How exactly brown trout came to be in the Dillon Reservoir is somewhat of a mystery.

The reservoir, built in 1963, is relatively new, Colorado Parks and Wildlife aquatic biologist Jon Ewert explained during a discussion on Wednesday, May 15, in Frisco about tightening local fishing regulations on the species.

Ewert said it is possible brown trout were stocked in the reservoir when it was first constructed. It’s also possible the fish had already been living in the Blue River and took up residence in the reservoir when it was dammed, he said.

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A local angler chimed in. Randy Ford, owner of Alpine Fishing Adventures, said he has heard that a family living in old Dillon stocked the fish in several ponds that were inundated when the dam was built.

But no matter how brown trout came to be in the Dillon Reservoir, Ewert said that his studies appear to show a local decline over the last decade regarding the number of brown trout that measure over 15 inches. For a reservoir once known as a destination for catching big brown trout, that suggests local regulations to protect the species may be necessary, he said.

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“I don’t have the impression that there is a huge amount of harvesting going on out there and that harvest is the culprit,” Ewert said. “But when you’re trying to restore the large fish in a fishery like this, you need to enact something to rule out the possibility that harvest is playing a role.”

That is why Colorado Fish and Wildlife is proposing regulations requiring that all brown trout above 14 inches caught on the Dillon Reservoir must be returned immediately and closing a section of the Blue River to all fishing in the fall to protect brown trout spawning grounds. The proposed closure along the Blue River would be from Sept. 1 to Dec. 1 from Coyne Valley Road to the reservoir.

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Some local anglers at the meeting proposed stricter regulations, including a potential full tributary closure during the fall to protect spawning. Ewert noted that this is only the beginning of the regulation process and there is plenty of time for Colorado Parks and Wildlife to hear feedback.

The public can submit comments about the proposed regulations anytime before they are introduced to the Colorado Parks and Wildlife Commission in August or before they are finalized in November, Ewert said.

Stocking fingerling

For most of the 80s, 90s and early 2000s, Colorado Parks and Wildlife didn’t stock rainbow trout fingerlings — a known food source for large brown trout — in the reservoir, Ewert said. Shortly after the agency made the decision in the late 1970s not to stock rainbow fingerling, another study found a decline in the populations of large brown trout, he said.

“Well what are the factors that led to the crash in the brown trout fishery?” Ewert asked rhetorically. “Well, it’s not going to take too much rocket science to figure out that we produced an artificial brown trout trophy fishery through stocking of fingerlings.”

When Ewert became the aquatic biologist for Summit and Grand counties around 2007, he said he moved away from stocking full grown rainbow trout and back to the practice of stocking fingerlings. Every year since, he said he has stocked the reservoir with about 300,000 3-inch rainbow trout.

“One thing that I knew would probably happen, and that I hoped would happen is that a side benefit of this would be that we would bring back the brown trout fishery — that large brown trout would become more prolific in the lake,” Ewert said.

For several years, that appeared to happen. Every other year, Ewert said he conducts a study to estimate species composition in the Dillon Reservoir where he deploys gillnets overnight in eight locations around the reservoir to gather a sample of fish species in the waterbody.

Outranked only by sucker fish, brown trout are the second most common fish in the reservoir, Ewert said. But brown trout aren’t stocked in the reservoir, so the entire population is sustained by natural reproduction, he said, noting the Dillon Reservoir has good spawning tributaries like the Blue River, Tenmile Creek and Meadow Creek.

In 2008, data showed a drop off of brown trout larger than 15 inches, Ewert said, noting that trout tend to start eating vertebrates when they reach 15 inches or larger. That suggested a lack of larger prey for brown trout to switch to after reaching 15 inches, he said.

“The browns got up to that size, and then they just stopped growing. There’s a lot of browns that size and very, very few beyond that size,” Ewert said. “So that big a drop-off — you don’t see that typically in most fish populations.”

By 2012, after a few years of stocking rainbow fingerlings, the drop-off in the data disappeared as more brown trout over 15 inches were observed, Ewert said. But the body condition of the large brown trout was poor, and many of the fish were obese, suggesting a few large fish were eating most of the fingerlings, he said.

Over the next few years, the body condition of the large brown trout moderated, and the fish being observed weren’t as obese, Ewert said. Then, in 2022, the body condition jumped back up again with more large brown trout being obese, he said.

Because there is a set prey base of about 300,000 rainbow trout fingerlings, the jump in body condition suggests there are less brown trout competing for prey, since the few that are eating fingerlings are becoming more obese, Ewert said. Also in 2022, the percent of brown trout above 15 inches observed in the Dillon Reservoir dropped to below 18%, the lowest in more than a decade, he said.

That simultaneous increase in overweight brown trout and overall decrease in brown trout over 15 inches being observed suggests the need for changes to rule out that fishing is having an impact on the species’ population, Ewert said.

“There are fewer large brown trout today than there were 10 years ago,” Ewert said. “So now I am no longer concerned about overshooting the prey base of predators that are present because I think we have less and less predators present.”

Tributaries

While observing fewer large brown trout in the Dillon Reservoir, Ewert said he has also observed a decline in brown trout populations in the Blue River above the reservoir, typically a strong spawning area for the fish.

In 2022, brown trout biomass in the Blue River was about 82 pounds per acre, still above the biological criteria for Gold Medal status, Ewert said, but it was a far cry from less than a decade earlier, when it had been nearly twice that.

“We’re seeing this 10-year decline of brown trout biomass,” Ewert said.

That is why Ewert suggested a seasonal closure to all fishing along the Blue River in the fall. He noted that such a closure had been in place along the Blue River and some other tributaries of the Dillon Reservoir until 2006.

Some local anglers and fishing guides, however, suggested that a stricter tributary-wide seasonal closure may be needed since other tributaries of the reservoir could become more heavily pressured if one area is closed.

“You got to do the Tenmile Creek too because its just going to focus all the pressure on Tenmile,” Joe Kerekffy, a fly fishing guide who has been fishing in the county for 22 years, said. “That’ll be devastating, instantly.” 

Ewert said that his proposal for seasonal restrictions on the Blue River is based on data and he doesn’t have the same data for other tributaries of the Dillon Reservoir. Still, the argument that the closure would shift fishing pressure to other tributaries is something worth considering, he said.