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


For years, Andrew Murray has watched the small schools of goldfish live in the storm pond near his home in suburban London, Ontario. He never thought much of them until this spring, on a sunny April day, when the normally greenish pond had an unusual, if not slightly orange, tint.

Passionate about nature photography, he made some close-ups. It wasn’t until he got home and looked at the pictures that he realized how many goldfish must be swimming in there.

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

Researchers at the University of Toronto believe this scenario plays out hundreds of times in suburban storm basins across the province. Originally built to reduce flooding in neighborhoods and relieve pressure on Ontario’s city sewer systems, these pools have become ground zero for what Nicholas Mandrak calls “super invaders.”

Ground zero ponds for goldfish invasion

“What we’re seeing is the proliferation of goldfish in suburban southern Ontario, in fact across Canada,” said the University of Toronto Scarborough professor who studies fish conservation. fresh water.

WATCH | Professor Nicholas Mandrak discusses the possible link between the explosion of wild goldfish in Ontario and climate change:

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

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

With infestations reported in Alberta, British Columbia, Saskatchewan, Manitoba and the United States, goldfish are quickly becoming one of the most prolific invasive species in North America. The reason they’re so prevalent is largely down to how people think of goldfish: harmless and disposable.

“People who release goldfish into urban ponds may think they’re doing a good thing, ‘I’m not killing a creature,'” Mandrak said. “There shouldn’t be any fish in these urban ponds.”

Once introduced to a storm pool, goldfish begin to feed on the insects that would normally colonize it. They reproduce quickly, and their numbers combined with their large appetites mean they out-compete amphibians and other creatures, reducing the diversity of native species.

They also escape when ponds are flooded, releasing the fish into wetlands, streams, rivers and lakes where Mandrak says the goldfish become predators, eating the eggs and fry of other fish.

Rare native species endangered goldfish

Goldfish also root, almost like an aquatic version of a pig, destroying habitat by uprooting plants and clouding the water for rare and endangered native species such as the spiny softshell turtle, green stinger and rainbow mold.

Goldish has been in Ontario’s wilderness waterways since the 19th century, according to Mandrak. What has changed, however, is the number of them, and he said it likely has to do with the recent addition of suburban stormwater ponds and a changing climate, providing conditions for fish. more favorable to prosper.

“It’s only recently that they’ve become more abundant,” Mandrak said. “It’s largely thanks to these urban ponds.

“We also believe that climate change is also involved. The waters of the Great Lakes and urban ponds are warming more than they would have done before. The water seems to be more suitable for goldfish.

“As the conditions become more extreme, meaning hot, oxygen-poor water, one of the few fish that does really well in this type of habitat is the goldfish.”

The climate change nexus

That’s why Mandrak thinks goldfish deserve our attention. Not only are we releasing them back into the wild, but human-caused climate change is creating the conditions for them to thrive, through warmer waters and their spread to nearby wetlands, through flooding caused by higher rainfall. intense.

Climate change is causing suburban stormwater ponds to hold warmer water that holds less oxygen, the kind of conditions that allow goldfish to thrive while other species perish. (Cole Burston/University of Toronto Scarborough)

That’s why goldfish are increasingly plentiful in Ontario’s protected wetlands, from Westminster Ponds in London to Cootes Paradise in Burlington.

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

“We are seeing small annual increases in mean annual temperatures leading to much larger and disproportionate increases in goldfish numbers because climate change is driving these more extreme environments.”

Part of Mandrak’s research also involves trying to chart the life cycle of a goldfish infestation in rainfed ponds, i.e. how many generations it takes before the fish take over. a body of water.

Scientists can determine how long a goldfish infestation has spent in a body of water by the size of its largest fish, which, unconfined by the size of their bowls, have been known to grow to the size of a two liter soft drink bottle. and weigh up to two kilograms.

“Last year we sampled a 100m by 100m pond and it had about 20,000 goldfish,” Mandrak said. “We thought they had been there for five to 10 years.”