Small goldfish can grow into a large problem in the wild

By Kim Smiley

Believe it or not, the unassuming goldfish can cause big problems when released into the wild.  I personally would have assumed that a goldfish set loose into the environment would quickly become a light snack for a native species, but invasive goldfish have managed to survive and thrive in lakes and ponds throughout the world.  Goldfish will keep growing as long as the environment they are in supports it.  So while goldfish kept in an aquarium will generally remain small, without the constraints of a tank, goldfish the size of dinner plates are not uncommon in the wild. These large goldfish both compete with and prey on native species, dramatically impacting native fish populations.

This issue can be better understood by building a Cause Map, a visual format of root cause analysis, which intuitively lays out the cause-and-effect relationships that contributed to the problem.  A Cause Map is built by asking “why” questions and recording the answers as a box on the Cause Map.  So why are invasive goldfish causing problems?  The problems are occurring because there are large populations of goldfish in the wild AND the goldfish are reducing native fish populations.  When there are two causes needed to produce an effect like in this case, both causes are listed on the Cause Map vertically and separated by an “and”.   Keep asking “why” questions to continue building the Cause Map.

So why are there large populations of goldfish in the wild?  Goldfish are being introduced to the wild by pet owners who no longer want to care for them and don’t want to kill their fish.  The owners likely don’t understand the potential environmental impacts of dumping non-native fish into their local lakes and ponds.  Goldfish are also hardy and some may survive being flushed down a toilet and end up happily living in a lake if a pet owner chooses to try that method of fish disposal.

Why do goldfish have such a large impact on native species?  Goldfish can grow larger than many native species and they compete with them for the same food sources.  In addition, goldfish eat small fish as well as eggs from native species.  Invasive goldfish can also introduce new diseases into bodies of water that can spread to the native species.  The presence of a large number of goldfish can also change the environment in a body of water.  Goldfish stir up mud and other matter when they feed which causes the water to be cloudier, impacting aquatic plants.  Some scientists also believe that large populations of goldfish can lead to algae blooms because goldfish feces is a potential food source for them.

Scientists are working to develop the most effective methods to deal with the invasive goldfish.  In some cases, officials may drain a lake or use electroshocking to remove the goldfish.  As an individual, you can help the problem by refraining from releasing pet fish into the wild.  It’s an understandable impulse to want to free an unwanted pet, but the consequences can be much larger than might be expected. You can contact local pet stores if you need to get rid of aquarium fish; some will allow you to return the fish.

To view a Cause Map of this problem, click on “Download PDF” above.