Wildlife officials monitoring a Minnesota lake made a troubling discovery last year when they looked in an inlet.

They found thousands upon thousands of goldfish swimming in the Big Woods Lake in Chaska in the Minneapolis suburbs.

“It was the most densely populated discovery of goldfish staff had seen,” according to the Carver County Water Management Organization.

The most likely culprit behind the invasive species? Pet owners who illegally dumped goldfish, officials say.

Now the organization is removing goldfish from the lake — by the truckloads. An estimated 500,000 goldfish are in the lake, uprooting plants and fighting with native fish for food, officials say. They can live up to 25 years, reproduce rapidly and survive through the frigid Minnesota winters.

Last week, the organization netted about 50,000 goldfish from the lake and loaded them into a truck. A video shows staff blasting loud sounds from an underwater speaker to drive the fish into nets.

All in all, the organization says it’s removed 100,000 goldfish this week.In the past, the organization has stocked the lake with pike to prey on the goldfish, which they have tagged to study their movements.

The work will continue, but the organization may never be able to remove all the goldfish, Minnesota Public Radio reported.

“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,” Andrew Dickhart, aquatic invasive coordinator for the organization, told MPR.

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Source: USA Charlotte Observer