Researchers believe a small parasite may be responsible for diminishing returns of steelhead around Puget Sound, and they're testing the theory in a USGS lab on Marrowstone Island.

The parasite is called nanophyetes and it enters the kidney of steelhead. Researchers are still unsure how exactly it’s affecting mortality.

"One of the things we're really concerned about with this particular parasite is first of all, whether it's killing the fish directly. If it's not killing the fish directly, if it's then somehow decreasing their swimming performance and making them more susceptible to predation," Paul Hershberger said.

Hershberger and others at the USGS field station are trying to figure out why steelhead are dying. Steelhead only number half of what they did in the 1980s.

There are two test groups: healthy steelhead and those infected with nanophyetes.

Some of the fish are monitored in tanks to see how long they live. Kidneys are removed and tested for the parasite.

Other fish were recently tagged with hydroacoustic monitors and released into the wild to see how the parasite affects them in the wild.

Scientists believe predators are certainly to blame, but this particular parasite may make the fish more susceptible.

"The most immediate cause is they're getting eaten. Well, ok. But if the underlying cause is something that's occurring in the fresh water, that takes our management in a different trajectory. We need to manage in the marine system but we need to think about what's happening in the fresh water system," said WDFW Biologist Erik Neatherlin.

Neatherlin works with the Marine Survival Project, a 5-year $40 million partnership with Long Live the Kings in Washington and the Puget Salmon Foundation of Canada. The groups hope to restore dwindling populations of salmon and steelhead.

"The ecosystem of Puget Sound is shifting. The health of Puget Sound is in decline," Neatherlin said.

Nanophyetes is most prevalent in the south Sound, and that's also where steelhead have the lowest return rates.

Researchers expect initial results within a month.

“If in fact this parasite is contributing to the decline there are possibly management strategies to we can do to mitigate the effects of the parasite and keep the fish from getting infected,” Hershberger said.