Colorado Parks and Wildlife staff members will try to remove a population of several hundred koi fish from Thunderbird Lake in east Boulder's Admiral Burke Park on Monday.
The koi, members of the carp family and related to goldfish, were illegally released into the lake over the past year, city officials said. There's a concern the population may be naturally reproducing.
Koi aren't native to Colorado and thrive in widely ranging water conditions, giving them the ability to displace more desirable fish species, officials said. Koi also are known to carry several diseases that can impact wild fish.
Jennifer Churchill, spokeswoman for Colorado Parks and Wildlife, said the manmade lake is stocked with native fish to allow for recreational fishing. While it's possible the koi wouldn't survive the winter, she said, the decision was made not to wait and risk damage to the existing fish population.
"They can really change the whole dynamic of the fishery," she said.
Churchill said wildlife officials aren't sure how such a big number of the large, decorative fish -- which generally are used by hobbyists to stock outdoor ponds -- ended up in the small lake.
"We have some strange things happen in our waters," she said.
Once the koi have been removed, they will be used at a local bird rehabilitation facility to feed osprey and great blue herons that are recovering from injuries, she said.
Even without a koi infiltration, keeping the lake healthy has been a challenge.
Boulder has pumped millions of gallons of potable water into the Thunderbird Lake to maintain water levels there since launching a pilot program in 2009. Now that the lake has stabilized, Boulder spokeswoman Jennifer Bray said the city's Parks and Recreation Department is proposing cutting back on the amount of water that will be pumped in.
Bray said the proposal will maintain the lake at a slightly lower level, with a goal of not adding water after July 20 each year.
The plan is expected to go the Boulder City Council for approval early next year.
"There would be ongoing monitoring of the site to maintain water quality and clarity," she said.