“Please don’t release your pet goldfish into ponds and lakes!”

Far from being an innocuous domestic animal, a goldfish freed in fresh water is an invasive species, an organism that is introduced to an environment, can quickly reproduce, outcompete native species and destroy a habitat. And even though they get less attention than invasive organisms such as Asian carp or zebra mussels, goldfish appear to be a growing problem in bodies of water across the United States and around the world, triggering warnings from government officials in Virginia, Washington state, Australia, Canada and elsewhere...

Also known by the scientific name Carassius auratus, goldfish can live to be 25 years old, weigh as much as four pounds and measure well over a foot long. They’re also surprisingly resilient: They can survive in severe conditions and can weather winters in bodies of water that have frozen over, living for months without oxygen. This quality, Bajer said, “makes them really, really tough and allows them to dominate certain types of ecosystems.”

Floods in Australia sent spiders scrambling for dry land. A town is now covered in webs.
Goldfish, like their common carp relatives, feed at the bottom of lakes, where they uproot plants and stir up sediment, which then damages the water’s quality and can lead to algal blooms, harming other species.