They were once pets. Now giant goldfish threaten the Great Lakes.
In a fishbowl, the goldfish – a species of carp native to East Asia, bred for aesthetic pleasure and traditionally believed to bring good luck – is little more than a home decoration. Usually only a few inches long, it is one of the easiest pets to keep.
But when released into the wild, the seemingly unassuming goldfish, freed from glass confines and no longer limited to meager meals of flakes, can grow to monstrous proportions. They can even kill native marine animals and help destroy fragile and economically valuable ecosystems.
“They can eat anything and everything,” says Christine Boston, an aquatic research biologist with Fisheries and Oceans Canada.
For the past several years, Ms. Boston and her colleagues have been monitoring invasive goldfish in Hamilton Harbour, at the western tip of Lake Ontario, about 56 kilometers southwest of Toronto. The bay has been decimated by industrial and urban development and by invasive species, making it one of the most common the most ecologically damaged areas of the Great Lakes.
Their studies, published last month in the Journal of Great Lakes Research, could help pinpoint goldfish populations for culling, said Ms. Boston, the lead author. “We found out where they are before they start spawning,” she said. “That’s a good chance to get rid of them.”
The fast-growing female goldfish, Ms. Boston noted, can also reproduce several times in one season. “They have the resources,” she added, “and they can use them to their advantage.”
Goldfish were first seen in Hamilton Harbor in the 1960s, but largely became extinct in the 1970s due to industrial pollution. In the early 2000s, their population seemed to recover. Goldfish can tolerate a wide range of water temperatures, reach sexual maturity quickly and eat almost anything, including algae, aquatic plants, eggs and invertebrates, Ms. Boston said.
Their football-shaped bodies can grow so large that they are too big a meal for predators – up to about 18 inches long. “A fish must have a very large mouth to eat it,” she said.
The wild goldfish are also destructive, uprooting and consuming plants that are home to native species. They help spawn harmful algal blooms by consuming the algae and expelling nutrients that promote its growth, Ms. Boston said, creating conditions intolerable for native fish.
To track the goldfish, the researchers captured and anesthetized 19 of the larger adults and surgically implanted tags the size of AA batteries in their abdomens. The tags, which sent signals to acoustic receivers around the bay, provided researchers with a map of their locations.
Eight of the fish died, but in the remaining 11, Ms. Boston and her colleagues found that the fish tended to spend the winter in deep waters and move to shallower habitats in the spring, where they prepared to spawn.
Some options for removing the goldfish, she said, include catching them with specialized nets deployed under winter ice, or using “electrofishing,” which involves stunning the fish with an electric current and removing them from the water. fetched. Both techniques, she added, would avoid killing the native fish.
Nicholas Mandrak, 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 “increased dramatically” over the past two decades. Their spawning boom, he said, was partly the result of people in densely populated areas releasing pets into urban ponds.
Climate change may play a role, due to the 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,” said Dr. Mandrak.
Despite the threat, he added, environmental managers tend to forget about the goldfish. “They just assume, ‘It’s been there for 150 years, there’s nothing we can do about it.'”
The problem is not unique to Canada. In Australia, a handful of unwanted goldfish and their offspring took over a river in the south-west of the country. Wild goldfish have flooded the country’s waterways United Kingdomand in Burnsville, Minnesota, the discovery of football-sized creatures in a lake in 2021 led officials beg their voters: “Please do not release your goldfish into ponds and lakes!”
Anthony Ricciardi, a professor of invasion ecology at McGill University in Montreal, noted that not all invasive goldfish grow to super large sizes, but even the small ones are problematic, outcompeting native fish populations and damaging the environment.
People mistakenly think that because goldfish are “small and cute” they won’t cause problems if released into the wild, says Dr. Ricciardi. “It’s the ‘Free Willy’ syndrome.”
Goldfish, he added, are just a small part of a massive invasion of non-native species whose outcomes can be unpredictable and in some cases exacerbated by climate change.
“Under human influence, beasts are moving faster and further in greater numbers, reaching parts of the planet they could never reach before,” he said. “We are talking about the redistribution of life on Earth.”