For decades, Dillon Reservoir has been a place where anglers could hook the fish of a lifetime — a 10-pound, 30-inch wild brown trout.
But the brown trout population in one of Colorado’s most visible and accessible mountain reservoirs has declined in recent years, prompting state wildlife officials to consider stricter fishing regulations on the reservoir and seasonal closures on nearby waters.
It’s unclear exactly what is causing the decline, said Jon Ewert, an aquatic biologist at Colorado Parks and Wildlife. But increased fishing during the pandemic, and after, may be a factor.
“We don’t know for certain whether harvest or fishing pressure is playing a large role, but we know the angling traffic has increased in the last few years since COVID,” Ewert said. “We want to rule out things that could be limiting the production of large brown trout, and harvest could be one of those things.”
Other potential causes include a change in water quality, development along the rivers and streams where the trout spawn, and stress from higher water temperatures caused by drought, Ewert said.
“We’re not in a drought today but we have been drought-prone over the last decade — it could be as simple as drought stress,” he said.
The number of brown trout measuring more than 14 inches long has declined for four consecutive testing years, according to population surveys conducted by Colorado Parks and Wildlife. The agency conducts surveys every two years.
In 2014, trout larger than that size made up 62% of all brown trout caught in the survey nets. By 2022, they made up only 33%.
The brown trout in the Blue River upstream from the reservoir also have experienced an “obvious and significant decline,” according to a 2019 CPW report.
“We’re in a position where we’re ahead of the curve and we can start protecting these fish before it gets really bad,” said Randy Ford, the owner of Alpine Fishing Adventures in Dillon.
He’s among fishing guides who support potential new regulations under consideration by Colorado Parks and Wildlife. The Dillon Reservoir Recreation Committee also backs the proposals.
The proposed rules would require anglers to immediately release brown trout that are longer than 14 inches, with the rule applying to the reservoir, to sections of the Blue River south of the reservoir and to Tenmile Creek. Fishing would be banned entirely from Sept. 1 to Dec. 1 in two places where the trout spawn in the fall: the Blue River between the reservoir and three miles north of Breckenridge, and West Tenmile Creek from Copper Mountain to the reservoir.
Restoring the large brown trout population is the agency’s top priority in the reservoir, according to a March 2023 Parks and Wildlife report.
Ford has been fishing in Dillon Reservoir for nearly 40 years. He said he caught his first brown trout from the reservoir when he was about 11 years old. He has run his guiding service for the last 10 years and started to notice a decline in the number of brown trout about seven years ago, he said.
He’s noticed the increased fishing on Dillon Reservoir — especially since the beginning of the pandemic — and on the nearby waters where the trout spawn. He said it “was time to adapt and meet the times.”
“It’s not like we’ve seen this population crash and they’re not there,” he said of the brown trout. “But it’s still nothing like it used to be.”
The other fish managed by Colorado Parks and Wildlife in the reservoir — rainbow trout, arctic char and kokanee salmon — have not experienced similar declines, Ewert said. However, rainbow trout are stocked in the water every year and state regulations mandate the release of all arctic char under 20 inches long.
Brown trout are not native to Colorado but have spread throughout the state’s waters since their introduction from Europe in 1890. The population in Dillon Reservoir is self-sustaining and wild, unlike the stocked rainbow trout.
“They’re very cool, very wild,” Ford said of the brown trout.
The Colorado Parks and Wildlife Commission first heard the new regulations at its Aug. 24 meeting. The commission is expected to make its decision this fall and, if approved, the new rules would go into effect when the state issues new fishing licenses in April.
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