While goldfish may seem perfectly harmless, it has been found that they can become an invasive species if left in the wild.

Goldfish are regularly given out as prizes at fairs or bought as pets, before they are dumped when the initial excitement wears off. Most people think that the best way to get rid of the fish is to release them into a nearby pond or river, but it turns out this can cause great damage to local ecosystems, according to a new study published in the journal Ecology of Freshwater Fish.

Goldfish
Photo by stux (Pixabay)

Humble goldfish an ecological menace

Scientists have found that goldfish can become an invasive species, which damage ecosystems in which they have no natural predators. Free from predators, goldfish can have a great impact on local resources and even kill other species in their new environment.

Various invasive species started life as pets and the goldfish is no different. The new study looks at huge football-sized goldfish in the Vasse River, in the South West region of Western Australia, where the fish have traveled large distances to their new home.

Goldfish are native to eastern Asia, but now rank among the worst invasive aquatic species on the planet. They are usually tossed out of people’s homes into catchment lakes, entering local ecosystems thanks to human actions.

Disruption of local ecosystems

Study author Stephen Beatty, from the Center of Fish and Fisheries, said that the fishes could have once been owned by families who were moving house and didn’t want to take the goldfish aquarium with them. Once the goldfish establish a population in the wild, it can be hard to get rid of them.

This leads to problems such as declining water quality, disturbed habitats, new diseases and more competition with native species.

“Once established, self-sustaining populations of alien freshwater fishes often thrive and can spread into new regions, which is having a fundamental ecological impact and are major drivers of the decline of aquatic fauna,” said Beatty.

“Our research discovered the fish displayed a significant seasonal shift in habitats during breeding season, with one fish moving over 230 kilometre [142 miles] during the year,” Beatty said.

Massive goldfish discovered in Australian river

The researchers used acoustic receivers to analyze the movements of goldfish populations over the course of the study, the results of which were published August 12. Some of the goldfish discovered by the researchers had reached gargantuan proportions.

Carassius auratus, the domestic goldfish, can reach over 4 pounds if it is released into a major waterway where there are plenty of resources. One problem arises from its feeding habits, which involve eating the eggs of competing fish.

It is thought that the study could better inform strategies for dealing with invasive species such as goldfish. “The results of this study will have important direct management implications, enabling more strategic development of effective control programs for the species such as targeting migratory pathways,” said Beatty.

It’s worth thinking twice before you release your small fairground goldfish into the wild. Before long it might grow into a huge monster, bullying other fish and wreaking havoc in the ecosystem.

Other recent reports have seen sightings of the pacu fish, which is closely related to the piranha. The fish was most recently reported in the Michigan Lakes, where swimmers have been surprised by its human-like teeth. The release of former pets can lead to devastation for other fish, and the best way to get rid of unwanted animals is to humanely put them down.