A cyclist rides on a bike path at Dillon Reservoir near Frisco, Colorado, on July 1, 2023. (Photo by Patrick Traylor/The Denver Post)

For decades, Dillon Reservoir has been a place where anglers can catch the fish of a lifetime – a 10-pound, 30-inch wild brown trout.

But brown trout numbers in one of Colorado’s most visible and accessible mountain reservoirs have declined in recent years, prompting state wildlife officials to consider stricter fishing regulations at the reservoir and seasonal closures of nearby waters.

It’s unclear exactly what’s causing the decline, said John Ewert, an aquatic biologist with Colorado Parks and Wildlife. But increased hunting during and after the pandemic may be a factor.

“We don’t know for sure if harvest or fishing pressure plays a big role, but we do know that fishing traffic has increased in the last few years since coronavirus,” Ewert said. “We want to rule out things that could limit the production of large brown trout, and harvest can be one of those things.”

Other possible causes include changes in water quality, development along rivers and streams where trout breed, and stress caused by higher water temperatures due to drought, Ewert said.

“We are not in a drought today but we have been vulnerable to drought over the past decade – it could be as simple as drought stress,” he said.

The number of brown trout 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 this size made up 62% of the total brown trout caught in survey nets. By 2022, they account for only 33%.

Brown trout upstream from the reservoir’s Blue River have also seen a “clear and significant decline,” according to a 2019 CPW report.

“We are in a position to be ahead of the curve and can start protecting these fish before things get really bad,” said Randy Ford, owner of Alpine Fishing Adventures in Dillon.

He is among the hunting guides who support potential new regulations under consideration by Colorado Parks and Wildlife. The Dillon Reservoir Recreation Commission also supports the proposals.

The proposed rules would require anglers to immediately release brown trout longer than 14 inches, with the rule applying to the reservoir, to portions of the Blue River south of the reservoir and to Tenmile Creek. Fishing will be completely prohibited from Sept. 1 to Dec. 1 at two places where 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 significant brown trout populations is a top priority for the agency at the reservoir, according to a March 2023 Parks and Wildlife report.

Ford has been fishing 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 said he has run his guiding service for the past 10 years and began noticing a decline in the brown trout population about seven years ago.

He has noticed increased fishing at Dillon Reservoir — especially since the start of the pandemic — and in nearby waters where trout spawn. He said it was time to adapt and face the times.

“It’s not like we’ve seen this population collapse and they’re not there,” he said of brown trout. “But it’s still the same as before.”

Other fish managed by Colorado Parks and Wildlife in the reservoir — rainbow trout, arctic char and kokanee salmon — have not seen similar declines, Ewert said. However, rainbow trout are stocked in the water every year and state regulations require that all Arctic char less than 20 inches in length be released.

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