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This undated photo provided by the U.S. Department of Agriculture shows a group of zebra mussels. The invasive species of small mollusks is growing in Colorado. Federal officials have offered a $100,000 prize in a crowdsourcing effort to find a way to kill invasive quagga and zebra mussels. (U.S. Department of Agriculture via AP, File)

A year after Colorado Parks and Wildlife staff first discovered invasive zebra mussels at Highline Lake — a reservoir north of Loma, near the Utah border — they found even more while doing routine work to remove buoys from the water this month in preparation for winter.

The first adult zebra mussel was found in the reservoir on Sept. 14, 2022, while staff were routinely testing for aquatic nuisance species in the lake, Parks and Wildlife officials found almost a dozen more mussels there the next month.

Five more adult zebra mussels were found Oct. 1 on the bottom of buoys.

Wildlife officials said the discovery was troubling, though not surprising. The agency has been working for 15 years to keep the state free of the nuisance species that can cause millions of dollars of damage to water distribution systems.

“Eradicating invasive mussels is extremely difficult, which is why we focus so heavily on preventing their introduction,” said Robert Walters, Colorado Parks and Wildlife’s invasive species program manager. 

“While this is disappointing, we are not considering this first attempt a failure,” he said. “We believe if we had done nothing we would be looking at a much larger infestation today with fewer options for eradication and a greater chance of zebra mussels spreading to other bodies of water in Colorado.”

Colorado Parks and Wildlife staff are currently evaluating options to eradicate the mussels at Highline Lake and stop the spread into other bodies of water in Colorado. All of the options will have significant impacts on fishing, boating and other water-based recreational activities at the lake. 


Colorado Parks and Wildlife officials could potentially lower or drain Highline Lake in 2024. 

“Highline is filled by a canal system that is connected to the Colorado River, so there would be a lot of moving parts to that,” said Rachael Gonzales, northwest region public information officer for Colorado Parks and Wildlife.

Additional work to contain and reduce the mussels’ spread is scheduled for Oct. 12. It will include removing boat docks and two Colorado Parks and Wildlife boats for inspection and decontamination, CPW staff said in a news release Friday.

The plan to eradicate and contain the mussels will harm the lake’s fishery by killing some or all of the fish there, Gonzales said, so starting Monday, an emergency fish salvage is in place at Highline Lake. This means that until further notice anglers can catch and keep as many fish as they want, as long as they’re fishing legally. 

“It’s only shoreline fishing, so they can’t bring boats into the lake, they can’t use nets and they need to have fishing licenses,” she said. “But if they catch 25 fish, they can take them. And they have to make sure that they are dead before they leave Highline, which is part of the regulations. We’re trying to give anglers an opportunity to put the fish to good use.”

This fish salvage is only in effect at Highline Lake. All catch limits remain in effect at nearby Mack Mesa Lake, Gonzales said.

Anglers and waterfowl hunters are reminded to clean, drain and dry their gear before traveling to and entering any new body of water to stop the spread of the mussels and other nuisance species, CPW staff said.

Tiny organisms with big, expensive impacts

On Sept. 14, 2022, a single adult zebra mussel was discovered during routine invasive species sampling. It was clinging to a square piece of plastic attached to a rope inserted into the bottom of the lake. Additional zebra mussels were discovered on docks and boats. 

The discovery of additional mussels prompted Colorado Parks and Wildlife to change the status of Highline Lake to “infested” from “suspect.” This was the first time a Colorado body of water was categorized as infested with zebra mussels, CPW staff said.

In November, Highline Lake began a two-phase approach to eradicate the zebra mussels by lowering the lake by 25 to 30 feet to expose areas along and near the shoreline and lake bed to kill zebra mussels by drying and freezing them.

In March, Colorado Parks and Wildlife applied three treatments of EarthTech QZ, a copper-based molluscicide, to the lake, they said. The compounds that are toxic to zebra mussels, such as Earthtec QZ, are toxic to other aquatic organisms, which makes eliminating the zebra mussels while preserving the other species in the water challenging, Roberts said.

The new infestation could lead to millions of dollars in damage to water-based infrastructure, and can threaten water quality and limit recreational opportunity, Roberts said last year after the first few mussels were found.

A single mussel can produce up to 1 million babies a year, making it challenging to contain the species, and nearly impossible to eradicate them once they’re introduced, he said.

The aquatic nuisances attach to solid or semisolid surfaces and clog boat engines or distribution pipes at water treatment facilities. If knocked off, the mussels can drop into and infest a body of water, he added.

The mussels can live outside of water for 27 days under the right conditions, and can pass almost a liter of water through their shells each day, consuming nutrients needed by other fish and plants in the ecosystem, Roberts said.

Their high levels of filter feeding can also alter food sources in lakes and reservoirs. 

“The challenge with eradicating zebra mussels is complex,” Roberts said. “You are targeting small organisms in large bodies of water. The adults prefer areas that are difficult to reach. They are extremely prolific reproducers and even missing a small number of them in a treatment can result in a re-infestation.”

Tatiana Flowers is the equity and general assignment reporter for the Colorado Sun and her work is funded by a grant from the Colorado Trust. She has covered crime and courts plus education and health in Colorado, Connecticut, Israel and Morocco....