Pet aquarium fish are being dumped in rivers where they damage unique local ecosystems by growing up to twenty times their regular size.

Goldfish are being caught weighing up to 2kg and koi carp up to 8kg and one metre in length, in the waterways of Western Australia amongst other exotic introduced species.

'A lot of these fish are much larger than the native fish, so they prey on them and compete for habitat,' Dr David Morgan, the Director of the Centre for Fish and Fisheries Research at Murdoch University, told Daily Mail Australia.

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Goldfish (pictured) weighing up to 2kg are some of the exotic aquarium fish invading and destroying WA's waterways

Koi carp (pictured) weighing up to 8kg and up to one metre in length are another of the 13 introduced pests

The native fish in the region, such as this Balston's Pygmy Perch which has been attacked, are only up 10cm

'Our native fish are generally less than 10cm in length like the pygmy perch or the native minnows.

'In the south-west of Western Australia 90 per cent of the fish and 100 per cent of the crayfish are only found there, so the rivers are refuges for the native fish and when they are forced to coexist with introduced species, they just don't do very well.'

Aside from feasting on local fish and taking their homes, introduced species are also bringing exotic parasites and disease with them.

'In Perth we're aware of a couple of parasites. One is pretty devastating on native fish, it's an anchor worm which is a little crustacean that leaves a hideous, hideous impact on native fish and was probably introduced by goldfish.

A huge Murray River Cod was removed from a Perth wetland in Novemeber last year: 'A lot of these fish are much larger than the native fish, so they prey on them and compete for habitat,' Dr David Morgan, the Director of the Centre for Fish and Fisheries Research at Murdoch University, said

Pest carps: 'In the south-west of Western Australia 90 per cent of the fish and 100 per cent of the crayfish are only found there,' he added

'We've now found it in three rivers around Perth and south of Perth.

'It's been only in the last decade that we've discovered a lot of parasites and disease that we don't know a lot about - there's a lot of things that have come in to Australia that we really don't know anything about.'

Research conducted by Dr Morgan and Stephen Beatty found that at least 13 species of fish had been introduced into south-western Australia, with pet owners believed to be the biggest culprits.

In 2012, the world's biggest ever specimen of a South American cichlid was found living in WA.

'Goldfish in captivity generally remain orange, grow to about 10-15 cm and weigh less than 100g – in the wild in WA they get to 2KG and 40cm,' Dr Morgan said

Pet owners who have dumped their aquarium fish in rivers are the culprits behind the invasion

In 2012, the world's biggest ever specimen of a South American cichlid (pictured) was found living in WA

'Effectively people are unknowingly doing it - or occasionally they deliberately do it for fish to catch - but they're basically released by pet owners, so people aren't really aware what damage they're going to have in the ecosystem,' Dr Morgan said.

'What you see up and down Western Australia's coast is a lot of pest fishes are right in town centres and almost every town has one.'

The problem is not unique to Australia. Many countries with Mediterranean climates have also had their waterways invaded by aquarium fish which, due to being extremely easy to breed, take off when in the wild.

'Education is our best solution, once fish get in the waterways they're extremely difficult to eradicate,' Dr Morgan, whose organisation euthanizes hundreds of the pests after catching them through electric fishing, said.

Introduced species also bring exotic parasites and disease with them