Once They Were Pets. Now Giant Goldfish Are Menacing the Great Lakes.

Their football-shaped bodies can swell to a size that makes them too large a meal for predators — up to about 16 inches long.

We really do make everything fat, don’t we?

This is the best summary I could come up with:

Inside a fishbowl, the goldfish — a species of carp native to East Asia, bred for aesthetic delight and traditionally believed to bring good fortune — is hardly more than home décor.

But released into the wild, the seemingly humble goldfish, freed from glass boundaries and no longer limited to meager meals of flakes, can grow to monstrous proportions.

Over the past several years, Ms. Boston and her colleagues have been tracking invasive goldfish in Hamilton Harbour, which is on the western tip of Lake Ontario, about 35 miles southwest of Toronto.

Goldfish can tolerate a wide range of water temperatures, reach sexual maturation quickly, and can eat nearly anything, including algae, aquatic plants, eggs and invertebrates, Ms. Boston said.

They help spawn harmful algal blooms by consuming the algae and expelling nutrients that promote its growth, Ms. Boston said, creating conditions that are intolerable to native fish.

Nicholas Mandrak, a professor of biological sciences at the University of Toronto Scarborough, said that while goldfish were introduced to North America in the late 1800s, the wild population had begun to “dramatically increase” in the past two decades.

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