Viral images of giant goldfish in U.S. lake spotlight dangers of invasive aquatic pets | CBC News

“Please don’t release your pet goldfish into ponds and lakes!” the city said on Twitter. “They grow bigger than you think and contribute to poor water quality by mucking up the bottom sediments and uprooting plants.” The post was shared thousands of times.

The situation is part of a larger trend underway north of the border, two Canadian ecologists said. 

Goldfish numbers in the Great Lakes especially have “really spiked since 2015,” said Jennifer Bowman, an aquatic ecologist for the Royal Botanical Gardens based in Burlington, Ont. “It’s getting a lot worse. They are very successful when they get together; a couple of goldfish can get access to a pond and you have thousands the next year.”

In one single marsh near Hamilton, her group has counted 2,000 goldfish. They’re “a little more tolerant of some of the chemicals in the water that would impact our native fish,” she added, allowing the invasive species to proliferate.  

Dumping an unwanted goldfish into a nearby lake or stream might seem harmless enough, but those invasive pets are disrupting local ecosystems, turning once-clear lakes brown and murky, says Nick Lapointe, a conservation biologist with the Canadian Wildlife Federation, an Ottawa-based advocacy group. 

Lake ecology

As bottom feeders, they stir up mud and sediment at the bottom of freshwater lakes. This clouds the water and makes it more difficult for plants and native fish species to thrive there, he said.

“In the worst-case scenario, you go from a clear lake dominated by plants, to a muddy lake dominated by algae.”

“Goldfish can be over a foot long. They eat fish eggs and snails and compete with our native species,” Lapointe told CBC News. “We have about 180 aquatic invasive species in the Great Lakes [alone] and every one of those adds to the problem and changes the ecosystem.” 

WATCH | Giant, disruptive goldfish taking over Canada’s waterways:

If goldfish are discarded in the wrong way, they can grow into destructive monsters. Researchers in Ontario have launched a unique catch and release operation in hopes of getting the upper hand. 4:25

Canada-wide data on the number of goldfish dumped into lakes and rivers annually doesn’t exist, Lapointe said. However, the numbers are thought to be significant. 

In Montreal alone, some 10,000 aquarium fish are released annually, according to a 2008 study published in the Canadian Journal of Fisheries and Aquatic Sciences.

Lapointe thinks Canadians might be more aware of the dangers of dumping aquarium pets into freshwater ecosystems today than they were 30 years ago.

But with population growth and more goldfish being sold, he said the numbers in lakes and rivers are almost certainly growing, especially considering the creatures are now breeding on their own in the wild, often at the expensive of other species. 

Most goldfish aren’t actually gold in colour, he added — that’s a rare genetic trait comparable to albinism. When goldfish breed in lakes or rivers, their offspring are far more likely to be brown, he said. 

“The declines in freshwater biodiversity are a crisis,” Lapointe said. “The major causes of that are: climate change, habitat loss and invasive species … We are starting more and more to see a standard list of five to six species found everywhere.”