“Be gone, I tire of you,” you tell your goldfish.

You can’t bear to kill it, so you dump it in a river or pond, or flush it.

Enter a growing scourge — ­dinner plate-sized goldfish that do just fine in the wild, growing to enormous size with all that room and elbowing out native fish.

It’s no urban legend — the big fish are increasingly showing up in the Thames River, says John Schwindt, an aquatic ­biologist with the area conservation authority.

“The Department of Fisheries and Oceans sampling crew found them in the main Thames, from Delaware right up to London,” he said.

Aquarium fish are often tossed into waterways or flushed down the toilet live, passing through the sewage system during high waters or flooding.

Goldfish stay small in contained areas, but can grow and reproduce fast with the glass walls gone, said Schwindt.

So far, the Upper Thames River Conservation Authority has found 36 goldfish in 22 locations along the Thames or areas it drains.

“Goldfish have remarkable reproductive potential. They spawn ­throughout the spring and summer and lay lots of eggs, so you just get a whole lot of goldfish competing with everything else for food.”

Goldfish can eat just about anything they can get in their mouths and can survive very poor and harsh environmental conditions.

“They manage to survive for many winters. They seem to be quite hearty in this area,” Schwindt said.

But the outsider fish can also squeeze out native fish.

“One of the ponds we worked on, we had bass and white ­suckers in the pond but they weren’t able to reproduce,” said Schwindt. “The goldfish fed on their eggs.”

Goldfish blend easily into new environments, eventually turning grey and dull brown colours.

Losing their bright orange hue makes goldfish less likely to

be eaten themselves, said Schwindt.

Schwindt said he’s seen goldfish about 40 centimetres long, but they can reach up to half a metre.

The problem is only growing, he said.

“Don’t release them and don’t flush them. If you have to get rid of them, euthanize them. Or try and find a new home for them,” he said.

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GOLDFISH