While seemingly innocuous as pets, goldfish are a potential menace to indigenous ecosystems when set free in the wild. This has occurred in virtually every region within the United States, where feral goldfish have been troubling freshwater lakes and rivers for many years.
Goldfish are members of the family Cyprinidae, one of the largest and most diverse families of fish, which also includes common carp. They are originally from Asia but have become widely introduced throughout the world as ornamental fish. Contrary to what most people believe, goldfish depend on how much food available in the habitat rather than the size of their tank. When kept as pets, due to poor care, they usually stay to about 1-2 inches long, while wild ones average about 14 inches in length. There have even been reports of captured wild goldfish that can grow as big as 19 inches and as heavy as 9 pounds.
Goldfish are hardy fish, able to bear different temperatures and water conditions, which in turn enable them to survive in many areas that are beyond their natural habitat. They are also remarkable specifying their ability to thrive in anoxic water bodies where there is very little, if not totally devoid of oxygen. Experiments have proved a goldfish can live for three to five months without oxygen; in winter conditions it should be able to survive frozen bodies of water. This is one property that makes goldfish capable of thriving in water habitats that are not suitable for other species of fish.
Food is the next least or nil criterion in the goldfish. Goldfish will virtually eat anything. This ability to exploit a wide range of resources enhances the potential of the fish to establish first in a new environment. Being bottom feeders, goldfish stir up mud and sediment at the bottom of freshwater lakes. This clouds the water and makes it more difficult for plants and native fish species to thrive there. In the worst case, you go from a clear lake dominated by plants into a muddy lake dominated by algae.
For decades now, feral goldfish have been a part of the invasion of a lot of North America involving the Great Lakes with their populations apparently rapidly increasing as of late. No one knows the reason behind this uptick, but it would most likely be because invasive carp seem to be better controlled, and the goldfish are taking the place of the fish.
The best way to prevent the spread of invasive goldfish is to avoid releasing these fish into the wild. Owners who no longer want their goldfish should contact their local pet store or aquarium for advice on how to rehome them. Otherwise, they could humanely euthanize them by putting them into a container of water and adding some clove oil, which sedates them, stops breathing, and finally kills them. Flushing them down the toilet or tossing them into a garbage can is also not a good idea, for there is still the likelihood that they will get into the waterways eventually.
The invasive goldfish are familiar reminders that even some small, seemingly harmless creatures can have a very big effect on the environment. Responsible pet ownership is where it begins if we want to assure the natural balance of our aquatic ecosystems.