many as 90 percent of adult salmon and steelhead that enter some Northwest
rivers will die before they can complete their spawning cycle.
will die from ocean conditions (ocean mortality), while others will succumb to
predators or warm water before they even reach their natal spawning grounds
will die near their spawning areas as they await a spawning opportunity or as
they arrive to spawn, according to a recent study.
mortality is a widespread phenomenon that affects Pacific salmon populations
from California to Alaska, according to Tracy Bowerman, postdoctoral researcher
at the University of Idaho.
rates of PSM have been attributed to urban runoff, as well as high spawner
densities and high water temperatures, which can reduce oxygen availability and
increase susceptibility to pathogens,” Bowerman said. “Unusually high rates of
PSM have contributed to the rapid decline of several important salmon fisheries
and pose a substantial extinction risk to other populations listed under the
Endangered Species Act.”
and summer chinook salmon, along with some coho and sockeye salmon, enter
freshwater several months before spawning and so are susceptible to “energetic
depletion” and environmental stressors (poor water quality, high temperatures
and disease) before spawning. Other
salmon, chinook, pink and chum, enter just prior to spawning, but often spawn
in higher densities and may be more susceptible to density dependent processes,
the study says.
the demands of salmon migrations, it’s not unusual to see some mortality prior
to spawning. “But unusually high PSM rates have been associated with steep
declines in some populations” and the study tries to get a handle on measuring
pre-spawn mortality and, to a certain extent, the reasons for pre-spawn
declines in the numbers of spawning Fraser River sockeye salmon, for example,
have been linked to early upstream migration timing, increasing the sockeyes’
exposure to pathogens and stressful holding conditions.
Sound coho have experienced 50 percent to 90 percent pre-spawn mortality due to
exposure to toxins from urban stormwater runoff.
mortality in Willamette River spring chinook salmon is caused by high water
temperature and poor fish conditions, according to the study.
reasons for PSM are intense crowding (pink and chum salmon), high spawner
density and pathogens.
Salmon Prespawn Mortality: Patterns, Methods, and Study Design Considerations,”
was published online December 6, 2016, in the journal Fisheries (http://www.tandfonline.com/doi/abs/10.1080/03632415.2016.1245993?journalCode=ufsh20).
co-authors are Christopher Caudill, assistant professor, and Matthew Keefer,
research scientists, both with the University of Idaho.
study reviews the ways in which PSM has been estimated across salmon
populations by interviewing fisheries professionals and shows how methods of
estimating PSM are sensitive to results.
PSM estimates are important for fisheries quotas and management of at-risk
populations,” Bowerman said. “Our study found that differences in methodology
and reporting could lead to substantial biases in PSM estimates.”
authors described study design considerations that could help improve the
accuracy and precision of prespawn monitoring programs. Standardized PSM
monitoring could help facilitate comparisons among populations and over time,
the factors that contribute to PSM will become increasingly important as stream
temperatures warm as a result of climate change and we have more years like
2015, during which we saw large-scale die offs in several Columbia River salmon
populations,” Bowerman said.
92 percent of fisheries managers surveyed estimated prespawn mortality by
collecting carcasses and checking for egg retention (carcass-based estimates).
The other 8 percent used escapement based estimates “using independent
estimates of female escapement.”
authors found in a comparison of the two methods in the South Fork Salmon River
in Idaho that generally carcass-based estimates were considerably lower (11
percent to 57 percent) than escapement-based PSM estimates (47 percent to 83
percent). One method may lead to underestimation, the other to overestimation,
the study concludes.
it comes to data collection, the study recommends:
spatially-balanced design to survey all of a spawning area where possible.
temporal design covering the time from spawning initiation through the spawning
at least once a week.
both male and female PSM data is included, they should be separated.
a female carcass has more than 75 percent of eggs remaining, it should be
considered unspawned; 25 percent to 75 percent is partially spawned and less
than 25 percent retention is spawned.
possible, list cause of death.