Chum salmon appear to have a cockroach-like resistance to pollution.

Scientists at Washington State University’s stormwater research center in Puyallup recently made the surprising discovery that chum are unaffected by the same levels of toxic road runoff that quickly kills their coho cousins.

“Overall, the results were very surprising because I thought chum would show some signs of getting sick, like we saw with coho — but that didn’t happen,” WSU ecotoxicologist Jenifer McIntyre said.

Also known as dog salmon, chum are the most plentiful salmon species in Puget Sound. They’ve maintained relatively healthy populations while coho and chinook numbers have plummeted.

Since 2012, the center has taken coho raised at a Suquamish Tribe hatchery and exposed them to stormwater captured from busy roadways. The coho showed almost immediate signs of illness and were dead within 24 hours.

The same toxic cocktail appears to have little affect on chum.

“Leading up to their deaths, (coho) grew lethargic and seemed confused, swimming erratically near the water’s surface and turning onto their sides,” McIntyre said. “(Chum) remained healthy looking and alert. Even their blood chemistry was relatively unaffected.”

The chum also were from the Suquamish Tribe’s hatchery on Grover’s Creek near Indianola.

The results match a decade of urban creek monitoring. Coho in Seattle-area creeks were observed dying at high rates after heavy or light rainfall. Rain picks up oil, heavy metals and other containments from roofs, roads and parking lots and funnels them into creeks.

Chum’s runoff resistance is a mystery.

“We do not have the answer, but I’m incredibly curious,” McIntyre said.

Chum are considered the most robust of the salmon species. Even as newborns, they leave protected freshwater streams and head to open water earlier than other salmon species.

They appear generally less sensitive to water quality. Even the problem of low dissolved oxygen, which has long plagued coho and other marine species in Hood Canal, is taken in stride by the resilient chum.

They might be tough, but they’re not especially tasty. With flesh some describe as “mushy,” chum are unpopular with anglers and have a comparatively low market value. The ailing coho, on the other hand, is a prized catch.

McIntyre hopes to learn how other salmon species react to stormwater pollution. Their survival rates could be severely impaired as the sound’s urban areas continue to expand, McIntyre said.

But there is a solution. A four-year study by WSU’s stormwater center showed that simple filtration systems can greatly reduce stormwater runoff’s toxic effects.