Unwanted pet goldfish are being dumped into waterways and growing as large as 1.9 kilograms, researchers in Western Australia have found.

Dr Stephen Beatty from the school of Veterinary and Life Sciences at Perth's Murdoch University has been working on a control program for Busselton's Vasse River for the past 12 years.

He said he and his colleagues regularly found goldfish that weighed over 1kg, and the largest they had found weighed 1.9kg.

Dr Beatty told 720 ABC Perth the goldfish in most cases had been innocently released there but were now choking the habitat for native fish.

"Perhaps they were kids' pets where the family have been moving house and their parents, not wanting to take the aquarium, have dumped them in the local wetlands," Dr Beatty said of the monster goldfish.

"Unfortunately a lot of people don't understand that wetlands connect up to river systems and introduced fish, once they get in there, can do a lot of damage to native freshwater fish and the aquatic habitat."

A 1.9-kilogram goldfish found in the Vasse River. ( Supplied: Murdoch University )

Goldfish cause algal blooms, disease

Goldfish, like carp, can cause a host of problems once they become established in a freshwater system.

"They cruise along the bottom stirring up the substrate with their feeding strategy," Dr Beatty explained.

"This can re-suspend nutrients into the water column which exacerbates things like algal blooms.

"They can also disrupt aquatic plants and eat other fish's eggs."

Goldfish and other introduced species also have the potential to bring unknown diseases to the native populations.

"We know that one disease has been introduced and we think it has probably come in on goldfish," Dr Beatty said.

"It causes lesions on the skin, it's pretty horrible to look at."

Freshwater fish unheralded but vital

Most native freshwater fish are small and go largely unnoticed by the public, but they perform a vital role in the ecosystem.

"They really help us because they are all carnivores and what they love eating is mosquito larvae in the water, so they actually help control mosquito and midge populations we think," Dr Beatty said.

"They do us a really good service, we just don't see them."

Dr Beatty's most recent research involved attaching acoustic transmitters, similar to those used to track sharks in the ocean, to 15 goldfish in the Vasse River in WA's South West.

He tracked their movements for 12 months.

"What we found is that they actually migrate off the main channel into the Vasse River into a wetland system to spawn," he said.

"Given that there is only a narrow channel to get into the wetland, we think that could be targeted to control the species in that system a lot more efficiently than we have been doing."

Don't dump pet fish

His team has had success eradicating carp in the Darch Brook, which flows into the Margaret River.

"We did actually eradicate them there before they became established in the Margaret River," he said.

"You can have success but you have to act really early and throw quite a lot of resources at it.

"And quite often it is still unsuccessful."

The ideal strategy is to prevent these invasive species from entering freshwater systems in the first place.

Dr Beatty advised people who had unwanted fish in tanks at home to either return them to a pet shop or to humanely euthanase them by putting them in the freezer.