by Paul Pegany, Spring 2019

Photos & video by John F. Williams except where noted


By Paul Pegany, Summer 2019

Photos &  video by John F. Williams except where noted

can rain gardens help save salish sea coho salmon?

The answer is a definite…maybe. One hidden mystery of the Salish Sea is an explanation for the precipitous decline in coho survival rates over the past three decades. Data covering the last 45 years indicates that coho survival rates on the Pacific Coast of Washington have remained relatively stable, but once they enter the Salish Sea, the picture changes drastically.

Currently, the mortality rate for salmon in Puget Sound is forty to ninety percent.

​​Several possible causes of coho mortality have been proposed by researchers:  food unavailability, predation, even changes in plankton due to climate change. However, pinpointing an exact cause has remained somewhat elusive. ​​

Developed, impervious surfaces: Seattle, WA. Photo by Steve Stolee
Developed, impervious surfaces: Seattle, WA. Photo by Steve Stolee
One possible contributor to coho spawner mortality is stormwater runoff. As our autumn rains settle into the Salish Sea region, they wash a toxic brew of automotive contaminants from our roads into local streams, increasing pollution in these waterways just as coho are returning to spawn. In addition, coho stay in their natal streams a full summer and winter cycle prior to heading out to the open ocean, making their young vulnerable to land-based pollutants as well.

Recently, a team of researchers led by WSU’s Jen McIntyre exposed coho to stormwater runoff coming directly from a Seattle area freeway. ​

The results were sobering. Within 24 hours of exposure, one hundred percent of coho exposed to the freeway stormwater runoff died, while their counterparts in clean water, as well as filtered storm water, had a one hundred percent survival rate.

McIntyre’s team later isolated the chemicals in tire dust, the fine particles that wear from our tires as we drive, as the leading cause of coho spawner mortality from stormwater runoff within the Salish Sea region. Furthermore, these researchers noted that coho are particularly susceptible to the effects of chemicals that leach from the tire dust, more so than other species of salmon. ​​​​

Simply looking at the black stew of stormwater that flows from a typical urban freeway, it might not be too surprising that coho and other marine species would have high mortality rates. However, one of the key findings of this research shows that when toxic stormwater was filtered using soil ​​​​infiltration columns, one hundred percent of coho mortality was prevented. Soil infiltration had the ability to capture pollutants and return the water sufficiently clean for coho survival. ​

See more about salmon in The Currency of Rainfall in this issue.
Photo courtesy of Kitsap Conservation District
So, what can individuals, neighborhoods, and municipalities do to help staunch the flow of tire dust and other toxic chemicals into the Salish Sea? ​How can we better filter our rain water prior to returning it to its source? Rain gardens can offer a partial, and aesthetic, solution.

​From small homeowner gardens that filter rooftop and driveway flows to larger municipal gardens designed to channel runoff from city streets, rain gardens effectively filter toxins from stormwater runoff, including the tire leachate that has proven so harmful to our native coho.

See more about rain gardens and runoff in Rainbows in this issue.
See more about the importance of soil in this issue.
All marine wildlife are pressured from an increasingly urban human presence around the Salish Sea, as well as the challenges from changing temperature and rainfall patterns due to climate change. As such, it becomes our job to be stewards of the valuable rains here in Western Washington and to return that water back to the land and sea in its purest form for the benefit of all living inhabitants with whom we share our beautiful Salish Sea.
Paul Pegany is a retired elementary educator who has over 30 years of experience in helping students better understand the world around them through science and literature. He splits his time between the Sonoran Desert of Arizona and the forests and seas of the Pacific Northwest. His goal is to help people of all ages better appreciate the beauty of the world, as well as understand the stewardship role that humans play as part of the greater ecosystem. Along with his wife, Sharon, he works to build his own knowledge through coursework and experiences in whichever biome he finds himself.