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Study Details How Contaminated Highway Runoff Deadly To Coho, Loss Rates 60 To 90 Percent
Posted on Friday, November 03, 2017 (PST)

In some Puget Sound streams up to 100 percent of coho salmon die before they spawn and a new study has identified the cause – contaminants from cars and trucks on high-use roads – a nonpoint source of pollution that flow as stormwater from impervious surfaces into streams.


The contaminants are so deadly that the coho can die within 24 hours after exposure.


“Affected adult males and gravid females become disoriented and show surface swimming, gaping, a loss of equilibrium, and finally death on a timescale of a few hours,” the study says.


Loss rates in streams most impacted by toxic road runoff are typically 60 to 90 percent of an entire fall run in a given urban stream, the study says. “Initial modeling has shown that wild Puget Sound coho, presently a species of concern under the U.S. Endangered Species Act, cannot maintain population abundances at such high mortality rates.”


With the help of the U.S. Fish and Wildlife Service, NOAA Fisheries’ Northwest Fisheries Science Center and Native American Tribes in the area, the study looked at 51 distinct coho salmon spawning reaches in various stages of urbanization in Puget Sound and found that up to 40 percent of those river basins that support coho salmon are in highly urbanized areas and “are predicted to have adult mortality rates that substantively increase the risk of local population extinction,” the report says.


“The study concluded that adult coho spawners are likely affected by the urban runoff mortality syndrome throughout much of their freshwater range in Puget Sound,” said Nathaniel Scholz, research zoologist, and head of the Ecotoxicology Program at NOAA’s Science Center in Seattle. “Moreover, the land use analysis suggests that motor vehicles are the likely sources of (one or more) contaminants that are causing recurring spawner die-offs. Our goal is to help identify where clean water strategies (e.g., green infrastructure) will be most beneficial, in terms of coho conservation.”


The study found that contaminants in stormwater runoff from the regional transportation grid likely caused the coho die-offs, the study says.


“Further, it will be difficult, if not impossible, to reverse historical coho declines without addressing the toxic pollution dimension of freshwater habitats,” the study concludes.


Among salmon, it seems that the toxic runoff harms only coho, not chinook, pink or chum salmon in the same way and, according to Scholz, scientists don’t know why. He said that in preliminary studies that have exposed both adult coho salmon and adult chum salmon to stormwater, only the coho are affected by the mortality syndrome.


Green stormwater infrastructure changes will likely be the saving grace for coho in urban areas. That includes bio-infiltration where contaminants filter through soil removing contaminants sufficiently so that the salmon will not die from runoff, the study says.


“Roads to ruin: conservation threats to a sentinel species across an urban gradient,” was published online October 18 in Ecological Applications, Journal of the Ecological Society of America,


In addition to Scholz, co-authors are Blake Feist, landscape ecologist with the Nearshore Ecology Team in the Ecosystem Science Program with the Conservation Biology Division at the NOAA Science Center; Eric Buhle with Quantitative Consultants; David Baldwin and Julann Spromberg, Environmental and Fisheries Science Division, NOAA Science Center; and Steven Damm and Jay Davis in the Washington Fish and Wildlife Office of the U.S. Fish and Wildlife Service.


The authors developed predictive maps that identify areas where toxic runoff in Puget Sound is impacting coho the most, but also areas where urbanization is in an earlier stage. Even those coho populations, Feist warns, will eventually feel more urbanization pressure and that doesn’t bode well for the fish. The maps also identify the current rates of mortality of the fish.


“I’d like to add that the predictive maps suggest that currently healthy coho populations in less developed areas will almost certainly experience increased mortality in the future as urbanization envelopes the basin where these populations reside,” Feist said. “The good news is that the maps can be used to identify hotspots where future development needs to integrate green infrastructure.”


Die offs of coho occur elsewhere to some degree and is a problem across the nation, Scholz said.


“We have received anecdotal reports of coho die-offs in urban watersheds from Northern California to British Columbia, and we believe - based on the data we have – that coho spawners in most or all west coast (urban or urbanizing) spawning habitats are vulnerable to the stormwater mortality syndrome. Additional research to test this assumption is planned.”


That includes areas in the Columbia River basin, such as the Portland/Vancouver urban area lower in the Columbia River and the Tri-City urban area in eastern Washington.


Feist said that the geospatial data used in the analyses are available nationally, which would enable scientists to run an analysis anywhere in the United States.


“It’s always exciting and satisfying to do research that has such clear applications to environmental problems, so I’m optimistic that the results from this research will be used to improve the livability of urban systems in the Pacific Northwest and elsewhere,” Feist concluded.


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