Mount Rainier’s fish-stocking legacy threatens native aquatic species

For more than a century, fish stocking in Mount Rainier National Park has reshaped waters that once had no fish at all. Between 1915 and 1972, more than nine million fish entered the park’s lakes and streams in an effort to create recreational fishing opportunities.

The change brought new pressures to native species. Before fish were introduced, the park’s lakes offered amphibians relatively secure breeding sites and habitat. Brook trout and other non-native fish altered that balance by feeding on amphibians, competing with native species and changing aquatic food webs.

The effects also reach bull trout, a federally threatened char species that depends on cold-water streams and lakes. Brook trout compete with bull trout for resources, prey on young bull trout and can hybridize with them. Scientists are also investigating whether Dolly Varden live in the park, although the species has not yet been documented there.

The National Park Service report says about 35 lakes and many streams still support breeding populations of non-native fish. Conservationists view the continued spread and persistence of introduced fish as one of the major potential threats to amphibian species of concern in the park.

Researchers are turning to environmental DNA, or eDNA, to find clues in remote waters. Fish and amphibians leave genetic material in water through feces, mucus, skin, scales and carcasses. Scientists can collect those traces by pumping water through a fine filter, offering a way to study populations that are difficult and expensive to survey directly.