest Rapids dams) averaged 73% for spring–summer Chinook salmon, 61%
for fall Chinook salmon, and 63% for steelhead and can vary
significantly among years (Figure 1). Fish that do not reach spawning
areas are lost to harvest and other causes. In recent years, losses to
predation from pinnipeds downstream from Bonneville Dam, and
temperature-related migration mortality upstream from the Hydrosystem
have been areas of concern.
● Harvest. Harvest in the
main stem Columbia and Snake rivers have averaged 9% for spring–summer
Chinook salmon, 22% for fall Chinook salmon, and 15% for steelhead. An
additional 3–6% of radio-tagged fish have been reported harvested in
lower river tributaries. These estimates should be considered minimums,
because harvest reports have been voluntary, with reward incentives.
Although considerable effort has been invested in
monitoring harvest, accurate estimates are difficult to collect and
verify. Unreported and illegal harvest does occur in the basin.
Delayed mortality associated with fisheries (i.e. following sport
releases, contacts with gill nets, etc.) has not been well studied.
● Non-Harvest Mortality.
After accounting for fishing mortality, an average of 12–17% of adults
died before reaching spawning tributaries or exiting the monitored
Hydrosystem. Root causes for these mortalities are unknown, but likely
include extended migrations, stress from elevated water temperatures,
energetic exhaustion, disease, unreported harvest, delayed mortality due
to injuries sustained during fallback over dams or from encounters with
fisheries, or other factors. Survival for all runs has tended to
be lowest in the lower Columbia River and higher through lower Snake
River reaches. Factors associated with fish loss are discussed in more
Figure 1. Average
escapement for adult spring/summer Chinook salmon, fall Chinook salmon,
and steelhead through the Federal Columbia River Power System, adjusted
for known commercial and sport harvests.
● Pinnipeds. The proportion of adult
salmonids consumed by pinnipeds in the near vicinity of Bonneville Dam
during spring (1 January to 31 May) has trended upward, from < 1% in
2002 to > 5% in 2007, as reported by USACE biologists. Stock-specific
migration timing patterns derived from radiotelemetry and PIT-tag
projects suggest that early migrating spring Chinook populations have
the highest losses to predation. Precise population-level impacts from
pinniped predation on salmon are difficult to estimate because predation
levels downstream from Bonneville have not been documented.
Several measures have been or are being evaluated to deter
pinnipeds from feeding on salmon at Bonneville, including fishway
exclusion devices, hazing, and deterrents.
● Pre-Spawn Mortality. Additional
mortality occurs upstream from the Hydrosystem and prior to spawning,
but quantitative summaries of these components of adult survival are
conspicuously limited. Intensive surveys of spawning areas in the South
Fork Salmon River, Idaho, reveal 15 to 60% of ‘successful’ Chinook
salmon migrants can die before spawning. A study is underway to
determine the relationship between late migration mortality and
migration history, water temperatures in and upstream from the
Hydrosystem, density, and energetics for this population.
Surveys of other Columbia River populations indicate consistent
temperature effects on prespawn mortality, and a multi-population
synthesis is in development.
● Straying. Permanent inter-basin
straying is a challenging component of assessing adult survival because
they could be considered either successful migrants (they reached
spawning areas) or unsuccessful migrants (they did not home to natal
sites). On average, 2–4% of known-origin spring–summer and fall Chinook
salmon and ~7% of steelhead in the radiotelemetry studies strayed into
non-natal basins where they may have spawned with native stocks.
Hatchery fish and fish transported (barged) from the Snake River as
juveniles were more likely than other groups to stray. Many strays
entered the Deschutes and John Day rivers, where some wild populations
are considered at risk.
● Wandering. In warm years
and during warm periods within years, large portions of summer and fall
Chinook salmon and summer steelhead runs encounter temperatures
considered stressful for salmonids. In response, many fish seek cool
thermal refuges—and particularly cool non-natal tributary streams.
During the warmest times, majorities of the fall Chinook and steelhead
runs concentrate in these refugia, where they may be highly vulnerable
to harvest or other mortality agents. While the behavior likely has
immediate energetic benefits, delays and elevated harvest risks during
warm water periods may lead to overall decreases in Hydrosystem
survival. There is some evidence that survival consequences of high
temperature exposure may be greater for obligatory migrants like Chinook
salmon than for steelhead, which migrate many months in advance of
Effects of dams and operations
● Passage Rates. Most adult salmon
and steelhead pass individual dams in 1 to 2 d, and pass quickly through
reservoirs. However, a proportion of each run (typically between 2–12%)
has taken several days to weeks to pass individual dams. Fish that take
a relatively long time to pass individual dams were less likely to
migrate successfully to spawning tributaries. Similarly, relatively
slow passage through the lower Columbia Hydrosystem (multiple dams and
reservoirs) has been associated with unsuccessful migration. This
association may have resulted from either inadequate passage facilities
‘delaying’ some individuals, requiring the expenditure of large amounts
of energetic stores, and resulting in premature death. Alternatively,
individuals in poor condition at river-entry may have been both slow and
less likely to reach spawning grounds, regardless of passage conditions
at dams. Studies are on-going to determine the relative roles of these
Improving passage efficiency has been a management goal, and
incremental improvements to fishways (transition pools, count windows,
entrance and exit conditions) and operations (spill, fishway
temperatures) are being studied and implemented.
● Fallback. Some fish from all
runs pass dams and then fall back downstream. Approximately 22% of
spring–summer Chinook salmon, 15% of fall Chinook salmon, and 21% of
steelhead fell back at one or more dams during migration over several
years of the telemetry research. Fallback has been associated with both
direct and delayed mortality, slowed migration rates, and increased
likelihood of straying. Fallback rates have been highest in years with
high river flow and high spill at dams, at least in part because most
fish fall back via dam spillways. Fish also appear to fall back as a
result of orientation errors, including failure to locate natal
tributaries and imprinting problems associated with juvenile barging.
