Though Cute as Pets, Goldfish Are Highly Invasive Once in the Wild

The animal’s insatiable appetites, bold behaviors and adaptability make them harmful to new environments

An image of a USFWS offical holding a goldfish that was released into a river. The fish is orange, and colossal at about a foot long.
Goldfish, as voracious eaters, will devour snails, small insects, fish eggs, and young fish, out-competing native fish. They can also grow over a foot long outside in the wild. (Pictured: A 14-inch goldfish removed by the U.S. Fish and Wildlife Service from the Niagara River that flows north from Lake Erie to Lake Ontario) USFWS

When pet owners release unwanted animals into the wild, they can wreak havoc on native biodiversity. Goldfish, for instance, make wonderful pets and add a bit of shine to a desk, but they are best left in their tanks. Their voracious appetites, tolerance for cold waters, and bold behavior compared to other native species in the water make them a "triple threat,” reports Fiona Harvey on new research for the Guardian. The study, published this month in NeoBiota by researchers at Queen’s University Belfast, looked at the ecological impacts of goldfish.

To see what effects fish have on local environments when they are released into the wild, researchers observed two commonly traded fish which frequently appear in United Kingdom water systems, the goldfish and the white cloud mountain minnow (Tanichthys albonubes). For the purposes of the experiment, the study was conducted in a tank environment in a lab. Both fish belong to the carp or minnow family, Cyprinidae, and are native to East Asia.

The team assessed and compared the impact the fish species had on the environment by looking at their availability as pets (and therefore how often they’re released into the wild), feeding rates and foraging behavior. Goldfish outcompeted white cloud minnows in these three areas, the Guardian reports. The goldfish was domesticated more than 1000 years ago and is invasive globally, while the white cloud minnow has only been commercialized in the last few decades, per Qamariya Nasrullah in Cosmos. Goldfish were more willing to aggressively take on other competing species in the wild for food and had the highest feeding rates with other fish they shared a tank with, per Cosmos. Scientists also observed goldfish as having the greatest boldness. They were more active during the study, and were the most likely to approach a novel object presented to them in the tank environment.

Goldfish, as voracious eaters, will devour snails, small insects, fish eggs, and young fish—and will wildly out-compete native fish. They can also stir up mud when they feed, increasing cloudiness in the water, which lowers the amount of sunlight that penetrates beneath the surface and affects aquatic plants' growth. The team deemed the goldfish a high risk as an invasive species and a high priority for management. In the United States, goldfish have been found to grow more than one foot in length in some water systems, per the Guardian.

Many pet owners think they are acting humanely by releasing goldfish into the wild. But, James Dickey, lead author and freshwater ecologist at Queen's University Belfast, says that this is destructive. 

"Our research highlights that goldfish are high risk, but we hope that the methods developed here can be used to assess others in the pet trade across Ireland and further afield,” Dickey says in a statement. “Readily available species are most likely to be released, so limiting the availability of potentially impactful ones, alongside better education of pet owners, is a solution to preventing damaging invaders establishing in the future."