BOULDER, Colo. (CBS4) – It’s a fish that can brighten a backyard garden, but when koi take over a lake, it can be trouble.

On Monday Colorado Parks and Wildlife removed close to 2,000 of the invasive fish from Thunderbird Lake in East Boulder. Where the koi came from is a mystery.

“It wasn’t something we released; we had no knowledge of it, so it’s a big question mark,” Joy Master with Boulder Parks and Recreation said.

It was earlier this year when officials with Boulder Parks and Recreation began to notice changes in the pond’s ecosystem. That’s when the non-native fish were spotted.

“Unfortunately we think someone may have dumped them here,” Jennifer Churchill with Colorado Parks and Wildlife said.

What may have been just a few fish dumped in the lake a few years ago has now turned into a big problem.

“They’ve pretty much taken over the fishery,” Churchill said.

koi fish pond 6map transfer Nearly 2,000 Koi Pulled From Lake In East Boulder

(credit: CBS)

So workers with Colorado Parks and Wildlife have been fishing them out.

“What we do is we shock the fish, they rise to the surface,” Churchill said. “They’re still alive, they’re just stunned and so we’re able to scoop them out with a net.”

Churchill says without their removal it could harm the lake by killing off native fish species and spreading diseases.

“It really just throws the ecosystem out of balance,” she said.

All of the fish taken out of the lake will be given to the Birds of Prey Sanctuary, which is a raptor rehabilitation program in Broomfield.

Colorado Parks and Wildlife says it’s never a good idea to release non-native species into the wild. Contact the Humane Society if you can no longer keep your pet.


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