Operational changes may help reduce fallback and therefore
increase overall survival, but it is unclear how large a reduction is
possible given the management constraints.
● Juvenile Barging. Currently,
about 70% of juveniles migrating seaward in the Snake River are
collected and transported by barge below Bonneville Dam. Returning
adults which had been barged as juveniles were ~10% less likely to
migrate to spawning grounds, exhibited less direct migrations, and
strayed to non-natal tributaries at rates that were approximately twice
that of adults that had migrated in the river as juveniles. Barging
probably interrupts the ‘sequential imprinting’ process whereby adults
use olfactory memories from the juvenile seaward migration during
● Spill. Results from the
adult radiotelemetry project during three years when spill levels were
manipulated at Bonneville Dam suggest that high spill volumes (>100 to
kcfs at Bonneville) increase adult passage times compared to moderate
spill volumes of ~75 kcfs. Higher spill was also associated with higher
fallback rates in spring and summer Chinook and in steelhead. As noted
above, longer passage times and fallback events have been associated
with lower survival for individual fish.
● Fishway Temperatures. Elevated
water temperatures and large temperature differentials (between top and
bottom of ladders) in dam fishways can deter passage. Mean passage
times for spring-summer Chinook at Lower Granite Dam increased from 6.6
hours when temperatures at the ladder exit were similar to those at the
base of the ladder, to 19.1 hours when exit temperatures were ≥ 2˚C
warmer than at the base. Similarly, the proportion of fish requiring
more than one day to pass the dam increased from 32.7% with no
temperature barrier to 71.4% when temperatures differed by ≥ 2˚C.
Similar patterns have been evident at other projects, including at John
Day Dam, where many fish reject fishways when water temperatures exceed
18ºC and temperature differentials exist between the tailrace and
fishways. Temporary temperature barriers contribute to adult passage
delay that may result in permanent straying to downstream sites or
● Dissolved Gas. High spill at dams
can create supersaturated dissolved gas condition in tailraces and
downstream areas, and there is concern that fish that encounter these
conditions may develop gas bubble disease. Results from an archival tag
study that monitored fish swimming depths suggest that adults do not
avoid plumes of high dissolved gas and frequently experience high
dissolved gas conditions. However, most adults remained at depths that
provided adequate “hydrostatic compensation” and consequently prevented
expression of gas bubble disease. Little is known about the effects of
the observed frequent, but short, exposures to supersaturated
FERL research biologists prepare a Chinook salmon to return to the
Columbia River after analysis at Bonneville Dam. Fish are captured in
the fishway, anesthetized, weighed, measured, and evaluated for health
and robustness before being released back into the river. This data has
been collected for more than a decade and contributes to knowledge of
the species and recommendations for its conservation management.
River and ocean environment
● Flow and Survival. Adult Chinook
salmon appear to have lower Hydrosystem survival in years with high flow
(discharge). This pattern is probably the result of higher fallback and
slower migration rates in high-flow years, two energetically demanding
aspects of migration. Survival for runs that migrate during typical
low-flow times (most fall Chinook salmon and steelhead) has not been
correlated with river flow.
● Temperature and Survival. Water
temperatures in the Columbia and Snake Rivers have been increasing since
dam construction began due to development and management of the
Hydrosystem as well as from regional climate patterns, resulting in
longer summers and higher summer temperatures. Adults returning in the
late spring, during summer, and in the early fall frequently choose the
coolest water available to them to migrate in, but still frequently
encounter stressful temperatures. Higher temperatures have been
associated with altered migration behavior and lower migration success.
Predictions for continued warming will probably adversely affect
migrating adults because migration through stressful temperature
conditions requires more energy and may contribute to higher rates of
A study is underway to evaluate effects of temperature exposure
on gamete quality and spawning success.
Cool water releases from Dworshak Reservoir, Idaho, can moderate
temperatures somewhat in the lower Snake River.
● Temperature and Straying/Wandering Behavior.
Interactions between river temperatures and wandering/straying
behaviors were outlined above. Use of non-natal cool-water refugia will
continue and possibly increase if current temperature trends persist.
Cool-water refugia streams appear to be critical habitat for many
● Ocean Conditions. Ocean conditions
have strong effects on salmon. Much of our monitoring program has been
conducted on adults returning after “good” ocean conditions.
Consequently, the results may represent the “best case scenario” because
fish were probably returning in good condition. If so, a future
downturn in ocean conditions may be associated with a downturn in adult
condition with subsequent effects on adult survival.
We are in the process of trying to evaluate effects of ocean
conditions and initial fish energetic state on reproductive success.
Endangered species act considerations
● Survival and Harvest. Many of the
topics described above have important implications for ESA-listed
populations in the Columbia basin. Many regulations are in place to
protect listed stocks, but it is not clear that all are effective, as
many populations continue to decline. The radiotelemetry results
suggest that some listed populations may be harvested at unexpectedly
high rates (i.e. upper Columbia River steelhead, Snake River fall
Chinook salmon), and low survival to spawning for these groups may be
● Straying. High water temperatures,
juvenile barging, and fallback at dams have all been associated with
increased straying by adult salmon and steelhead. Straying is an
important management concern in the basin, due to the potential for
increased interbreeding between ESA-listed stocks and non-listed stocks,
especially those of hatchery origin.
The RV Nerka, one of FERL's salmon tracking vessels, prepares to follow
tagged fish through the Columbia and Snake Rivers. Field technicians
track tagged fish by boat on the rivers, by truck on roads near rivers,
and by foot using backpack-based receivers when streams are too shallow