Releasing pet goldfish into the wild has serious consequences

The fish can weigh nine pounds, rapidly reproduce, and disturb ecosystems like the Great Lakes.
A man holes a 14-inch goldfish removed from the Niagara River.
A 14-inch goldfish removed from the Niagara River. USFWS

The Great Lakes are facing invasive species that originated in homes across the United States. Common goldfish (Carassius auratus) released into lakes, rivers, and ponds by pet owners have been wreaking havoc for decades and scientists are trying to follow the fish to solve the problem. The species of freshwater fish native to Asia is usually only about five inches long in a home aquarium, but can grow to up to 19 inches long and tip the scales at nine pounds when they swim freely. 

[Related: Whatever you do, don’t set your pet goldfish free in a stream.]

In the Great Lakes, they’re an invasive species that will “eat anything and everything,” Fisheries and Oceans Canada aquatic research biologist Christine Boston told The New York Times. These seemingly benign household pets can harm native marine wildlife, uproot plants, contribute to harmful algal blooms, and eat vegetation in the environmentally and economically sensitive region. 

According to The Ohio State University College of Food, Agricultural, and Environmental Sciences, invasive goldfish will eat the eggs of native fish species. They will also take over the spaces that other fish will use for reproduction and shelter. The goldfish are even able to reproduce with the native common carp to create bigger hybrid species that are equally harmful.

A study published in November in the Journal of Great Lakes Research details how an invasive population of goldfish in Hamilton Harbour has been changing the area. The harbor in Lake Ontario southwest of Toronto is one of the most environmentally degraded areas of the Great Lakes due to industrial activity and building. The team captured 19 adult goldfish between June 2017 and October 2018. They implanted transmitters in the fish that the scientists used to track where the goldfish swam. 

The team saw that the goldfish spent time in the same area over the winter and returned to the same places to spawn every year. They predicted when the fish would swim into spawning areas by studying how many fish were present, where they were congregating, and the water temperature. They then built models that accurately anticipated the tagged fish and saw that they tended to move into their spawning grounds when temperatures hit about 50 degrees Fahrenheit

The study helped point out goldfish populations for culling. Some of the options for catching them include using nets beneath winter ice, shocking them with electrical currents, or scooping them out of the water. Catching the goldfish before they start spawning is a good opportunity to get the fish out of the lakes, particularly since they reproduce several times in one mating season and they can even live up to 25 years. 

[Related: How goldfish use booze to get through a hard winter.]

 “They have the resources and they can take advantage of them,” said Boston, who is a co-author of the study.

Lake Erie is also not exempt from this problem. In August 2022, a foot-long goldfish was reported in Fairport Harbor north of Cleveland, according to the Ohio Division Wildlife. Goldfish have also been spotted in waterways in Minnesota and in Illinois.

If you can no longer care for a live goldfish, the United States Fish & Wildlife Service recommends trying to find another caregiver, donating a fish to a pet store or school, and checking online forums and social media dedicated to pet adoption.