Two decades ago, someone dropped a handful of unwanted pet goldfish into a creek in southwestern Australia. Those goldfish grew, swam downstream, mucked up waters wherever they went and spawned like mad. Before long, they took over the whole river.
Researchers from Murdoch University believe this scenario, or something like it, is the cause of a feral goldfish invasion in Australia’s Vasse River. Since 2003, they have been running a goldfish tracking and control program that involves catching fish along the length of the river, freezing them to death and studying them in the lab. Despite this program, goldfish in the Vasse are thriving, with some fish growing as long as 16 inches and weighing up to 4 pounds – the size of a two-liter soda bottle.
Goldfish are one of the world’s worst invasive aquatic species, with outbreaks also having been reported in Nevada, Colorado and Alberta, Canada, in the last several years. Goldfish in the Vasse River, though, “have the fastest known growth rate of goldfish in the world,” said Stephen Beatty, a researcher at Murdoch University who helps lead the control program. If his team gets the Vasse’s goldfish problem in order, their work could inform goldfish management efforts far beyond Australia.
Goldfish invasions start with a disconnect between how people view goldfish and what goldfish are like in the wild, Beatty said. “Once you introduce something into a new environment – even if it’s a cute, cuddly aquarium fish – it can have quite unexpected, serious biological consequences.”
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The goldfish is a domesticated carp, first bred in ancient China for ornamental gardens. For centuries, goldfish were prized symbols of luck and fortune. Shortly after they made their way to the United States in the mid-1800s, however, they transitioned from the exotic to the mundane.
The U.S. government played a large role in this, according to Katrina Gulliver, a historian who has chronicled the goldfish. For decades in the late 1800s, the newly established U.S. Commission on Fisheries gifted free goldfish to Washington residents as a publicity stunt, handing out as many as 20,000 fish in some years.
In a Times article from 1894, a reporter jested, “The business of distributing free goldfish to the people of the District of Columbia has become such a tax on the Fish Commission that it appears they must choose between running a goldfish bureau for Washington exclusively and conducting the legitimate work of the bureau.”
This, and the later practice of giving out goldfish at carnivals, spawned the harmful notion that goldfish are disposable and inconsequential. In fact, when tossed into waterways – particularly warm, nutrient-rich and relatively stagnant ones like the Vasse – goldfish behave in unexpected ways.
For one, they look different. Freed from the constraints of a tank, goldfish balloon to the size of footballs. Within a few generations, they revert to natural yellow and brown colors, in place of the bright orange breeders try to achieve.
They’re also an ecological nightmare. Goldfish swim along the bottom of lakes and rivers, uprooting vegetation, disturbing sediments and releasing nutrients that trigger excess algal growth. They feed broadly, eating algae, small invertebrates and fish eggs. To add insult to injury, they transmit exotic diseases and parasites.
Females produce up to 40,000 eggs each year – much more than most freshwater fish species – and are capable of interbreeding with other species of wild carp. With no natural predators, a large portion of goldfish offspring survive to reproductive age, continuing a cycle of rampant overpopulation.
So how do you get rid of them in a lasting way? Once they’re established somewhere, eradicating goldfish is a notoriously difficult undertaking – which is why Murdoch scientists recently spent a year tracking the movement of the fish in the Vasse. Their study, published last month in The Ecology of Freshwater Fish, yielded some unexpected findings.
For starters, goldfish are long-distance swimmers – Beatty’s team saw goldfish routinely travel the length of multiple footballs fields in a day, and even observed one fish that traveled more than 140 miles in a year.
For another, goldfish migrate to spawn. That’s right, the same fish that are often kept in tiny bowls, swimming in circles, navigated in droves to an off-channel wetland during breeding season.
It’s perhaps a surprising finding for a domesticated species, but the behavior seems to be innate, Beatty said, and points to goldfish having complex cognitive abilities.
“We think of goldfish as not being very intelligent – more like furniture or home accessories than sentient creatures,” said Dean Pomerleau, an engineer from Pittsburgh. But his family has trained pet goldfish to perform complicated tricks, such as nosing a tiny soccer ball into a net, and researchers have shown that goldfish can discriminate between classical music by Bach and Stravinsky. (Yes, goldfish can hear – they have evolved a bone structure that translates changes in pressure from sound waves from their swim bladder to their inner ear.)
A better understanding of goldfish behavior can inform management strategies, Beatty said, such as trapping fish en masse after they’ve migrated to their breeding grounds.
Meanwhile, to ensure goldfish invasions don’t get worse, it’s crucial that pet owners get rid of unwanted fish responsibly, said Linda Walters, a biology professor at the University of Central Florida who has helped produce two children’s books on the dangers of emptying home aquariums into local waterways.
The best strategy is to give healthy fish away, to a responsible aquarium, pet store or hobbyist, Walters said. In Florida, the Fish and Wildlife Conservation Commission also takes unwanted exotic pets off people’s hands on regularly scheduled amnesty days.
If your fish is sick, the most humane way to kill it is probably to put it to sleep in an ice slurry. As for whether or not you should flush your fish down the toilet, experts recommend against it. Not only is there a slight chance your fish could survive a journey through the septic system and end up in the wild, but, in general, it’s just not a very pleasant way to say goodbye to Bubbles.