Colorado Division of Wildlife biologists are hopeful that the successful natural reproduction will lead to re-establishing wild, self-sustaining rainbow trout populations in Colorado where whirling disease has precluded wild rainbow trout recovery efforts.

The fish, a cross of the Hofer rainbow trout and other rainbow strains that are used for stocking, appear to be resistant to whirling disease.

The fish were "young of the year", and hatched in May 2007. They were captured in October when DOW aquatic researchers conducted electro-fishing operations in the two areas.

"They were plump, colorful fish, they looked good," said Barry Nehring, an aquatic researcher for the DOW in Montrose who has been working on whirling disease since 1994.

"This is indicative that we've had successful reproduction."

Several of the fish were then sent for genetic testing to a laboratory in Boulder that verified the fish were offspring of Hofer-cross rainbows stocked in the river and the ponds in 2004 and 2005.

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 that fish grow more slowly in the Colorado River because the water is very cold. Consequently, researchers speculate that Hofer crosses stocked there might not yet have reached sexual maturity. They'll look for young fish again this fall.

The Gunnison River is lower in elevation, water temperatures are warmer and it is renowned for producing large trout. Brown trout - which are resistant to whirling disease - thrive in the river.

The ponds on the Frying Pan River also provide relatively warm water.

George Schisler, another DOW aquatic research scientist, is hopeful that the next positive milestone will come in late 2008. "The fish need to make it to age one and beyond, so we'll see this fall," Schisler said.

But judging from research conducted on the Hofer strain, scientists are confident that the fish will survive and continue to reproduce.

Whirling disease is caused by a microscopic parasite that passes through the fish's skin. The organism attacks the cartilage of young fish and distorts the spine. The affected fish move in a whirling motion, basically swimming in circles when excited or when trying to escape predation. This type of behavior greatly reduces their ability to survive in the wild.

The disease was found in Colorado in the mid-1990s and it devastated most wild rainbow trout populations throughout the state.

During a whirling disease seminar in 2002 in Denver, a German scientist delivered a research report about a rainbow strain that was resistant to whirling disease.

The Hofer rainbow trout was raised in a German hatchery. 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 the 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 parasite spores had developed. Schisler said researchers were stunned by what they saw. Spore counts in Colorado River Rainbows ý which have been used for stocking by the DOW for years - exposed to the disease could reach 4,000,000 per fish.

The highest count in the Hofers reached only about 3,000 and did not affect the fish.

DOW aquatics staff then started crossing the Hofers with existing rainbow stock and conducting more tests. Not only were the new strains of fish resistant to the disease, they also grew faster than traditional stocker strains. Hofers grew to catchable size ý about 10 inches ý in about 14 months, four months faster than the other rainbow trout strains.

In 2004 fingerlings of the new cross strains were first released into the Gunnison River. They were first released into the Frying Pan River ponds in 2005 and into the Colorado River in 2006.

Some catchable-size crosses were also stocked in two reservoirs near Berthoud in the spring of 2006. The fish continued to grow in the reservoirs and anglers were successful in catching them.

Mark Jones, aquatic research leader for the DOW, said Colorado leads the nation in the whirling disease war.

"No other state has conducted more research into identifying real solutions to the whirling disease problem," Jones said. "We could tell this was a good thing from the start."

Based on the extensive research, the DOW hatcheries are expanding production of the various crosses. In 2008 more than 1 million sub-catchable and catchable fish of the Hofer crosses are planned to be stocked in lakes and rivers throughout Colorado.

Research to examine the resistance of the Hofer crosses to whirling disease and their ability to survive in the wild is ongoing

News Source: Colorado DOW - Feb. 01, 2008