OUT: Cutthroats threatened by brook trout competition and whirling disease

Division of Wildlife aquatics researcher Kevin Rogers (in yellow) examines a healthy male brook trout held by Gregor Deklava during a trapping project last fall at Trappers Lake. The project was designed to determine an estimated population of brook trout in the lake.

A project to restore Colorado River cutthroat trout to Trappers Lake may have more unanswered questions than solutions.

Trappers Lake, located at 9,600 feet elevation near the head of the White River about 30 miles east of Meeker and just inside the Flat Tops Wilderness Area, is a popular fishing destination, offering both cutthroat trout and sizeable brook trout.

Once home to pure Colorado River cutthroat, today most of the trout carry a mix of Colorado River and Yellowstone cutthroat genes.

Despite the lake’s placid appearance, the cutthroats face two serious threats: competition from brook trout and the deadly parasite of whirling disease.

Several years ago,  the Colorado Division of Wildlife began a two-part project aimed at restoring pure Colorado River cutthroat to Trappers Lake.

First, Kevin Rogers, a DOW aquatics researcher based in Steamboat Springs, began reducing the brook trout population by trapping brook trout in the fall, when the spawning adults are most susceptible.

At the same time, the DOW attempted to over-ride the Yellowstone genetics by stocking up to 45,000 pure Colorado River cutthroats each year from 2003–2007.

“Our efforts at Trappers initially was to mitigate the brook trout expansion and to preserve the cutthroats,” said Rogers. “In 2003 (while trapping some fish) we noticed some cranial deformities in the cutthroat and submitted them for testing.”

Such deformities are common in whirling disease-infected trout and sure enough, tests showed about half the fish submitted were infected.

“The infection at that point was not too bad,” Rogers said. “But since then the intensity of the infection has gotten higher.”

Trappers Lake isn’t the only high-elevation lake in Colorado infected with the deadly whirling disease parasite.

Over the past six years, researchers have found whirling disease in 17 lakes and streams in six wilderness areas. Last summer,  DOW aquatics researcher Barry Nehring and his crew sampled 45 lakes between 11,000 and 13,000 feet in elevation and found the parasite and the disease in two lakes between 11,000 and 12,000 feet and two more in lakes above 12,000 feet.

Nehring is considered one of the nation’s most-knowledgeable whirling disease experts, and his latest work is focusing on the life cycles of the Tubifex tubifex worm, a common aquatic worm that acts as the intermediate host for the whirling disease parasite.

Among Nehring’s findings is the predominance in Colorado of the lineage III tubifex worm, the lineage most susceptible to whirling disease. Other lineages of the worm aren’t susceptible, and where those lineages are found WD is absent.

“The parasite that causes whirling disease has been in Trappers Lake for decade now, so it’s not a big secret,” Nehring said recently. “We passed the point of no return in 2006 when it wasn’t just a low-grade infection anymore.”

He said the level of infection might be affecting the survival of young cutthroat trout.

“We don’t know that for sure but we do have 100 percent prevalence in the young cutthroat fry in the fall of the year,” said Nehring.

Similarly, young rainbow trout fry, which also are highly susceptible to whirling disease, emerge from the nests in early summer but are dead from whirling disease by the fall.

Anglers might not notice an immediate decline in Trappers Lake cutthroat trout because anglers are catching adult fish, which are resistant to whirling disease.

Again, something similar happened in the early 1990s, when whirling disease first started wiping out the state’s rainbow trout. Anglers were catching trout old enough to be resistant to the disease and many anglers dismissed the threat of WD.

But as the older trout were caught or died, there were no young rainbow trout to replace them.

The infection at Trappers Lake is considered moderate to severe, Nehring said.

“If there’s more than a 50 percent infection it’s considered in the moderate to severe class,” he said. “Most of the wild fish probably going to die before they get to that point.”

Rogers said tests indicate the parasite got into the lake in 1999 but no one knows how.

He hypothesized an angler may have caught an infected fish in the river and cleaned it in the lake.

“Or it could be a fish climbed the barrier (at the end of the lake) and got in the lake or maybe a bald eagle” carried the parasite into the lake, he said. “All the headwater lakes going into

Trappers are still negative so probably the easiest explanation would be someone catching a fish down below and dumping the guts in the lake.”

As the level of infection rose, concern also rose about the cutthroat trout and the brook trout.

Rogers said he’s noticed a decline in the brook trout populations but he’s unsure if it’s due to trapping or whirling disease.

So what happens now at Trappers Lake?

“There are some Band-Aid solutions in terms of stocking, but historically the lake held one of the biggest and most-robust populations of cutthroat trout and you don’t want to lose that wild capability,” Rogers said.

Initially there was hope of the trout developing natural resistance, but again like rainbow trout, if all the cutthroats die off, there will be nothing to grow resistant.

“Right now, there is no answer,” said Nehring. “I compare (Trappers Lake) to Yellowstone
Lake where the parasite has been for more than a decade. And in the Yellowstone the cutthroat fishery is not doing very well.”


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