In what has become an all-too-common occurrence at Colorado’s most productive fishery, the fate of trout in Antero Reservoir is hanging by a shoestring.
With an estimated six weeks of ice cover remaining, oxygen levels at the popular impoundment south of Fairplay have dropped well beyond the danger level.
Despite the best efforts of Colorado Division of Wildlife crews, conditions continue to worsen.
“It’s been a harsh winter,” area biologist Jeff Spohn said of a condition caused by thick ice and a heavily crusted snow cover.
These conspired to squelch light penetration necessary to keep abundant aquatic vegetation alive. Decaying vegetation robs the water of oxygen while releasing toxic hydrogen sulphide.
“Even at low concentrations, hydrogen sulphide is lethal to fish,” Spohn said. “It’s a double-edged sword.”
Spohn measured oxygen levels at various points in the lake Wednesday and was dismayed at what he found: readings of only 1.5 parts per liter or less. The highest reading was just 1.5 parts per million, well past the danger point.
“We start seeing issues with fish of all sizes once it gets to 3 parts per liter,” he said.
Large fish — the lunkers that so delighted anglers when Antero opened in July following a five-year, drought-induced drawdown — are most susceptible to oxygen deprivation.
The biologist wouldn’t speculate on how many trout already might have died or on the prospects for survival through the remainder of winter.
“I’m not saying we’re going to lose all our fish, but I am concerned about it. We won’t know until the ice goes off and we set our gill nets,” Spohn said.
What has been a brutal South Park winter delivered a final blow even when Spohn and his crew two weeks ago launched an emergency measure to reverse the oxygen nosedive. Using a plow attachment, they scraped snow from a part of the 2,200-acre reservoir in an effort to let more light in.
Spohn noted an immediate and encouraging gain of up to 0.5 ppl, lending hope that trout might gravitate to these areas of refuge.
Then winter dealt yet another blow. Three significant snowstorms, coupled with hurricane-force wind that reached 109 mph at Kenosha Pass, again coated the surface with sticky snow. Accelerated vegetation decay did the rest.
Winter mortality scarcely is a new phenomenon at Antero. Never exceeding 15 feet in depth, the reservoir always is susceptible to the current combination of conditions.
“The same thing that makes those fish grow so big can cause this in winter time,” Spohn said. “It grows fish fast, but every year you’re toying with this.”
A dense layer of vegetation covering the entire lake serves as a rich nursery for a variety of trout food: scuds, snails, crayfish and four major species of aquatic insects.
During spring and summer, trout grow an astonishing 1 1/2 inches a month. Trout stocked at 12 inches the previous year measured up to 24 inches when the reservoir reopened July 17.
The real surprise came when anglers caught several monster trout — including a cutthroat-rainbow hybrid weighing 18.5 pounds, a state record. Spohn opined that these outsized fish had survived in spring seeps or at the South Fork of the South Platte River inlet during the refilling of the reservoir. He hopes many among the current crop will discover similar areas of sanctuary.
Meanwhile, fishing activity has come to a virtual halt. On Wednesday, four Denver-area anglers were the lone specks on a platform of solid white. Jim Bechtle, a Division of Wildlife creel clerk, reported that not a single trout had been caught in three days. The anglers didn’t see any fish, in contrast to frequent reports of sightings and a few sporadic catches a couple of weeks earlier.
“They’ve gone into survival mode,” Spohn explained. “The fish are bunched up somewhere, trying to find a refuge somewhere.”
Spohn and other DOW fish managers have explored ways to alleviate inevitable future occurrences, but with little success.
“The only things that might work involve closing the lake to ice fishing, and they’re also very expensive,” said Spohn, who took an optimist’s view of the near future.
“This is disappointing, but I’d like to pass along to the people that we’ll try our best with our resources to rebuild it again.”
Charlie Meyers: 303-954-1609 or cmeyers@denverpost.com
