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Assessing Pre-Spawn Mortality: In Some NW Rivers 90 Percent Of Salmon/Steelhead Die Before Spawning
Posted on Friday, February 10, 2017 (PST)

As many as 90 percent of adult salmon and steelhead that enter some Northwest rivers will die before they can complete their spawning cycle.


Some will die from ocean conditions (ocean mortality), while others will succumb to predators or warm water before they even reach their natal spawning grounds (enroute mortality).


Others will die near their spawning areas as they await a spawning opportunity or as they arrive to spawn, according to a recent study.


Prespawn 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.


“Elevated 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.”


Spring 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.


Given 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 die-offs.


Steep 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.


Puget Sound coho have experienced 50 percent to 90 percent pre-spawn mortality due to exposure to toxins from urban stormwater runoff.


Pre-spawn mortality in Willamette River spring chinook salmon is caused by high water temperature and poor fish conditions, according to the study.


Other reasons for PSM are intense crowding (pink and chum salmon), high spawner density and pathogens.


“Pacific Salmon Prespawn Mortality: Patterns, Methods, and Study Design Considerations,” was published online December 6, 2016, in the journal Fisheries (


Bowerman’s co-authors are Christopher Caudill, assistant professor, and Matthew Keefer, research scientists, both with the University of Idaho.


The 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.


“Reliable 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.”


The 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, Bowerman added.


“Understanding 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.


Some 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.”


The 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.


When it comes to data collection, the study recommends:


--A spatially-balanced design to survey all of a spawning area where possible.

--A temporal design covering the time from spawning initiation through the spawning season.

--Sample at least once a week.

--If both male and female PSM data is included, they should be separated.

--If 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.

--If possible, list cause of death.


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