Narrowing Down Pre-spawning Mortality Factors for Coho Salmon

The annual pre-spawning salmon mortality study at the Suquamish Tribe’s Grovers Creek Hatchery takes a different twist each year.

After six years of learning how coho and chum salmon are affected by runoff from urban streets, scientists are narrowing down which pollutant is killing fish. This year, they focused on how tire residue in water affects juvenile and adult coho and chum salmon.

“We want to figure out which concentration of the tire residue in the water will kill fish and how long after exposure do the fish become sick and die,” said Jen McIntyre, aquatic ecotoxicologist for Washington State University, who has overseen the last few years of the project. Other partners include U.S. Fish and Wildlife Service, University of Washington and National Oceanic and Atmospheric Administration.

Fish are exposed to the polluted water for 24 hours or less, and then pulled from the tank and observed for normal or abnormal behavior. Fish that appear to be dying have their blood and organ tissues sampled to determine which organs are most affected, and at which point the blood chemistry starts to change and affect the fish.

Scientists also are observing how the polluted water affects chum and coho differently. In the past, chum haven’t been fazed by polluted water, but coho have died within hours.

“Chemicals that leach from tire particles are part of the complex chemical mixture of urban runoff,” McIntyre said. “But we don’t know yet how important a part.”

The yearly work at Grovers Creek is part of a larger effort to understand the causes and consequences of coho pre-spawn mortality in urban watersheds.

Other regionwide studies have included a land-use analysis of stormwater runoff to coho spawning habitats throughout Puget Sound, using data from stream surveys collected from 2000-2011 by the Suquamish and Stillaguamish tribes, and other private groups and federal agencies, including Wild Fish Conservancy and U.S. Fish and Wildlife Service.

The new models developed from the survey work represents the best estimates where coho spawners are likely the most vulnerable to the stormwater mortality syndrome, said Nat Scholz, lead for the Ecotoxicology Program at NOAA’s Northwest Fisheries Science Center.

“A major take-home of the work is that it looks like the chemical causing the most problems are coming from motor vehicles,” Scholz said. “Put simply, the greater the traffic density within a given geographic area, the stronger the association with the mortality syndrome. Rainfall appears to be playing a role, but in the more urban areas this influence is swamped out by vehicles.”

In the 2017 experiments, scientists were focused on how tire material in urban runoff affects salmon. Nearly a dozen tires were ground up to a fine powder, then added to well water. Salmon were exposed to the polluted water and then observed for signs of stress and death. Photo: T. Royal