Disenchanted fish owners are dumping their unloved pets into lakes and rivers in Western Australia, unleashing a ginormous threat to native populations.

These unloved pets grow as large as their environment allows, and can turn into disease-carrying bullies.

For example, the normally five to 10cm koi can grow to a metre long.

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Swan River Trust river science environment officer Jeff Cosgrove told the ABC it is a massive environmental issue for the region.

“Through the drainage system, these fish simply spread and they go out into our beautiful rivers,” Dr Cosgrove said.

“They are eating up the food resources and using up the habitat that our native fish would otherwise be using.”

Exotic fish are also prone to introducing and spreading exotic diseases and parasites, and can damage natural habitats.

“This is an added pressure, on top of all the other pressures our aquatic wildlife is experiencing,” Dr Cosgrove said.”

“Once the fish is in the system, they are extremely difficult to eradicate.”

—with ABC