Paul Hladick catches koi fish at Thunderbird Lake in Admiral Arleigh A. Burke Park where Colorado Parks and Wildlife staff attempt to remove a population
Paul Hladick catches koi fish at Thunderbird Lake in Admiral Arleigh A. Burke Park where Colorado Parks and Wildlife staff attempt to remove a population of koi fish in Boulder on Monday Nov. 19, 2012. DAILY CAMERA/ JESSICA CUNEO. (JESSICA CUNEO)

A crew from Colorado Parks and Wildlife cruised around Thunderbird Lake on Monday scooping up hundreds of bright orange koi that were discovered in the lake earlier this year.

An electric current stunned the fish first, making them easy catching. Each pass of the net delivered anywhere from one to a dozen fish into waiting buckets.

Parks and Wildlife officials believe there are roughly 1,500 koi living in Thunderbird Lake, a man-made water body in Admiral Burke Park in Frasier Meadows in southeast Boulder.

As a non-native species, koi have the potential to upset the ecological balance of the lake, Parks and Wildlife spokeswoman Jennifer Churchill said. In particular, koi eat zooplankton, which could contribute to overgrowth of algae. That's already been a problem at the lake as water levels have fallen in recent years.

Exotic species also can out-compete native fish species, Churchill said. Parks and Wildlife stocks the lake with several fish species, including bluegill, bass and catfish.

"We're trying to balance an entire aquatic ecosystem," Churchill said, from algae and zooplankton to top predator fish and birds. "When we have a situation like this where koi are dumped into a waterway, it can unbalance the system."

Wildlife officials assume the koi were put into the lake by people disposing of pet or ornamental fish. They originally thought there were a only few hundred fish, but further study showed there are more than a thousand.

Thunderbird Lake does have an outlet to other waterways, leaving open the potential for the koi to spread if they are not removed.

Koi, a colorful fish often used by hobbyists to stock outdoor ponds, are an ongoing problem for wildlife officials. In the 1990s, Parks and Wildlife removed 17 dump trucks full of koi from Arbor Lake in Arvada, Churchill said.

Lisa Pedersen, CEO of the Humane Society of Boulder County, said exotic fish stores and Craigslist can be good resources for finding homes for unwanted fish. While the Humane Society is focused on dogs and cats and cannot actually take fish, workers there can help fish owners find people who can help.

Don and Janette Lenschow, who live in the nearby Keewaydin neighborhood, watched the wildlife crew work Monday and recalled how the water in the lake once was clear and children used to watch catfish swim in schools near the shore.

"I'm not surprised," Lenschow said of the koi. "We know that carp tend to be everywhere. It's a good thing that they're trying to restore it."

Neighbors have lobbied the city to also the water at the lake to previous levels, but city parks officials believe they can stabilize the lake without continuing to pump millions of gallons of potable water into it, as they have since 2009.

The electric current that passed through the water does not kill the fish. The wildlife crew tossed back bass and catfish caught in their nets, but they were far fewer than the koi.

Churchill said it will be hard to know if they've gotten all the koi. Another analysis of the lake will be done after Monday's harvest.

The koi will be delivered to the Birds of Prey Foundation in Broomfield as food for recovering raptors.

"They are going to benefit wildlife in the end," Churchill said.

Contact Camera Staff Writer Erica Meltzer at 303-473-1355 or meltzere@dailycamera.com.