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.

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