Monster feral goldfish in the Vasse River WA

Original story by Paige Taylor, The Australian

The former pets and their offspring, some 40cm in length and weighing 2kg, are among invasive feral fish entering the southwest waterways, where for the first time unique local fish are outnumbered by alien species.

The invaders eat native fish eggs and stir up algal blooms as they feed on the bottom of the rivers and estuaries.

There are just 11 native freshwater fish species left in Western Australia’s south. They compete with 14 introduced species, many of them thriving in degraded ­environments.

Of the invading fish, 80 per cent are aquarium species thought to have been released by owners.

The Vasse River south of Perth has the world’s fastest-growing goldfish known to science, according to Stephen Beatty, a research fellow at Murdoch University’s School of Veterinary and Life Sciences.

He has helped remove more than 1600 goldfish from the Vasse over the past decade and monitored the growth rate of others. “They can grow to 18cm in the first year, which is quite ­incredible,” he said.

In an unusual study last summer, Dr Beatty and colleagues implanted 21 feral goldfish with tiny versions of the pingers the state’s Fisheries Department has inserted under the skin of great white sharks.

The scientists installed listening posts along the river, similar to the technology Fisheries use to learn when a white pointer is near a beach. The findings, soon to be published, were a little surprising.

The goldfish are more mobile than previously thought; they were tracked covering more than 2km within 24 hours, a long way for a fish of its size.

Their breeding ground was tracked to a wetland attached to the Vasse.

Dr Beatty said the inform­ation was useful because his ­department, the Freshwater Fish Group and Fish Health Unit, examined ways to prevent invasive species from crossing into other waterways.

Some appeared to be highly tolerant to salt water, another worrying discovery.