Scientists overseeing the Grovers Creek coho salmon stormwater experiment are closer to determining what kills salmon exposed to polluted runoff.

“It’s a ‘rule out’ year,” said Jen McIntyre, a toxicologist with Washington State University (WSU) who has been leading the project the past few years. “We’re doing more intensive blood and tissue analysis this year to narrow down possible causes of death.”

Last fall at the Suquamish Tribe’s Grovers Creek Hatchery, the Puget Sound Stormwater Science Team exposed adult coho salmon to either unfiltered stormwater runoff collected from a highway in Seattle, or well water. The team consists of scientists from WSU, University of Washington, National Oceanic and Atmospheric Administration (NOAA) and U.S. Fish and Wildlife Service (USFWS).

Over a four-week period, a total of 61 adult coho were exposed to the road runoff. One half were sampled after 30 minutes of exposure; the others a few hours later when they showed advanced symptoms of pre-spawn mortality (PSM). Another 61 salmon were exposed to well water from the hatchery and sampled at the same intervals.

Organs – including gills, heart and kidney – were preserved for later testing. Blood samples were tested on site at the hatchery in a mobile lab.

“We’re looking at oxygen levels, white blood cell levels, metabolites, proteins, immune responses – indicators of organ failure,” McIntyre said. “We want to see how blood changes over a period of time – at what point in the body does the blood change? Before or after it goes through a particular organ? And how long after exposure does that happen?”

In recent years the team, working with the tribe, has published several peer-reviewed studies showing that most or all of the adverse impacts on species like salmon can be prevented by pre-treating urban runoff with experimental soil columns, such as bioinfiltration or rain gardens, said Jay Davis, a USFWS environmental toxicologist.

“However, if through our studies we are able to isolate the chemical or chemicals that are causing PSM, controlling the source of the compounds is a more effective way to improve water quality and salmon habitat, and thus help recover the species,” he said.

The yearly forensic science at Grovers Creek is part of a larger effort to understand the causes and consequences of coho pre-spawn mortality in urban and urbanizing watersheds, said Nat Scholz, lead for the Ecotoxicology Program at NOAA’s Northwest Fisheries Science Center.

For example, the Suquamish Tribe and other regional partners have conducted stream surveys to monitor adult coho deaths in spawning habitats throughout Puget Sound. The stormwater team is using these field data to refine sophisticated models, to predict where coho are most at risk based on the density of road networks and other types of impervious surfaces.

“We’re trying to answer four major questions with the ongoing science,” Scholz said. “First, why are coho dying, and what chemical or chemicals are the smoking gun? Second, this is a pavement problem, so where in Puget Sound are coho most vulnerable to future changes in development and growth? Third, which clean water strategies in our stormwater treatment toolbox work best? Fourth, how much green infrastructure will we need at the watershed scale to ensure people and coho salmon can coexist in the coming decades?

“We’re racing against the clock.”

The team receives support from various agencies, including the U.S. EPA’s National Estuary Program.