Return of the rainbow?
SUMMIT COUNTY – Local guide Trapper Rudd well remembers the heyday of rainbow trout fishing in Colorado, back before whirling disease spread into the state in 1987, crippling and deforming the beautiful speckled fish. In just 10 years, many wild rainbow trout populations across the state were decimated.”It truly was a magnificent time,” Rudd said, recalling rainbow trout fishing sessions on the Blue River and other hot spots.”You could get into the double-digits pretty quick,” Rudd said. “The rainbow populations here were as strong as anywhere in the country.” Rainbows are known for their willingness to rise to a dry fly, and for their splashy acrobatic moves once they’re hooked, he explained.”We started seeing it before we know what it was,” Rudd said, describing the onset of the whirling disease epidemic in the late 1980s. “When it was in its bloom, we started catching these small fish that were deformed. We noticed them acting funny around the banks,” he said.
It didn’t take biologists very long to figure what was going on. Parasitic spores – part of a complex life cycle involving mud-dwelling worms – were infecting the fish and spreading like wildfire. Before long, both state hatcheries were infected, and researchers pinpointed a shipment of infected trout from Idaho as the source. Worst of all, there seemed to be no way to stop it. Initial reports from biologists suggested that it 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 infection.By the early 1990s, rainbow populations simply collapsed, disappearing entirely from some rivers and lakes, with only a few remnant populations holding on. Some rainbow populations in the High Country also managed to avoid the worst of the disease, based partially on the fact that fast-running mountain streams don’t have the layer of mud on the bottom that provide the ideal environment for the parasite-hosting worms.Rudd said other fish quickly filled the niche, with brown trout, for example, thriving in some areas where rainbows previously dominated. In other cases, brook trout populations started to increase, and that’s not always a good thing. The brookies can crowd out populations of native cutthroat trout in some places, Rudd said.All the while, researchers looked for answers, with the goal of someday re-establishing rainbow trout in Colorado. In recent years, some of the most promising research has focused on breeding Colorado River rainbows with another strain, called Hofer rainbows, from a hatchery in southern Germany. The Hofer trout, it turns out, are highly resistant to whirling disease.
Working first at Colorado Division of Wildlife (CDOW) research facilities, and recently moving on to field trials, aquatic biologists are mixing the two strains, hoping to blend the resistance of the Hofer trout with the desirable sport fish qualities of the Colorado River strain.”We’re closer now than we have been to finding a solution. There’s some light at the end of the tunnel,” said George Schisler, one of the CDOW scientists involved in the effort.HistoryRainbow trout, or course, are not native to Colorado. They all stem originally from the West Coast, primarily 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. Those that were introduced to Colorado developed characteristics that made them into desirable fish, including their spunky nature, as well the fact that they are relatively long-lived.
“It’s an outstanding fish in the absence of whirling disease, but it’s very susceptible,” Schisler said. Rainbows were also exported to Europe, where whirling disease was introduced into the mix. The spores that infect the fish are part of a complex lifecycle involving mud-loving tubiflex worms. But the key thing is that, over time, some of the European rainbows evolved with a resistance to the disease.Working with scientists through an international collaborative, the CDOW researchers started to do some genetic testing to try and find the exact genes that make the German trout resistant. 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. Currently, there are two field trials in progress, in the Gunnison and South Platte rivers, Schisler said.
The researchers are finding a range of resistance in the various families. Using common animal husbandry techniques, they are re-crossing 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 said the differences are even apparent in a lab setting, where the Colorado River rainbows tend to try and hide in the farthest corner of a tank.But the Hofer rainbows are nearly tame.”You can practically pick them up,” Schisler said.
The Colorado research has important implications for fisheries across the country. Whirling disease is still spreading,” said Dave Kumlien, director of the Montana-based Whirling Disease Foundation. “This is the only tool resource managers have for managing rainbow trout fisheries in the wild,” he said.The outcome of the Colorado research is still uncertain, but holds some promise. Schisler said that this year they are hoping to find some reproduction among the fish that have been released in the Gunnison, and that would take the program a big step forward.But based on the early results, it seems that rainbow trout may one day again flaunt their brilliant colors against the crystal-clear waters and cobalt-blue Colorado skies.Bob Berwyn can be reached at (970) 331-5996, or at email@example.com.
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