Dumped Pet Goldfish In Colorado Lake Now Number 3000, Threaten Ecosystem

Think twice before you dump your no-longer-wanted pet fish in a lake or even flush them down the toilet, biologists say. Hardy fish can survive septic systems and make it to waterways where they can multiply and seriously affect the indigenous life there, including native fish and delicate ecosystems.

A few Koi goldfish were dumped into a lake in Colorado approximately three years ago, likely by a well-meaning but misinformed private owner and began to reproduce at a massive rate, with no end in sight. The estimate of three years ago is made by looking at the biggest fish that are evident in the lake. Koi goldfish are not native to the lakes.

Biologists are concerned about how to humanely remove the Koi goldfish from the Boulder-area lake before they do serious damage and threaten other species. Ben Swigle, a fish biologist at the Colorado Parks and Wildlife, told Live Science that the fish were first noted by a park ranger three or four weeks ago, and have been a significant concern since the discovery.

“Based on their size, it looks like they’re 3-year-olds, which were probably produced from a small handful of fish that were illegally introduced into the lake. If they escape and move downstream, they’ll directly compete with our native species, all of which were here before the land was even settled. If they [the goldfish] explode and get downstream and potentially explode, there they are competing with these fishes not only for spawning habitat but also for foraging resources.”

While there are several options for removal of the fish, none are easy and some may be controversial. One possibility is to literally net them out as much as possible and give them to a raptor sanctuary that cares for injured Eagles and Owls. The problem with these particular goldfish is that unlike other fish that are sometimes purposefully “stocked” into ponds, they are not rigorously tested for disease. That means that the dumped goldfish could potentially carry viral hemorrhagic septicemia virus (VHSV), responsible for killing off thousands of fish in the Great Lakes, and bacterial kidney disease, which has been seen in aquarium fish of various species, Swigle said.

People may not be aware, but it is illegal to dump aquarium fish into public lakes. One way to deal with unwanted fish is to call the pet store where they were purchased and ask to return them. Other guidelines for unwanted fish disposal is set forth by the American Veterinary Medical Association, and includes euthanasia methods such as the injection of ketamine.