This giant goldfish was caught July 2 in Minnesota’s Keller Lake, 20 miles south of Minneapolis. Goldfish compete with native species for food, increase algae in lakes and, reportedly, are among the world’s worst invasive aquatic species. City of Burnsville, Minn. Photo

Invasive, football-sized goldfish are turning up in lakes and waterways out-competing native species for food and choking ecosystems with voracious feeding that kicks up mud and sediment, leading to harmful algae blooms.

Officials in Burnsville, Minn. captured nearly 30 giant goldfish, some measuring more than 18 inches and weighing up to 4 pounds in July. “People have unwanted fish in their aquariums, so they dump them into a pond, river or spring,” officials said.

“Released into their favored slow-moving fresh bodies of water, goldfish grow unhampered in ideal conditions not always available in the domesticated goldfish bowl. They adapt to their environment, with unique eating habits and with increased sensitivity in hearing and sight, producing chemicals for temperature regulation.

“These long-lived goldfish are an ecological nightmare. With no natural predators, they transmit disease and parasites as they swim along the bottom of lakes and rivers, uprooting vegetation, disturbing sediment and eating small invertebrates and fish eggs.”

Canadian authorities estimate that as many as 50 million goldfish may inhabit Lake Ontario. The population has exploded in recent years, which has impacted other species,” said Tys Theysmeyer of the Royal Botanical Gardens in Hamilton, Ontario. “Populations of frogs, fish, turtles, salamanders are all significantly down.”

Where did it all begin?

Sometime during the Jin Dynasty (265-420 AD), the ancient Chinese observed that some of the freshwater carp they were breeding as food tended to occasionally display mutated colors ranging from red to orange and yellow. A few centuries later, people developed ornamental water gardens, which they stocked with a gold variation of the silver Prussian carp, from which goldfish sprang.

During social gatherings, some of the finer specimens were temporarily showcased in smaller containers—the world’s first fish tanks.

Goldfish invasions start with a disconnect between how people view goldfish and what goldfish are like in the wild. “A cute, cuddly aquarium fish can have quite unexpected, serious biological consequences once introduced into a new environment,” experts say.

Environmental officials have been pleading with fish owners not to dump aquarium fish.

The U.S. Fish and Wildlife Service recommends putting an unwanted fish up for adoption, donating it to a school or humanely euthanizing it with a veterinarian or a pet store’s assistance. Whatever you do, don’t release it into a pond or lake—and don’t flush it down the toilet. In 2019, a 14-inch goldfish was reportedly caught downstream from a wastewater treatment plant on the Niagara River near Buffalo, N.Y.