eDNA Testing Reveals Something Fishy in Olympic Mountain Lakes

JULY 2026
Five hundred miles and 200,000 total feet of elevation gain—that's the distance traversed by field crews collecting environmental DNA from mountain lakes across Olympic National Park. Despite the remoteness of their destinations, another visitor had reached these lakes almost a century before them: fish.

Nonnative trout arrived in historically fishless lakes with a little help from their friends: sport anglers and public land agencies. Beginning in 1930, over 1.6 million fish were hiked in or dropped out of airplanes to stock lakes for recreational fishing in the park and surrounding National Forest lands.

A stock train carries trout to Moose Lake in 1940. Most lakes were planted with fish a single time, but some were stocked as many as 18 times. NPS

For years, trout swam unsupervised in an ecosystem that evolved without fish. Amphibians found themselves suddenly on the menu. Predation from introduced fish, combined with factors like climate-induced habitat loss, has contributed to amphibian declines in mountain lakes across the western U.S., including the disappearance of Cascades frogs from Lassen Volcanic National Park. Yet in Olympic, the legacy of fish introductions and their impact on amphibians remained understudied until the three-year survey began in 2019.

“There has been a growing recognition, particularly in wilderness areas and national parks, that the introduction of nonnative species is not consistent with park policy,” said Olympic National Park coastal ecologist and limnologist Dr. Steve Fradkin, who contributed to the study. “In a place like Olympic, which is 95% congressionally designated wilderness, that sort of thing is in conflict with what in a modern day we tend to think we’re all about.”

What's Out There?

Field crews set out to find answers to a question that has been unfolding for nearly a century. Advances in eDNA testing, which picks up traces of genetic material shed by an organism in its environment, offered a less-intensive way to determine the presence of species in remote areas at a parkwide scale.

A research technician collects an eDNA sample at Milk Lake by pumping lake water through a thin filter. Laboratory testing analyzes the genetic material collected to determine which species are present. NPS

Results published this winter reported introduced fish in roughly half of the 117 lakes sampled. This exceeded historic documentation—twenty-one lakes with fish had no records of planting, suggesting introductions by anglers who took matters into their own hands.

With fish relatively widespread, two species of amphibians showed cause for concern. Long-toed salamanders and rough-skinned newts were less likely to be found in lakes with fish. This echoes studies elsewhere where introduced trout impacted the growth and survival of long-toed salamanders.

Amphibians as a broader group, however, appeared to coexist with fish.

“Our expectation ... was that wherever you have fish, the amphibians are in jeopardy,” said Jeff Duda, ecologist at the USGS Western Fisheries Research Center and author on the study. “What we discovered is that there’s some nuance to that.”

Researchers found fish and amphibians coexisting in 45 lakes—over three times the number of lakes with only fish.

Graphic Description (Alt Text)

A map of Olympic National Park with eDNA detections marked with four differently colored icons: fish only, amphibians only, fish and amphibians, or none. Detections appear across the park with higher numbers on the northwest and east sides of the park. Amphibian-only lakes and lakes with both fish and amphibians are the most common parkwide. Lakes with only fish are much less common, with only a dozen occurrences. A handful of lakes throughout the park had neither fish nor amphibians.

Changing Habitats

To survive alongside a predator, amphibians have at least one advantage over fish: legs. Their ability to use different habitats offers a degree of refuge from fish-stocked lakes.

"A lot of these amphibians are not necessarily solely dependent on the aquatic environment. They move for part of their life history. They move to land, where they’re protected from fish. They can move around the landscape and occupy other ephemeral wetlands or nearby, smaller lakes that could not house fish,” Duda said.

Cascades frogs, for example, often spend time on lake shorelines out of reach of trout. They breed in seasonal wetlands, allowing vulnerable eggs, tadpoles, and froglets to grow up in ponds that lack fish. But these small bodies of water are vulnerable to a different threat altogether—increasing temperatures and shrinking snowpack.

“Those more ephemeral [water bodies] that hold amphibians are likely going to be changing over time,” Fradkin explained. “More permanent systems ... in the future might be more important for maintaining these species on the landscape.”

Long-toed salamanders were less likely to be found in lakes with fish. Habitat loss due to climate change adds another layer of stress. NPS / Scott Anderson

You don’t need to hike to an alpine lake to see these threats to amphibian habitat. A view of the Olympic Range reveals this year’s diminished snowpack, a mere 26% of the 30-year normal. Large mountain lakes will become crucial refuges in this changing landscape. In the face of habitat loss, Cascades frogs and other amphibians may have no choice but to spend more time in lakes where fish could eat their eggs and larvae.

Fish Out of Water

As amphibians face worldwide population declines, nearby parks are responding to compounding stressors with fish removals. In Mount Rainier and North Cascades National Parks, native amphibian populations have increased in the absence of fish.

In Olympic, knowing which lakes have nonnative fish is the first step in understanding their impact on the ecosystem. For the first time since trout were brought to the park nearly a century ago, scientists have a baseline of population distribution, allowing monitoring and stewardship as fish and amphibian relationships evolve in the face of change.

Further Information

• Learn about how NPS scientists monitor the health of mountain lakes in Olympic, Mount Rainier, and North Cascades National Parks
• Read about broader eDNA inventories of aquatic species throughout the North Coast & Cascades Network
• Explore the NPS Historical Photo Collection for a visual history of fish stocking in parks across the western U.S.
• Read more about mountain lake restoration in North Cascades National Park
• Create an interactive visualization of climate indicators in Washington state

Where this belongs

• Olympic National Park

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