goldfish are being caught weighing up to 2 kg and koi carp up to 8 kg 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 .
` 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 was probably introduced by goldfish .