Hofers vs. whirling disease: It's a knockout

By Cherry Sokoloski
North Forty News

Some recent German immigrants are making a big splash in Colorado. And like any ambitious immigrants, they're spreading out, mingling with the natives, and working hard to prove their worth.

Hofer rainbow trout were brought from Germany several years ago because of their resistance to whirling disease. Since then, researchers – most of them right here in Larimer County – have been busy testing these hefty trout in Colorado waters and crossing them with other varieties like the Colorado River rainbow and the Harrison Lake rainbow.

The goal of all this research is to restart the rainbow fishery in the state, after rainbows were virtually wiped out by whirling disease. To do that, researchers must develop crosses that can survive in Colorado's rivers and lakes. Furthermore, they must find some strains that can reproduce in the wild so that trout populations can once again sustain themselves and grow.

Fishing enthusiasts in Larimer County can be justifiably proud of the Hofer work that has been done in their backyard. Research got off the ground in 2003 at the Parvin Lake research station, owned by the Colorado Division of Wildlife, near Red Feather Lakes. That facility is still the hub of Hofer work.

The DOW researcher at Parvin Lake, George Schisler, is "our main guru on genetics," said Jim McKissick, assistant state hatchery chief.

The Poudre Unit in the Poudre Canyon has also been very involved in the Hofer work. At the Poudre hatchery, two dirt ponds that have been empty for several years are now being used to test disease resistance in several Hofer varieties. The hatchery has a new manager this year, James Ingram.

The Poudre Unit and the fish research hatchery in Bellvue, managed by Phil Schler, are the only two facilities in the state holding pure Hofer brood fish. McKissick credits the local facilities with being the "mainstay" of the DOW research on Hofers.

The research involves the tedium of record keeping, counting fry and regularly cleaning fish tanks, but it also has those days of celebration when workers capture Hofer crosses that have survived for several years.

That happened on the upper Colorado River last year, when the DOW found very good survival among Hofer-Colorado River rainbows that had been stocked in 2006. From young rainbows that were caught, researchers were able to tell that some reproduction had occurred as well. The Gunnison River has also seen good success with the Hofer-Colorado River strain.

Tracking on the Poudre

Closer to home, the DOW is experimenting with Hofers on the Cache la Poudre River. In 2008, the agency stocked 2,300 tagged Hofer-Harrison crosses in the river. This strain is more resistant to whirling disease than the Hofer-Colorado River variety, and it also grows faster under hatchery conditions, according to Schisler.

However, the Hofer-Harrison has done better in lakes than in rivers, and not many of the tagged fish were found in the 2009 sampling.

The next step for the Poudre is two-fold, Schisler said. "We are going to stock Hofer-Harrison rainbows in equal numbers with Hofer-Colorado River rainbows to see if we have better results with the latter, as we did in the Colorado River," he said.

The agency is also considering manually moving brown trout, which compete with rainbows, out of a one-mile reach of the river to see if that increases the rainbow survival rate.

The good news for anglers is that Hofer strains are larger than pure Colorado River rainbows, so this German cousin should improve the sport once it becomes established.

Counting the fish

Schisler explained how the DOW does a "fish census" for a given stretch of river when researchers want to measure the survival rate of stocked fish.

The most common method is called "mark-recapture." The DOW team first shocks a section of the river, then makes a run on both sides of the river with two different rafts, capturing all the fish they can find.

Workers mark each captured fish with a fin clip and weigh and measure every fish. In a couple of days, the procedure is repeated on the same stretch of river, and the marked and unmarked fish are counted separately.

"The ratio of marked fish to unmarked fish tells us how many total fish are in the stretch of river," Schisler explained.

When young fish are found in the river, researchers can assume that reproduction in the wild has occurred, since wild rainbows were virtually wiped out by whirling disease. Therefore, any 'bows found in the river are sure to be stocked fish.

Hofers on the move

Statewide, the DOW's goal is to stock as many Hofers and Hofer crosses as possible in Colorado rivers, streams and reservoirs. However, according to McKissick, it takes time to produce enough of these new strains to satisfy the state's stocking needs. At this point, other rainbow strains are still being stocked even though they are vulnerable to whirling disease.

The state is making progress, though. In 2009, Hofers and Hofer crosses made up one-third of the total rainbow fish stocked in the state. Sixty-two stretches of rivers and streams, as well as 114 lakes and reservoirs, were stocked with the German strain and its crosses.

To satisfy the demand for disease-resistant fish, 13 of the state's 19 fish hatcheries are involved in producing and stocking the Hofer strains. McKissick said the DOW is trying to determine which Hofer crosses grow best at each facility.

"We're making a lot of headway," he noted.

Local fishermen can help with the Hofer research. Anglers who catch tagged rainbows in the Poudre are encouraged to call Schisler at 881-2504 and let him know where they caught the fish, so the DOW can track movement of the trout.

At Parvin Lake, the Hofer crosses are marked with coded wire tags in their noses that are difficult to see. Schisler would like anglers there to turn in the heads of any rainbows they keep to augment the sampling done by the DOW.