Two decades ago, someone dropped a handful of unwanted pet goldfish into a creek in southwestern Australia. Those goldfish grew, swam downstream, mucked up waters wherever they went and spawned like mad. Before long, they took over the whole river.

Researchers from Murdoch University believe this scenario, or something like it, is the cause of a feral goldfish invasion in Australia’s Vasse River. Since 2003, they have been running a goldfish tracking and control program that involves catching fish along the length of the river, freezing them to death and studying them in the lab. Despite this program, goldfish in the Vasse are thriving, with some fish growing as long as 16 inches and weighing up to four pounds — the size of a two-liter soda bottle.

Goldfish are one of the world’s worst invasive aquatic species, with outbreaks also having been reported in Nevada, Colorado and Alberta, Canada, in the last several years. Goldfish in the Vasse River, though, “have the fastest known growth rate of goldfish in the world,” said Stephen Beatty, a researcher at Murdoch University who helps lead the control program. If his team gets the Vasse’s goldfish problem in order, its work could inform goldfish management efforts far beyond Australia.

Goldfish invasions start with a disconnect between how people view goldfish and what goldfish are like in the wild, Dr. Beatty said. “Once you introduce something into a new environment — even if it’s a cute, cuddly aquarium fish — it can have quite unexpected, serious biological consequences.”