Study Finds Long-Term Damage in Lakes Stocked With Nonnative Fish

Alpine lakes disturbed by humans may never be the same again.

That’s one conclusion from a study of high lakes at Mt. Rainier National Park, where people have been adding fish, then removing them, over the last century.

Deanne Drake and Robert Naiman, scientists from the University of Washington, took sediment samples from beneath the lakes and examined diatoms--tiny snowflake-shaped algae at the bottom of the food chain--to check the health of ecosystems over 500 years.

Two lakes where fish were added, then died out or were fished out, still showed changes in diatoms several decades after the fish were gone, Drake and Naiman said in a report published in December by the Journal of Conservation Biology.

That means the lakes still have not rebounded to their natural state without fish, Drake and Naiman concluded. The added fish eat zooplankton that eat diatoms, changing the populations of zooplankton and thus the diatoms as well.

Peter Leavitt, an expert in the ecological histories of lakes at the University of Regina in Saskatchewan, called the findings very significant.

“It’s important that people understand how easily these systems can be disturbed and how difficult it is to get them to recover,” Leavitt said.

Rainbow trout, brook trout and other species have been added over the last century to thousands of mountain lakes in the Cascades, Rocky Mountains and Sierra Nevada that originally had no fish, so that high-country anglers could catch them.

None of Mt. Rainier’s lakes originally had fish. Like many other mountain lakes, they lack enough nutrients and good spawning places, said park biologist Barbara Samora. Salamanders and frogs were kings of the food chain.

But visitors to Mt. Rainier started adding fish as early as 1890.

For 55 years, from 1918 to 1973, the National Park Service stocked Rainier’s lakes with cutthroat, rainbow and brook trout for the benefit of sports fishermen, following a national policy.

“Oh, sure, it was a mistake,” Samora said. “They didn’t know any better then, and they were trying to encourage people to come to the parks, using recreational fishing as one way to do that.”

That policy changed in the 1970s with growing scientific evidence that inserting nonnative species into national parks altered the natural systems that park managers were trying to preserve.

Adding fish to a fish-free lake is like adding a major predator on land, said Mt. Rainier superintendent Jon Jarvis. The fish eat salamander larvae and compete with salamanders for food, changing the ecology.

Plans to quit stocking lakes met with opposition, but the national parks generally stopped adding nonnative species in the 1970s. Mt. Rainier’s fish mostly died out or were fished out.

Drake and Naiman took their samples in 1996-97 from high-elevation lakes on the flanks of Mt. Rainier.

Two fishless lakes left undisturbed--Shriner and Dick lakes--had an unchanging community of diatoms over more than 300 years.

Four lakes with introduced fish --Clover, Eunice, Tipsoo and Owyhigh--changed their diatom populations substantially within 10 to 20 years of the arrival of fish, the study found.

Trout still live and reproduce in Clover and Tipsoo lakes.

Eunice and Owyhigh lakes-- where fish were last stocked in 1973 and were gone within five years--have not yet returned to the ecosystem from pre-fish days, the study found.

Recovery may take longer than several decades, Drake and Naiman suggested.

Or, the scientists wrote, “ecological conditions in stocked lakes may have been driven past a threshold of change--exceeding the bounds of resiliency--from which they will not return spontaneously.”

Disturbances such as loss of lake-shore vegetation may also have affected diatom communities in lakes over the last 30 years, they wrote.

Drake and Naiman said full ecological restoration will require more than simply removing fish.

A critic of the study, fish biologist Jim Johnston of the Washington Department of Fish and Wildlife, said the scientists did not sufficiently consider environmental changes such as global warming that could cut off any recovery.

Drake responded that any environmental damage should have been seen in the unstocked lakes but did not show up. Those lakes were relatively stable over a long time.

An expert on mountain lakes ecology, Robert Hoffman at Oregon State University, said other studies have shown that salamanders rebound and return to their old habits after fish are gone. But zooplankton, which he has studied, must be transported from lake to lake in a recovery process that takes longer, he said, and diatoms are likely similar.

“We need to clearly think through the kinds of changes that we are going to cause when we introduce nonnative species into these aquatic systems,” Hoffman said.

To fully restore a lake, said Leavitt of the University of Regina, scientists could reintroduce plankton and insect species that existed before fish were added. He said that method has been attempted with some success at Snowflake Lake near Banff National Park in Canada.

Taking a wider view, Drake and Naiman said the results imply that ecological restoration of other systems may be more difficult than managers expect.

“Clearly, resources dedicated to lake restoration are better spent on prevention than on attempting to fix environmental damage after the fact,” the scientists wrote.

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Mt. Rainier National Park:

https://www.nps.gov/mora