CPW working to ‘eradicate illegally introduced invasive fish’ in Gypsum Ponds

DENVER (KDVR) — Wildlife biologists say Colorado’s Gypsum Ponds have the potential to be a great fishery for anglers, but illegally introduced invasive fish described as “voracious predators that eat other fish” are dominating the fish population.

Colorado Parks and Wildlife said in a press release Wednesday that the ponds were originally managed for catchable trout, but over the years multiple species have “invaded the ponds, likely through illegal introduction.”

“Gypsum Ponds has the potential to be a great fishery with both warm water and catchable trout fishing opportunities,” CPW Aquatic Biologist Kendall Bakich said. “Recently, we discovered black bullhead catfish in the ponds. Bullhead catfish, smallmouth bass, and green sunfish currently dominate the pond fishery, and are voracious predators that eat other fish and limit the production of more desirable sportfish.”

Invasive fish ‘dominating’ Gypsum Ponds

Restricted invasive fish species were first documented in Gypsum Ponds in 2010, according to CPW. Since then, several efforts have been made to mitigate the potential for their escape and deplete the targeted species, but those efforts have not been successful.

Wildlife officers have begun draining ponds at Gypsum Ponds State Wildlife Area in their latest effort to “eradicate” the fish, and “eliminate their threat to the adjacent Eagle River.”

In June, CPW began lowering the ponds for outlet renovations and issued an emergency fish salvage on June 18 urging anglers to harvest fish from the ponds.

Lowering the pond for the project created an opportunity to attempt an eradication, CPW said.

“Attempting full eradication now while the ponds must be lowered for the outlet renovations is not only our best opportunity for successful eradication, it is also our best chance to prevent the spread of highly invasive fish species to the adjacent river,” Bakich said.

Crews are still working to drain the ponds, which CPW said are expected to be refilled in April 2025. Once the ponds are lowered, CPW said it will determine which method, or combinations of methods, it will use to get rid of the fish.

CPW may use fish toxicant to eradicate

CPW said in the release that it may consider using a fish toxicant called rotenone, an “organic compound derived from the root of a tropical legume,” which has been used by CPW for over 80 years. If that route is deemed necessary, CPW said it will occur in late September or early October.

“The purpose of using rotenone is to achieve ecological balance, control and remove non-native fish, and reintroduce native fish to lakes and streams. Rotenone is chosen for its immediate effectiveness and quick degradation in the environment. When applied, it is toxic only to gilled organisms such as fish and macroinvertebrates and will not threaten other species that visit the ponds or linger in the carcasses of fish or the treated habitat they may access,” CPW said.

If application is necessary, placards will be posted around the ponds identifying application and restrictions during treatment.

CPW said construction work to improve water control structures in the ponds is expected to begin in late September when the water is lowered and is set to last about two months. There will be road closures in the area during that time, but anglers will be able to access the river on foot.

Future of fishing at the Gypsum Ponds

The rotenone is expected to break down naturally over the fall and winter before refilling begins next April. CPW said the ponds will likely return to normal levels within a few weeks, and rotenone levels will be monitored throughout the process.

In the spring, macroinvertebrates like insects and gastropods will repopulate the ponds when they are refilled from the Eagle River diversion, CPW said in the release.

In the long term, CPW said it plans to develop a “diverse warm-water fishery in both ponds with distinctly different fish assemblages.” They are considering some fish that are not a threat to the Eagle River including largemouth bass, bluegill, yellow perch and black crappie.

CPW said it will engage with the local community and anglers about the future pond management strategy this winter before finalizing plans.

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