Introduction of sterile predatory fish meant to reduce population of non-native small-mouth bass, boost angling opportunities

If everything goes according to plan, Ridgway Reservoir will have some new residents moving in this spring – walleye.

The goal is for these predatory fish to help reduce the population of another non-native species previously introduced to the reservoir, while providing anglers more options for fishing.

The approval for introducing walleye comes after years of preparation and coordination – and significant investments at the reservoir’s spillway to ensure the fish can’t escape the reservoir and travel downstream.

You can’t just dump a new species into a reservoir – that’s how the problem with the illegal introduction of smallmouth bass started years ago. No one seems to know exactly when they were introduced, but wildlife officials have been trying to control the population ever since.

In purposefully (and legally) introducing a new species to the reservoir, there’s a prescribed set of studies, assessments and approvals from several different government agencies that must be met.

Colorado Parks and Wildlife Aquatic Biologist Eric Gardunio said he’s been working on the plan since at least 2017.

His first drafts of the lake management plan started about seven years ago, and those needed approval from the Upper Colorado River Endangered Fish Recovery Program and the U.S. Fish & Wildlife Service, among others, before they could move forward.

Once they cleared that hurdle, they waited for the installation of a new fish screen at the reservoir. It was constructed at the outlet, where water gets released downstream to the Uncompahgre River. That was completed in 2022.

Now, the goal is to stock the reservoir with 30,000 of these tiny fish, each measuring about 1 inch long. The hope is they’ll grow large enough to become an important – but controllable – part of the food chain in the reservoir and their voracious appetites will curb the population of invasive smallmouth bass.

This spring, they’ll carefully place the tiny walleye in the shallower, warmer parts of the reservoir that offer more protection for the babies. The fingerlings will start out their lives eating plankton, then moving on to aquatic invertebrates and bugs, then eventually they’ll eat crayfish and other fish.

Many steps to approval

Before anyone could move forward with the idea of introducing the walleye to the reservoir, they had to make sure they couldn’t escape downstream or reproduce.

First – the physical barrier to keep the fish in the reservoir was a problem. For years, those who managed Ridgway Reservoir’s water levels and releases had to be careful of how much the reservoir filled during the runoff season each spring. There was a “no spill” rule – they couldn’t let the water flow over and out of the spillway, because invasive smallmouth bass in the reservoir could escape downstream into habitat for native, endangered fish.

These fish – which live downstream within specific stretches of the Gunnison and Colorado rivers – include the razorback sucker, the Colorado pikeminnow, the humpback chub and the bonytail. These populations are susceptible to predation from other species.

The new fish screen at the reservoir ensures the operators of Ridgway Reservoir’s dam, Tri-County Water Conservancy District, can adjust water levels and let the reservoir fill to capacity without the fear of letting more than water go downstream.

The installation of a fish screen at Ridgway Reservoir, completed in 2022, was part of a yearslong plan to introduce walleye to the reservoir to help control the population of another non-native species while also giving anglers more options for fishing. The goal is to stock the reservoir with 30,000 walleye, starting this spring.
Erin McIntyre — Ouray County Plaindealer

Controlling the population

The walleye CPW plans to introduce to the reservoir will come from the Pueblo State Fish Hatchery, likely delivered by a truck.

The hatchery obtains the eggs and sperm, also called milt, from wild spawning fish at Pueblo Reservoir in the early spring. Wildlife officials then bring them to the controlled environment of the hatchery.

Then they take those just-fertilized eggs and put them in a hydraulic pressure chamber to sterilize them, which is carefully timed and monitored. The goal of this is to create what biologists call a “triploid” walleye, which is unable to reproduce.

These eggs hatch, and the tiny fry mature at the hatchery. It takes just a few weeks for them to get large enough to be tested to see if they’ve been properly sterilized – and a lab does that by testing their DNA.

“If the fish that are produced this year don’t meet the rates, they won’t stock them,” said Gardunio. “Our bottom line here is, we don’t want to create a wild, self-sustaining walleye population in Ridgway Reservoir.”

CPW already produces millions of sterile walleye every year, and will just add Ridgway to the list of water bodies it stocks. They will continue to stock the reservoir with rainbow trout and kokanee salmon.

Monitoring the population

In some ways, the work is just beginning for Gardunio.

“We’re trying to do this in as responsible a way as possible,” he said. “We’ll be monitoring it closely and making any adjustments we think need to be made.”

It will be a while before anglers catch any huge walleye, he said.

“Ten years from now, I expect there to be some really nicesized walleye in the fishery,” he said.

Gardunio is hoping anglers will be able to identify the small walleyes and let them grow bigger, so they can reach their potential and eat more smallmouth bass. The tiny walleye can look similar to yellow perch. “Make sure you know what the species looks like and don’t keep a small walleye by accident,” he said.

Initially, anglers will be able to keep up to five walleye per day, but there’s no size requirement.

For now, the agency plans to continue its annual fishing contest encouraging removal of smallmouth bass from the reservoir – and Gardunio is looking forward to seeing how those populations are affected as the walleye grow.

Walleye fishing is big in the Midwest and the Great Lakes region, said RIGS Fly Shop & Guide Service co-owner Tim Patterson. He thinks the species will appeal to local anglers.

“It’s an incredibly tasty fish,” he said. I think that will increase interest in visitors, targeting that species.”

Patterson said he’s glad to hear there will be another species for anglers to fish, while providing a possible solution to another problem.

“If they’re eating the invasive smallmouth (bass), that sounds like a real win-win.”