Inside an aquarium, goldfish – a species of carp native to East Asia, bred for aesthetic pleasure and traditionally believed to bring good luck – is nothing more than a simple decorative element of the home. Usually only a few centimeters long, they are among the easiest pets to keep.
But released into the wild, the seemingly humble goldfish, freed from glass confines and no longer limited to meager flake meals, can grow to monstrous proportions. They can even kill native marine wildlife and help destroy fragile and economically valuable ecosystems.
“They can eat anything,” said Christine Boston, an aquatic research biologist with Fisheries and Oceans Canada.
For the past few years, Ms. Boston and her colleagues have been monitoring invasive goldfish in Hamilton Harbour, which is located on the western tip of Lake Ontario, about 35 miles southwest of Toronto. The bay has been decimated by industrial and urban development, as well as invasive species, making it among the more degraded from an environmental point of view areas of the Great Lakes.
Their study, published last month in the Journal of Great Lakes Research, could help target goldfish populations to cull, said Boston, who is the lead author. “We found out where they are before they start laying eggs,” she said. “It’s a good opportunity to get rid of it.”
The fast-growing female goldfish, Boston noted, can also reproduce multiple times in a season. “They have the resources,” she added, “and they can take advantage of them.”
Goldfish were first spotted in Hamilton Harbor in the 1960s, but largely died by the 1970s due to industrial contamination. By the early 2000s, their population seemed to be recovering. Goldfish can tolerate a wide range of water temperatures, reach sexual maturation quickly and eat almost anything, including algae, aquatic plants, eggs and invertebrates, Boston said.
Their football-shaped bodies can swell to sizes that make them too large a meal for predators, up to about 16 inches long. “A fish would have to have a really big mouth to eat it,” she said.
Wild goldfish are also destructive, uprooting and consuming plants that host native species. They help generate harmful algal blooms by consuming algae and excreting nutrients that promote its growth, Boston said, creating intolerable conditions for native fish.
To track the goldfish, the researchers captured and sedated 19 of the larger adults and surgically implanted tags the size of AA batteries into their bellies. The tags, which send signals to acoustic receivers around the bay, gave the researchers a map of their locations.
Eight fish died, but the remaining 11 led Boston and her colleagues to discover that the fish tended to spend the winter in deep water and moved to shallower habitats by spring, where they prepared to spawn.
Some options for removing goldfish, he said, include catching them with specialized nets deployed under winter ice, or using “electric fishing,” which involves stunning the fish with an electric current and removing them from the waterfall. Both techniques, he added, would avoid killing 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 “increase dramatically” over the past two decades. The explosion in spawning, he said, was partly due to people in densely populated areas releasing domesticated animals into urban ponds.
Climate change could play a role, due to goldfish’s ability to adapt to warming and poorly oxygenated waters, he added.
“There are literally millions of goldfish in the Great Lakes, if not tens of millions,” Dr. Mandrak said.
Despite the threat, he added, environmental managers tend to forget about goldfish. “They just assume: ‘It’s been there for 150 years, we can’t do anything about it.’”
The problem is not unique to Canada. In Australia, a handful of unwanted pet goldfish and their offspring have taken over a river in the country’s southwest. Wild goldfish have flooded streams in the United Kingdomand, in Burnsville, Minnesota, the discovery of football-sized creatures in a lake in 2021 led officials to their voters implore: “Please do not release your pet goldfish into ponds and lakes!”
Anthony Ricciardi, a professor of invasion ecology at McGill University in Montreal, noted that not all invasive goldfish become supersized, but even small ones are problematic, outcompeting native fish populations and harming the environment.
People mistakenly believe that because goldfish are “small and cute” they won’t pose a problem when released into the wild, Dr. Ricciardi said. “It’s the ‘Free Willy’ syndrome.”
Goldfish, he added, are just a small part of a vast invasion of non-native species whose outcomes can be unpredictable and, in some cases, are made worse by climate change.
“Under human influence, animals are moving faster and in greater numbers, reaching parts of the planet they never could have reached before,” he said. “We’re talking about the redistribution of life on Earth.”
