by Kerry McGowan

Every fall marks the beginning of a unique migration in the Pacific Northwest, when Pacific salmon migrate from the open ocean to freshwater streams to spawn. Beginning in October, a population of coho salmon returns to Elliott Bay then enters the Duwamish River. The Duwamish is Seattle’s only river and a designated Superfund site, one of the most toxic sites in the country. Once in the Duwamish, the salmon spend some time adjusting to a lower salinity before migrating to freshwater streams. Some of the coho salmon enter a grated culvert south of Terminal 5 near Harbor Island and swim for several hundred meters through an underground pipe before daylighting in Longfellow Creek in West Seattle. Most of the coho that use Longfellow as spawning habitat are from hatchery stock and do not have a home stream to return to, so they navigate to Longfellow Creek — one of the first inputs of fresh water they encounter.

A culvert that salmon must jump through in order to enter Longfellow Creek.
A culvert that salmon must jump through in order to enter Longfellow Creek.

Longfellow Creek flows though urbanized areas in Roxhill, Delridge, and West Seattle, and drains over 2,000 acres of land. This means the coho salmon in Longfellow are often exposed to urban stormwater runoff after rainfall events, and this pollution impacts their health and spawning success. Coho are more vulnerable to stormwater pollution than other types of salmon. They suffer from a unique set of symptoms after exposure to chemical contaminants in the runoff, including disorientation, loss of swimming ability, and a gaping mouth. They usually die shortly after the symptoms begin. Researchers think the salmon are experiencing a type of hypoxia, meaning they are unable to use the oxygen in their tissues for respiration.

Scientists are not exactly sure why coho are more affected by the pollutants in urban stormwater runoff than other species of salmon. Coho spend more time developing in freshwater than other species, about one year, and they prefer to spawn in low-lying streams that are often closer to urbanized areas than more pristine, upstream habitats. They also tend to migrate to freshwater sources during the “first flush” of rain in the fall and are likely blasted with pollutants that had the chance to accumulate to toxic levels prior to the rainy season. Longfellow Creek suffers from “urban stream syndrome,” with frequent flooding after heavy rainfall and a reduced number of plant and animal species able to tolerate urban conditions.

Scientists also have yet to identify exactly which pollutant, or combination of pollutants, affect the coho so severely. Urban stormwater runoff contains a complex mixture of contaminants, including petroleum products, heavy metals, fertilizers, pathogens from dog feces, and occasionally human sewage after extreme rainfall events. The runoff is so toxic to coho that the adults often die before they have a chance to release their eggs or sperm. This phenomenon is known as pre-spawn mortality (PSM), and it is alarmingly high in Longfellow Creek. Collaborative studies done by academics and government organizations found that PSM was 67 to 100% in Longfellow over a period of eight years of surveying. For comparison in a non-urban stream, PSM cases were negligible, around 1%. Even if the coho salmon in Longfellow do manage to spawn, creating salmon nests called redds, many of the fertilized eggs are washed away by the stream’s frequent heavy flooding. For these reasons, Longfellow is not considered to be a self-sustaining stream.

A pre-spawn mortality case in a female coho salmon.
A pre-spawn mortality case in a female coho salmon.

For the past two years, Puget Soundkeeper has trained a group of dedicated volunteers to document the coho population that returns to Longfellow Creek and also to measure the level of PSM. In 2016, from October to December, volunteers surveyed a fourth mile stretch of Longfellow, recording all sightings of live coho and noting their behavior. Volunteers also dissected salmon carcasses found in the stream, taking measurements of body size and noting the spawning condition of females.

A post-spawn mortality case in a female coho salmon.
Post-spawn mortality – this salmon was able to lay eggs before it died.

Volunteers conducted 60 daily surveys during the 2016 salmon run, and recorded over 113 sightings of live coho in Longfellow Creek, only one of which had symptoms that matched those associated with stormwater toxicity. Volunteers also dissected 50 deceased coho. Of the 21 female salmon, 48 percent had died before spawning, lower than in past surveys of Longfellow.

While it is too early to suggest any sort of permanent downward trend in the level of PSM in Longfellow, the above average precipitation this past fall may have prevented pollutants from accumulating to levels toxic to coho. The spawning success this year may also be attributed to ongoing community restoration efforts in the Longfellow Creek Watershed. Delridge Neighborhood Development Association has restored a one acre natural wetland that filters runoff before it flows into Longfellow Creek. Seattle Public Utilities offers rebates through their RainWise program, which encourages homeowners to install rain gardens in their yards. Nature Consortium and other groups have removed invasive plant species like Himalayan blackberry and replaced them with native, shade-providing species on the banks of the creek. These restoration efforts have the potential to hugely impact the health of spawning coho salmon. Past studies have found that filtering urban stormwater runoff through a simple mixture of sand, compost, and mulched bark reduces coho salmon mortality from 100 to 0%. Continuing to restore Longfellow, as well as installing green stormwater infrastructure like rain gardens, swales, and permeable pavement can go a long way to improving the health of one of the Pacific Northwest’s most iconic species.

A huge thank you to the volunteers who surveyed the creek every week, trekking out in even the worst downpour to gather data. Their efforts help raise awareness regarding the urban plight of coho salmon and hopefully inform future behavioral and policy changes that can sustain healthy salmon runs in Puget Sound.

Read the full 2016 Salmon Survey report for Longfellow Creek. 

Thanks to The North American Native Fishes Association for their support of this project!