There was a time when Colorado’s lakes and streams were full of big, splashy rainbow trout that would eagerly rise to an angler’s lure and put up quite a tussle, breaching the water in a shimmer of green, red and gold.
But that all changed about 20 years ago, when state workers accidentally released a batch of fish infected with a deadly parasite that quickly spread throughout the state and decimated rainbow trout populations.
Initial reports from biologists suggested that whirling disease might not be a problem for wild rainbow trout populations, so the Colorado Division of Wildlife continued stocking infected trout for four or five years after they first discovered the disease. By the time they realized their mistake, it was too late. By the early 1990s, rainbow populations had simply collapsed, disappearing entirely from some rivers and lakes, with only a few remnant populations holding on.
In the Colorado River, for example, rainbow trout dropped to less than 1 percent of the total trout population. Historically, the balance was about 80 percent rainbows and 20 percent brown trout, says Jon Ewert, an aquatic biologist with Colorado Parks and Wildlife.
“We started seeing it before we knew what it was,” says Trapper Rudd, a longtime Summit County guide who well remembers the heyday and subsequent decline of Colorado River rainbow trout.
“When it was in its bloom, we started catching these small fish that were deformed. We noticed them acting funny around the banks,” he says, describing the onset of the epidemic in the late 1980s.
The disease essentially deformed the spine of the fish, causing them to swim in circles — hence the name “whirling disease.” With no known cure, and no way to eradicate the parasites, the future looked grim — until Colorado Division of Wildlife biologists started cross-breeding Colorado River rainbows with a strain of domesticated trout from southern Germany that are resistant to the parasites.
It’s important to remember that rainbow trout aren’t native to Colorado. They stem from the West Coast, pri marily from a few rivers in northern California and Oregon. In the late 1800s, when fish husbandry became widespread, rainbows were transported all over the world, and the Colorado version quickly evolved into a spunky, long-lived strain that made them a desirable fish for anglers.
Working through an international collaborative, Colorado biologists started some genetic testing to try to find the exact genes that make the German trout resistant to whirling. The first crosses between the Colorado River rainbows and the Hofer rainbows were made in 2003, with 35 different families. Using genetic markers, the scientists will be able track the offspring from those pairings, even after the fish have been released into the wild, biologist George Schisler says.
The new strains showed a range of resistance, so using common animal husbandry techniques, the biologists re-crossed the families to maximize the resistance while at the same time maintaining the desirable qualities of the Colorado River rainbows. It’s challenging in part because the resistant Hofer strain has long been bred as a docile food fish. Schisler says the differences were even apparent in a lab setting, where the Colorado River rainbows tried to hide in the farthest corner of a tank.
But the Hofer rainbows are nearly tame.
“You can practically pick them up,” Schisler says.
By 2008, the state’s fisheries experts were ready to release some of the fish back into Colorado rivers, with high hopes that they could rebuild the once-thriving rainbow trout fishery. And last year for the first time, they determined that the new strain of rainbows are reproducing in the wild — a hopeful sign as fisheries biologists try to rebuild selfsustaining rainbow trout fisheries throughout the state.
“We looked in good fry habitat, Ewert says, describing part of the rigorous and regular monitoring used to assess fish stocks.
“Last year for the first time we saw them prior to stocking,” he says, explaining that the timing of the observations makes it certain that they were observing fish born in the river rather than fish stocked from a hatchery.
“At one site, rainbow fry even outnumbered brown fry,” he says.
The new cross-bred strain is finding favor with anglers and guides, too.
“They’re really strong, exciting fish to catch. Even the ones that are a year old, maybe 10 inches, they are really energetic fish. ... You know what you’ve got on your line,” Ewert says, adding that the Hofer rainbows are growing especially fast, probably because of their genetic history as domesticated fish.
The new rainbows are even catching hold in places like the Arkansas River that historically have not been strongholds for the species.
“Before the Hofers, all our rainbows were stocked fish, series of different strains over the years. They did pretty well as catchable size, but if you planted them too small, the browns would eat them all,” says Greg Felt, a long-time guide with ArkAnglers.
“We’ve been seeing really good natural reproduction. We hadn’t really had that before. [Colorado Parks and Wildlife] really surprised me with how effective they were with this,” Felt says.
“The other thing that’s kind of cool about it is watching to see how the rainbows and browns get along. Will the rainbows compete with the browns directly? I’m sure there is some competition but rainbows make use of different habitat, and they’re active at different times,” he says, explaining that the addition of the Hofers enhances overall fishing opportunities on the Arkansas.
“Where I tend to look for them and target them is around riffles, or where there’s a cobbled shelf, that shelf just drops away. If you weight way your rig so it drops of the shelf, you can look down and just see those torpedoes stacked up,” he says, referring to the handsome speckled rainbows.
The same holds true for the Colorado River, says Mitch Melichar, a Silverthorne-based guide with Cutthroat Anglers.
“I have not seen a whirling disease fish in years, probably since late 1990s. We’re catching the Hofers on the Blue River and in the Colorado. ... It’s been a pretty big success from an angler’s standpoint,” Melichar says, highlighting few spots where state biologists have targeted their efforts, including the Upper Colorado near Parshall, where by some reports there are as many as 8,000 fish per acre of water in an area where state biologists have stocked up to 200,000 fish per year in a six-mile reach of the river. Downstream of Glenwood Springs is another area where rainbows have come back strong, with anglers reporting 25-inch fish, he adds.
The state wildlife agency has also crossed Hofer rainbows with a lakeadapted trout from Montana that also shows resistance to whirling disease. The Harrison-Hofer cross, which spawns in lake inlets, is being used to boost rainbow trout populations in Colorado lakes and reservoirs, Ewert says, adding that there are signs of success in Grand Lake.
Rebuilding self-sustaining populations is important because there’s always a limited supply of hatchery fish.
“We only have so many to go around, I’ve concentrated all of our stocking in the state wildife areas, around Parshall and Hot Sulphur Springs. We’re hoping they’ll seed themselves downstream. We’ve already seen some of that. Maybe at some point we can start taking eggs out of the river again,” he says.
There is even some potential that the Hofer rainbows could act as a biological filter, helping to reduce the overall incidence of whirling disease. Somehow, the resistant fish are able to make the parasitic spores nonviable, Ewert says.
“When the spores come drifting down the river, they stick to first fish they come to, but Hofers deform the spores. The spores that get released off a Hofer are non-viable. They could be a kind of super-fish, acting as a biological filter,” he says.
It’s still a little early to know for sure, but based on early results, it seems that rainbow trout are back, once again flaunting their brilliant colors against the backdrop of crystal-clear waters and cobalt-blue Colorado skies.