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YELLOWSTONE NATIONAL PARK - Predatory lake trout, drought and whirling disease continue to take a toll on Yellowstone National Park's resident population of Yellowstone cutthroat, but biologists say they are beginning to see subtle signs of hope improvement.

The increasing number of young cutthroat spotted in the park's Yellowstone Lake in recent years should be getting old enough to spawn in the next year or so, which could help rebuild the population, said Todd Koel, lead fisheries biologist for the park.

A decent snowpack in Yellowstone this winter, closer to long-term averages than the last several years, should increase the flow of the park's waterways, possibly reconnecting some drought-stricken streams with the lake again and cooling water temperatures. That could help impede the advance of whirling disease.

And an aggressive gill-netting program that kills thousands of lake trout each year at Yellowstone Lake may also be paying off. as evidenced by declining numbers of the non-native trout being hooked by anglers, Koel said.

"We're encouraged. That's what we need to see," Koel said.

There are still probably tens of thousands of Yellowstone cutthroat trout in Yellowstone, but it is a fraction of the population that once numbered in the millions, Koel said.

The biggest trouble has been the invasive lake trout, first found in Yellowstone Lake in 1994. One lake trout can eat 50 to 60 cutthroat a year.

Since 1994, more than 100,000 lake trout have been caught in gill nets on Yellowstone Lake and killed. Several thousand have been caught already this year, Koel said.

Fish biologists are hoping to refine their methods in the coming years, including learning more about where the lake trout are during certain times of the year so gill-netting operations can target those areas.

Biologists are also considering researching synthetic pheromones to attract lake trout so they can be caught, Koel said. Such a discovery would also help reduce lake trout numbers elsewhere in the West, including at Swan Lake, Lake McDonald in Glacier National Park and Lake Pend Oreille in northern Idaho.

Unless the lake trout are controlled, the cutthroat population could continue to decline precipitously in the coming decades, Koel said.

"Right now, we're in a situation where we can't let up," Koel said.

Earlier this year, the U.S. Fish and Wildlife Service denied a petition from environmentalists to protect the Yellowstone cutthroat under the Endangered Species Act. The agency said that despite shrinking habitat and threats to the fish, there wasn't enough evidence to indicate the fish will disappear in the next 20 to 30 years in the West.

Environmentalists say the fish is found in parts of Montana, Idaho and Wyoming, including Yellowstone National Park, and small parts of Nevada and Utah.