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A small fish that was once someone’s pet has turned into a big problem in a Twin Cities lake.

Carver County Water Management Organization

 removed tens of thousands of goldfish last week from an inlet connected to Big Woods Lake, part of the Grace Chain of Lakes in Chaska.

The brightly colored species commonly found in pet stores and aquariums was first discovered in Big Woods Lake in April 2019. It’s believed that a few of the non-native fish were intentionally dumped into the lake, where they have quickly reproduced.

"The most likely scenario is that somebody or a couple people released goldfish, and they're exceedingly hardy fish,” said Madeline Seveland, a spokesperson with the Carver County Water Management Organization.

Big Woods Lake has relatively poor water quality and few other predatory fish species, conditions that have allowed the goldfish to multiply. 

Carver County Water Management Organization staff netted and removed some goldfish from a small pool between Big Woods and Hazeltine lakes last spring. Then, on Oct. 26, they netted a massive haul — an estimated 50,000 goldfish from the inlet channel. 

The removal is part of a three-year plan to study and manage the invasive species, said Andrew Dickhart, aquatic invasive coordinator for the water management organization. Staff have captured and tagged about 500 goldfish to track their movements, “so we can monitor their behavior and where they're aggregating, where they're spawning,” he said.

They plan to use electrofishing, or temporarily stunning fish with an electric current, to get a better estimate of their population size, Dickhart said. And they plan to age-test some of the fish, to learn how quickly they are reproducing.

“If we're going to remove the fish, we need to keep track of a percentage of the population that we're removing, until we can get it down to a certain level where that population may no longer be harmful to the lake ecosystem,” he said. 

However, there’s been relatively little research done on goldfish, so it’s not clear what that threshold is.

Goldfish haven’t gotten as much attention as invasive carp, but they are causing significant problems in parts of Europe, Canada and Australia as well as the United States, said Peter Sorensen, a fisheries biologist professor at the University of Minnesota. 

“It just hasn't reached a high level of awareness,” he said. “They don't jump and knock people out of boats and break bones. But it's a global issue.”

They are extremely resilient fish that can survive harsh conditions, including lakes with poor water quality, Sorensen said. 

“They're perhaps the only vertebrate animals that can live without any oxygen, which is remarkable,” he said. “They can go what we call totally anaerobic, and they just live off sugars and produce alcohol.”

With that ability, goldfish are able to survive in shallow lakes that completely freeze over in the winter, Sorensen said.

“If there are not many other fish in the lake — because most fish can't stand the conditions that goldfish do really well in — then they just take off,” he said.

Minnesota DNR blames illegal goldfish release for virus that killed carp

Goldfish are a type of crucian carp and share many of the same habits — stirring up sediments at the bottom of a lake, uprooting plants while feeding and competing with native fish for food. There hasn’t been much research on the impacts of goldfish on a lake’s ecosystem, Dickhart said.

“The question that we're trying to answer is, how much is this rough fish problem contributing to the poor water quality of the lake?” he said. “If we remove a certain amount of these fish and all of the sudden we start to see the lake rebound, then we have evidence that this really was the driving factor.”

Once they’ve figured out where the goldfish are moving and spawning, staff also will be looking at ways to control them, Dickhart said. Possible methods include stocking native predator fish or installing barriers in areas where goldfish are known to reproduce.

Staff also hope to educate people to never release any kind of domesticated animal into the wild. Owners who can’t keep a pet any longer should return it to the pet store or take it to a surrender event, often hosted by the 

“We don't want to put all these resources in and remove these fish and get the lake back to a stable system, and then just have this happen all over again,” Dickhart said.

Minnesota News

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Water

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Kirsti Marohn

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A few of the aquarium pets were likely dumped into the lake in Chaska. The combination of relatively poor water quality and few other predatory fish species allowed the goldfish to reproduce in massive numbers.

Two men standing in a creek use a large net to catch goldfish.

Courtesy of Carver County Water 

A hand holding a goldfish in front of a truck bed full of goldfish.

Thousands of goldfish removed from Twin Cities lake

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