In 1930, Americans carried fish into Olympic's fishless mountain lakes; 1.6 million trout later scientists are finding the ecological cost

Beginning in 1930, more than 1.6 million fish were carried by horseback or dropped from aircraft into the naturally fishless mountain lakes of what is now Olympic National Park, creating recreational fishing opportunities in waters that had never supported fish populations before. Most lakes were stocked only once, although some were restocked repeatedly, with individual lakes receiving as many as 18 stocking events over the following decades. The introductions fundamentally changed ecosystems that had developed without fish predators, particularly for native amphibians that had evolved in these isolated mountain waters. Nearly a century later, researchers have completed the first comprehensive survey of fish and amphibian distributions across 117 remote lakes in and around Olympic National Park. Using environmental DNA, or eDNA, collected between 2019 and 2021, the team has now mapped where introduced fish remain and found that their presence is associated with lower detection rates for some native amphibians, while other species continue to coexist with fish by using habitats beyond their reach.How researchers used eDNA to survey 117 remote mountain lakesResearchers surveyed 117 lakes between 2019 and 2021, covering roughly 500 miles of travel and more than 200,000 feet of cumulative elevation gain while collecting water samples for environmental DNA. The technique detects genetic material organisms leave behind in their surroundings, allowing researchers to determine whether particular fish and amphibian species were present without having to capture every animal in each remote lake. The survey detected introduced fish in 59 of the 117 lakes examined. Amphibians were detected without fish in 52 lakes, while fish and amphibians were detected together in 45 lakes and fish were detected without amphibians in 14 lakes. Six lakes produced no detections of either group. The survey also identified fish in 21 lakes where researchers could find no historical record of stocking. The study notes several possible explanations, including incomplete historical records, unauthorised fish introductions or false-positive detections, while the National Park Service says the findings suggest some fish may have been introduced independently of the official stocking program. The results provide a much more complete picture of the distribution of introduced fish than historical stocking records alone could provide.Why some amphibians declined near fish while others continued to coexistThe survey found that long-toed salamanders and rough-skinned newts were less likely to be detected in lakes where introduced fish were present, consistent with previous research showing that nonnative trout can affect native amphibians. But the researchers did not find that every amphibian species disappeared wherever fish occurred. In fact, fish and amphibians were detected together in 45 lakes, about 3.2 times as many lakes as those where fish were detected without amphibians. Jeff Duda, an ecologist with the U.S. Geological Survey and a co-author of the study, said researchers had expected amphibians to struggle in lakes containing fish but found that some species appear capable of coexisting with them. One important advantage amphibians have is that they are not confined to the water. Species can use terrestrial areas around lakes or move into small, temporary wetlands and ponds where fish cannot establish themselves. Cascades frogs provide an example: researchers note that they may breed in ephemeral wetlands and ponds adjacent to lakes, allowing their eggs and tadpoles to develop in habitats without fish. That flexibility may help explain why some amphibians remain widespread even after decades of fish introductions, while species that depend more heavily on permanent lakes appear more vulnerable.Why climate change could make the fish problem harder for Olympic's amphibiansThe remaining question is what happens as the mountain environment itself changes. The small seasonal wetlands that provide amphibians with refuges from fish are particularly vulnerable to warming temperatures and declining snowpack because they depend on water remaining on the landscape long enough for breeding to occur. The National Park Service notes that the Olympic Range's snowpack was only 26% of its 30-year normal in the year of its accompanying report, illustrating the increasingly difficult conditions facing these temporary habitats. That creates a potential double pressure: amphibians already face predation and competition from introduced fish in permanent lakes, while the alternative wetlands that allow some species to avoid fish may become less reliable as the climate warms. Other national parks have provided evidence that removing introduced fish can benefit native amphibians. The National Park Service points to work at Mount Rainier and North Cascades national parks showing increases in native amphibian populations where fish are absent. For Olympic, however, the new eDNA survey provides something that has been missing for decades: a park-wide picture of where introduced fish and native amphibians are found today. That baseline gives researchers and park managers a way to track future changes and determine where fish removal or other conservation measures could have the greatest benefit as both introduced predators and climate change continue reshaping the park's high-elevation lakes.Catch the latest World News and Live updates. Download the TOI app.