OUT: Flooding from snowpack catches juvenile trout emerging from nest
The immediate effect of watching flood waters last spring roar from the bypass tubes at Blue Mesa Reservoir was amazement at the power of so much water.
That water’s ultimate power, however, might be felt in years to come if anglers notice what biologists already have seen: there are fewer fish in stretches of the Gunnison River.
Perhaps not enough to register on the sensitivities of anglers but sufficient to hint at what might happen should the Gunnison Basin see a series of high-water years.
“Last year’s hydrograph (with high water and prolonged cold temperatures) wasn’t ideal for the trout population,” said Dan Kowalski, aquatics biologist with the Colorado Division of Wildlife in Montrose. “Big water never is but the trout handled it pretty well.”
Last year’s electro-shocking surveys indicated the trout populations were down slightly from 2007, mostly due to what Kowalski called a “double whammy” against the brown trout population.
Low flows in February and March 2008, created while the Bureau of Reclamation was conducting dive inspections of the dams on the Gunnison, left brown trout redds exposed at the time the young browns were too small to leave their nests.
Then, a snowpack that climbed to record depths across the Western Slope came roaring downhill, the resulting high flows in late May and early June catching juvenile brown trout freshly emerging from the nest.
“It was tough on last year’s brown trout age class,” Kowalski conceded. “In some places brown trout populations were down 50 percent compared to 2007.”
Even so, the veteran biologist eyed the survey results with mixed emotions.
“But that’s not nearly as bad as I thought it might be,” he said. “Between the low flows in
February and March and the high flows in May, I was anticipating a higher impact, maybe up to a 90 percent impact. So it wasn’t as bad as it might have been.”
Having fewer brown trout in the Gunnison River isn’t necessarily a bad thing.
Brown trout numbers grew expansively during the recent drought, Kowalski said, putting more pressure on rainbow trout populations already stressed by whirling disease, conmpetition and low, warm flows.
The drought-reduced flows, in some cases as low as 300 cubic feet per second, contributed to “an over-population of brown trout year classes,” Kowalski said.
In Ute Park, brown trout outnumber rainbows 16-1, with approximately 1,104 browns per mile compared to 66 rainbows in the same distance.
The biomass – roughly the cumulative weight of the fish – is down a bit because many small fish didn’t survive last year’s down-and-up water flows.
“In 2008 the biomass of brown trout was down about 14 percent, (but) anglers won’t notice it because the number of 14- to 18-inch hasn’t decreased,” he said.
In Ute Park, there are an estimated 1,038 brown trout 14 inches and longer per mile, the survey shows.
Knocking back some of those brown trout plays into the Division’s plans to restore rainbow trout to the Gunnison using a unique hybrid, carrying the genes of a whirling disease-resistant rainbow trout.
“The 2008 scenario (fits) into our management strategy for the river,” Kowalski said. “But if we saw the same flow pattern for year upon year we would expect it to negatively impact the brown trout, and biologically it’s not good for the river.”
Surveys indicate the number of rainbow trout fry in stretches of Ute Park are about 10 percent the level seen in the years prior to whirling disease.
Recent efforts to restore rainbow trout numbers by annually stocking fish hasn’t succeeded in Ute Park although some benefits are being seen downstream below the Smith Fork and below the North Fork, Kowalski said.
One bright spot is the apparent success of the hybrid progeny of Hofer and Gunnison River rainbows, the young of which are growing well and reproducing, Kowalski said.
Best of all, their young also are surviving, he said.
“Rainbow trout fry numbers haven’t really increased, they’re still perhaps 10 percent or so of what we found in the mid-80 and early ‘90s,” he said. “But the fry we are finding have fewer clinical symptoms of whirling disease and some had genetic markers of the Hofer WD-resistant strain.”
And what pleases Kowalski even more, and what holds great promise for the future, is last year’s October survey revealed some wild rainbows surviving the initial effects of whirling disease.
“Fish that were spawned in 2007 had recruited to age 1 in 2008,” Kowalski said. “In previous years, the young fish would emerge but would be gone by the fall.”
Any impacts to angling might be too small to be noticed.
“Last year’s brown trout 1-year age class was plenty big enough to recruit to a large class of 2-year fish and so I don’t think anglers will notice any shortage of fish,” he said.