Image: Murdoch University, YouTube

For many people, goldfish are just harmless pets, but when introduced to the wild they are one of the worst invasive species in the world.

Goldfish management has become a widespread effort, with wildlife officials trying to contain outbreaks in parts of North America including Nevada and Colorado and more recently, Australia’s Vasse River, where their presence is becoming a detrimental hindrance.

“[The fish in the Vasse River] have the fastest known growth rate of goldfish in the world,” researcher Stephen Beatty told to the New York Times.

Goldfish are domesticated freshwater fish in the carp family, recognized primarily as common household pets. Although seemingly harmless, these animals have characteristics that make them extremely catastrophic to freshwater habitats.

Although they remain small when kept in captivity, goldfish can grow to massive sizes in the wild. The largest goldfish on record was measured at nearly 20 inches, discovered in the Netherlands.

In the wild, they graze the bottoms of lakes and streams while swimming, resulting in the accidental uprooting of necessary vegetation and excess algal growth. They are also opportunistic feeders that feed on a variety of invertebrates, fish eggs, and plant matter.

Goldfish are egg layers that deposit up to 40,000 eggs each year which then attach and hatch on dense aquatic vegetation within 48-72 hours. This mass amount of reproduction coupled with their lack of natural predators attributes to vast overpopulation.

The worst part? Once established in a particular location, goldfish are stubbornly difficult to uproot.

A study published in the Ecology of Freshwater Fish looks at the migration movements of goldfish, highlighting the need for further behavior analysis of this species in order to reduce their population.

Researchers suggest this issue stems from pet goldfish being dumped into rivers and streams. The takeaway message for owners: Dispose of your goldfish responsibly.