Giant Goldfish Are Bad News for the Great Lakes

Researchers are tracking invasive goldfish—which, often, were once kept as pets—in Lake Ontario to determine how best to manage them

a person in a USFWS uniform holds a large goldfish with two hands in front of a river
A 14-inch goldfish (Carassius auratus) pulled from the Niagara River USFWS

Monstrous invasive goldfish measuring more than a foot in length are popping up in the Great Lakes—and in waterways across the United States—gobbling up native species and sometimes wreaking havoc on local ecosystems. 

Often bought as ornamental fish because of their striking colors—or won as carnival prizes—goldfish are common “first pets” in many American households. But people who grow tired of caring for their goldfish will often release them into the wild, erroneously believing their five-inch pet is an innocent addition to a lake or pond. On the contrary, these animals are voracious eaters—given the chance, they can grow 19 inches long and weigh up to nine pounds

“You see goldfish in the store, and they’re these small little fish,” Caleb Ashling, a natural resources specialist in Minnesota, told the Washington Post’s Reis Thebault in 2021. “When you pull a goldfish about the size of a football out of the lake, it makes you wonder how this can even be the same type of animal.”

In the Great Lakes, abandoned goldfish and their kin are known to root up plants, contribute to harmful algal blooms and consume native vegetation, writes Livia Albeck-Ripka for the New York Times. Now, in a study published last month in the Journal of Great Lakes Research, scientists report more about the behavior of these released fish, in hopes that their discoveries will lead to better management practices. 

“There are literally millions of goldfish in the Great Lakes,” Nicholas Mandrak, a biological scientist at the University of Toronto Scarborough, tells the Times. “If not tens of millions.”

To track goldfish movement, scientists captured 19 adult goldfish from Lake Ontario’s Hamilton Harbour between June 2017 and October 2018 and surgically implanted transmitters into their bodies. These trackers sent unique acoustic signals to receivers located around the bay, which documented when and where the fish were swimming. The fish that were tagged in 2017 were tracked for two full years, while those tagged in 2018 were tracked for less than one year. Eight of the fish died, but after analyzing the data for the 11 surviving fish, the researchers saw some patterns emerge.

“They overwintered in the same areas each year and returned to the same spawning habitat each spring,” lead author Christine Boston, an aquatic research biologist with Fisheries and Oceans Canada, tells Brian Owens of Great Lakes Now

The team also predicted when the goldfish would move into spawning areas by examining fish presence, abundance and temperature, as well as by developing models to predict goldfish presence, per the study. Their models not only accurately anticipated the movement of the tagged fish, but they also found the animals tended to move to spawning areas when temperatures rose above roughly 50 degrees Fahrenheit. Previous research had identified higher thresholds of 59 to 71 degrees.

Before moving to their spawning grounds, the fish tended to congregate in certain areas—providing a good opportunity to cull them, Boston tells the Times. The fish could either be picked up with nets beneath winter ice or shocked with electrical currents and scooped out of the water, she tells the publication.

While not all goldfish grow to huge proportions, even the small ones can cause problems and spread disease. Scientists and lake managers have issued pleas for the public to not abandon pets outside. 

“It doesn’t matter whether it’s a snake, a turtle, a bird or a fish, you just can’t put it into the wild,” Brian Heise, an aquatic ecologist at Thompson Rivers University in Canada, tells Ashley Joannou of the Canadian Press. “It’s often going to be harmful to that animal because they’re not adapted to that place, but also we’re going to harm our local animals and plants.”

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