The Animas River meanders through the Animas Valley en route to Durango earlier this week. The river, along with several other state fisheries, will be stocked this year with a hybrid strain of rainbow trout that is resistant to whirling disease. The state’s rainbow population was virtually wiped out in the 1990s because of the disease, which is caused by a parasite./Photo by David Halterman

by Missy Votel

Decimated by whirling disease more than a decade ago, Colorado’s waterways may soon be teeming with wild rainbow trout. According to Colorado Division of Wildlife biologists, a new strain of rainbow has reproduced in the wild. The fish, a cross between the Hofer rainbow trout and other rainbow strains used for stocking, appear to be resistant to whirling disease.

The fish were first stocked in 2004 and 2005 in the Gunnison River and ponds along the Frying Pan River, near Basalt. In October, an electro-fishing survey in the two areas found young Hofer rainbows to be healthy and thriving. “They were plump, colorful fish; they looked good,” said Barry Nehring, an aquatic researcher for the DOW who has worked on whirling disease since 1994. “This is indicative that we’ve had successful reproduction.”

DOW biologists are hopeful that the success will lead to re-establishing wild rainbow trout populations in Colorado, including the San Juan River drainage in Southwest Colorado.

Joe Lewandowski, with the DOW in Durango, said the news is exciting from a local standpoint. With the exception of isolated wild rainbow populations at higher elevations such as Vallecito and Hermosa creeks, the local population was wiped out by the epidemic in the early ’90s. “Traditionally, there have been rainbow trout in the Animas, San Juan, Piedra and Dolores rivers,” he said.

Re-establishment of a wild rainbow population will have benefits for anglers as well as the DOW, which has been stocking waterways with hatchery-bred Colorado River rainbows for the last decade.

“Wild rainbow are more wary of predators and disturbances in the water, making them more of a challenge to catch,” Lewandowski said. “Plus, with an established wild population, we won’t have to stock as much.”

He said the Durango DOW received about 35,000 of the Hofer-strain fish eggs in December. Twenty-thousand of the fingerlings will be released in the Animas this spring with the rest being divvied up among the Piedra, upper and lower Dolores and San Juan rivers. Statewide, 1 million of the fish will be released this year.

Hofer cross fingerlings were also stocked in the upper Colorado River, near Kremmling, in 2006, but researchers did not find any young fish there in 2007. Biologists said fish grow more slowly in the colder water of the Colorado and will look for young fish again this fall. George Schisler, another DOW scientist, is hopeful that the next milestone will come then. “The fish need to make it to age 1 and beyond, so we’ll see this fall,” he said. But judging from research conducted on the Hofer strain, scientists are confident the fish will survive and thrive.

The disease-resistant strain first came to the attention of Colorado researchers during a whirling disease seminar in 2002 in Denver. There, a German scientist delivered a report on a rainbow strain, raised in a German hatchery that was resistant to whirling disease. The DOW moved quickly to determine if the fish could survive in Colorado. Early in 2003, DOW researchers worked with the University of California at Davis to import Hofer eggs and start a brood stock at the Fish Research Hatchery near Fort Collins. The fish were exposed to the disease and then dissected to see how many whirling disease parasite spores had developed.

Schisler said researchers were stunned by what they saw. Spore counts in Colorado River rainbows were as high as 4 million per fish. The highest count in the Hofers reached only about 3,000 and did not affect the fish. The DOW then started crossing the Hofers with existing rainbow stock. Not only were the new strains resistant to the disease, they also grew faster, reaching catchable size (10 inches or so) in about 14 months, four months faster than other rainbow strains. “The researchers really went out on a limb,” said Lewandowski. “Although the experiment was tightly controlled, they figured it probably couldn’t get any worse than it is.”

Lewandowski said although he expects the new strain to take hold eventually, the Animas faces certain challenges that could take the reintroduction effort longer than in other areas. “It’s probably going to take five to 10 years to re-establish them,” he said.

For starters, the Animas has a heavier metal load than other rivers, making it harder for fish to grow and thrive. Secondly, the new rainbows will have to contend with the local brown trout population, which is immune to whirling disease and well established. “The brown trout are pretty aggressive and will go after the rainbows. They’re going to have a lot of nice meals,” he said. “But hopefully, we’ll get a foothold, and the river will get cleaner as the years go by.”

Lewandowski noted that browns and rainbows are not native to Colorado. Rainbow trout were introduced from California by miners in need of a food source, and the browns from the East Coast for similar reasons. He said most of the Animas’ native fish, such as cutthroats, were wiped out by the ensuing mine run-off. “It’s hard for fish to make it in a place like the Animas,” he said.

As such, Lewandowski said the Hofer crosses will likely be reintroduced in stages over the next few years. Rainbow trout typically live to four to five years and can reproduce at the age of two. An electro-fish survey in a few years will determine whether or not this is happening.

“We expect to have some reproduction but the brown have a good hold on the Animas,” he said. “We’ll take a look at how the rainbows are doing in a few years.” •