Lurking beneath the calm surface of Toronto’s ponds and waterways lies an unexpected monster of the deep — giant goldfish.

It's no urban fish tale. Rick Portiss, a fish expert with the Toronto and Region Conservation Authority said goldfish consistently appear during Toronto fisheries surveys.

“They show up in the regular batches of fish, and every once in a while you get these big fat ones that look like pumpkins they’re so big and orange,” he said.

Most of these fish begin life in fishbowls or garden ponds as pets. They either escape during flooding or are released into public wetlands where they flourish.

Karen McDonald, a project manager with the TRCA said goldfish grow to the appropriate size of their environment.

“If you have a small goldfish bowl you’re not going to produce a three-pound goldfish. But when you release them into the wild and they have unfettered access to resources… they’re gonna gorge themselves. And it’s a large water body so they’re not going to be impacted by the small size of the container that they’re in,” she said.

While turning Toronto’s public waters into a giant fishbowl is golden for former pets, it can cause a problem for native species.

According to Fishes of Toronto, a city-published guide to local fish, when goldfish and their larger cousin, Koi, are released into lakes and streams they begin to uproot and feed on aquatic plants, destroying the habitat for other fish and wildlife.

Part of the reason the goldfish population is so prolific is its ability to adapt and reproduce.

“They breed like crazy. Even people that intentionally put them in their large natural ponds end up with thousands and thousands of them within a few years because they just multiply so fast,” said Chris Dahl, owner of Hydrosphere Water Gardens and Fisheries.

Dahl describes goldfish as “hearty” and said that unless they get trapped in ice and freeze solid, they’ll survive. He also said it’s not uncommon for goldfish to crossbreed with Koi, resulting in larger than normal fish.

Xiao Yang, a member of Toronto Urban Fishing Ambassadors, has caught gargantuan goldfish.

He said he regularly catches goldfish that weigh as much as a kilogram.

“If I fish with bread or worms, and use a larger hook so the smaller sunfish cannot take the bait, I would get a goldfish almost every other catch,” he said.

Despite catching so many, Yang said he’s never eaten a goldfish. They’re strictly catch and release as far as he’s concerned.

The TRCA has a different policy. “Sometimes staff take them home as pets,” said McDonald. “Other times they’re sent to the Royal Ontario Museum as specimens.”

The goldfish problem may be growing, but when it comes to giant invasive species McDonald said it could be worse.

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“Fortunately we’re not encountering pythons or boa constrictors or anything like that,” she said.

GOLDFISH FOUND IN TORONTO WATERS DURING TRCA SURVEYS