A goldfish the size of a small dog has been discovered in a river after it survived being flushed down the toilet by its owner.

The 14-inch-long fish was found downstream of a wastewater treatment plant in the Niagara River in New York.

It was caught in the river’s Black Rock Canal by a non-profit organisation called Buffalo Niagara Waterkeeper (BNW) that protects the river. In a Facebook post, the organisation wrote: “This is why you should never flush your fish!”

“Goldfish can survive year-round in our watershed and can destroy the habitat of native fish,” a BNW spokesperson wrote in the post, which was accompanied by a picture of the giant fish held in two hands by employee Marcus Rostens.

“Scientists estimate that tens of millions of goldfish now live in the Great Lakes. If you cannot keep your pet, please return it to the store instead of flushing or releasing it.”

The image of the fish is believed to be a few years old but it was recently posted online by BNW to educate people about the dangers of flushing fish.

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They are not native to North America and do not have a natural predator which means they outcompete the native fish.

“Aquatic invasive species that don’t naturally belong in the Great Lakes, like this goldfish, are a constant threat to the health of native wildlife populations and their habitats,” a BNW representative told Live Science.

Goldfish are part of the carp family and generally only reach one to two inches in length but can grow to between 12 and 14 inches in the wild.

The average life expectancy of a goldfish is 20 years but they rarely live that long in captivity.

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