Unloved pet fish are flooding Australian waterways, introducing diseases and bullying native fish. In large waterways, goldfish and koi can grow to up to 10 times the size of native species and compete with them for habitat and food. RN Afternoons reports.

In Western Australia's southwest, exotic fish bought for aquariums are continually being dumped in lakes and rivers, decimating native fish populations.

They are eating up the food resources and using up the habitat that our native fish would otherwise be using. Dr Jeff Cosgrove, Swan River Trust

Dr Jeff Cosgrove, Swan River Trust's river science environment officer, says it's a massive environmental issue for the region.

According to Dr Cosgrove, aquarium owners get sick of their fish and dump them into areas they sees as having little environmental significance.

'They're dumping them into a man-made lake, say, in a housing estate, and they think, "What's the harm?" At least it will have a nice home and I don't have to look after it."

'Through the drainage system, these fish simply spread and they go out into our beautiful rivers.'

Aquarium fish will typically grow as big as their environment allows.

In the case of koi, that can be up to one metre long, compared with five to 10 centimetre-long native species.

They are often more aggressive than native fish, and will bully and out-compete them for food.

'They are eating up the food resources and using up the habitat that our native fish would otherwise be using,' says Dr Cosgrove.

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,' says Dr Cosgrove.

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

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