Ontario’s wild goldfish population is exploding and climate change may be to blame

Over the years, Andrew Murray would see small schools of goldfish living in storm ponds near his home in the suburbs of London, Ontario. He hadn’t given much thought to it until this spring, on a sunny April day when the usually greenish pond took on an unusual, if not a little orange color.

An avid nature photographer, he captured several close-ups. It wasn’t until he got home and looked at the pictures that he realized how many goldfish were swimming there.

“When I looked at them on my computer later, it was clear there were only thousands and thousands of fish in there.”

Researchers at the University of Toronto believe this scenario repeated hundreds of times in suburban storm pools across the province. Originally built to reduce environmental flooding and reduce stress on Ontario’s municipal sewer system, this pool has become ground zero for what Nicholas Mandrak calls a “super-invader.”

Ground zero ponds for goldfish invasion

“What we’re seeing is breeding goldfish in suburbs in southern Ontario, even across Canada,” said a professor at the University of Toronto Scarborough who studies freshwater fish conservation.

VIEW | Prof. Nicholas Mandrak discusses possible climate change links to Ontario’s wild goldfish boom:

Ontario’s wild goldfish population is exploding and climate change may be to blame

Professor Nicholas Mandrak of the University of Toronto Scarborough discusses whether the recent boom in Ontario’s wild goldfish population is linked to climate change. 1:18

With reported infestations in Alberta, BC, Saskatchewan, Manitoba and in the United States, goldfish are rapidly becoming one of the most prolific invasive species in North America. The reason why they are so widespread is based on how people think of goldfish: harmless and disposable.

“People releasing goldfish into urban ponds might think they’re doing a good thing, ‘I don’t kill creatures,'” Mandrak said. “There shouldn’t be any fish in this urban pond.”

After being put into a storm pond, the goldfish begins to eat the insects that normally colonize it. They lay eggs quickly, and their numbers combined with their broad appetite means they compete with amphibians and other creatures, decreasing the diversity of native species.

They also flee when ponds flood, releasing fish into wetlands, creeks, rivers and lakes where Mandrake says carp become predators, eating the eggs and fry of other fish.

Rare native species threatened by goldfish

Goldfish are also rooted, almost like the aquatic version of the pig, destroying habitat by uprooting plants and clouding the water for rare and endangered native species such as spiny softshell turtles, green arrowheads and rainbow clams.

Goldfish have been in Ontario’s wild waters since the 19th century, according to Mandrake. What’s changed, however, are their numbers, and he says it may have something to do with the recent addition of suburban rainwater ponds and climate change, offering more favorable conditions for fish to thrive.

“Recently it’s just getting more abundant,” said Mandrak. “This is largely thanks to this urban pool.

“We also think that climate change is also involved. The waters of the Great Lakes and urban ponds are heating up more than ever before. The water seems more suitable for goldfish.

“When conditions get more extreme, which is warm, low-oxygen water, one of the few fish that actually lives well in such habitats is the goldfish.”

Climate change link

That’s why Mandrake believes that goldfish deserve our attention. Not only are we releasing them into the wild, but human-caused climate change is creating the conditions for them to thrive, through warmer water and spreading to nearby wetlands, through flood events caused by higher rainfall.

Climate change is causing rainwater ponds on the outskirts of cities to contain warm water that has lower oxygen, the kind of conditions that allow goldfish to breed, while other species die. (Cole Burston/University of Toronto Scarborough)

That’s why goldfish are found in greater numbers in the protected wetlands of Ontario from Westminster Ponds in London to Cootes Paradise in Burlington.

“There’s been a rapid increase in goldfish numbers and I really think that’s climate change,” Mandrak said.

“We saw a small annual increase in mean annual temperature leading to a much larger and disproportionate increase in goldfish numbers as climate change led to this more extreme environment.”

Part of Mandrak’s research also involved efforts to map the life cycle of goldfish infestations in storm ponds, that is, how many generations it took before fish took over a body of water.

Scientists were able to determine how long goldfish infestations had been in waters with the size of the largest fish, which, not limited by the size of their bowls, have been known to grow to the size of a two-liter pop bottle. and weigh up to two kilograms.

“Last year we sampled a pond measuring 100 meters by 100 meters, and in it there were about 20,000 goldfish,” said Mandrak. “We think they’ve been there for between five and 10 years.”

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