The discovery of foot­ball-sized goldfish in a Burnsville lake has officials pleading with pet own­ers to stop dump­ing their un­want­ed fish into local water­ways.

Burnsville officials found 10 fish, some a foot long, in Keller Lake earlier this month while surveying the fish population as part of a water quality project. On Monday, a second trip yielded 18 more fish, some 18 inches long and estimated to weigh about 4 pounds.

"Most of them were definitely bigger than you'd find in your typical aquarium," said Daryl Jacobson, the city's natural resources manager.

The proliferation of the bright orange fish, which don't naturally live in Minnesota waters, is a problem that's plagued communities around the metro as pet owners seeking a humane next chapter for their pets — which hail from east Asia and are a smaller cousin of the common carp — end up adding an invasive creature to their local waterways. It is illegal in Minnesota to release goldfish into waterways.

In Burnsville, a recent tweet from the city included photos of one especially large goldfish and several in a holding tank.

"Please don't release your pet goldfish into ponds and lakes!" the tweet said. "They grow bigger than you think and contribute to poor water quality by mucking up the bottom sediments and uprooting plants."

Goldfish, which reproduce rapidly and have few natural predators, im­pair wa­ter qual­i­ty by feed­ing along lake floors, dis­rupt­ing plants and stir­ring up sed­i­ment, which in turn re­leas­es phosphorus into the wa­ter, en­cour­ag­ing al­gae growth. The fish also com­pete with na­tive spe­cies for food.

"I would not be surprised if they're in a lot of lakes [around the metro], especially in low numbers," Jacobson said. "Goldfish are a pretty hardy species."

Burnsville officials worked with Carp Solutions, a six-year-old startup that develops new technologies for controlling carp, on their goldfish surveying effort. The company uses boat electrofishing to capture the fish, said founder Przemek Bajer, who is also a University of Minnesota research assistant professor at the Minnesota Aquatic Invasive Species Research Center. Wires reach out from a boat's bow, electrifying the water, and the stunned fish float to the surface to be netted and measured.

In Burnsville, the fish were ultimately killed.

Goldfish and carp can survive in frozen lakes and those with very poor water quality because they can live without oxygen for long periods. They also show up in healthier lakes.

"I think that they are getting more and more common," Bajer said.

Ecologists and lake managers are wondering how much they should worry about the proliferation of goldfish, Bajer said, and if there's something they still don't know about the fish that's allowing them to thrive in Minnesota waters.

In Carver County, a resident notified county staff about clouds of orange fish about two years ago. Since then, the staff has been trying to determine their habits and patterns, said Paul Moline, manager of the Carver County planning and water management department. The fish were first found on Big Woods Lake and then Lake Hazeltine, both in Chaska.

"This was the first time we'd seen a large population and fish the size they were," Moline said.

The department received an $88,000 state grant to study the goldfish. Officials are still trying to figure out why they're doing so well and how to control them, Moline said; last fall they caught and removed some with nets, using clove oil to put them to sleep before killing them.

In Burnsville, officials aren't sure of their next steps after finishing their Keller Lake fish survey. They may leave the fish or try to remove them, Jacobson said. Improving water quality is the goal.

In the meantime, goldfish own­ers who no longer want their pets are ad­vised to find them a new home with another person, or at a pet store — just don't release them in a local lake.

"That's really the last thing you should be doing," Moline said.

Erin Adler • 612-673-1781