European Carp species are taking over Australian rivers and streams and driving out native varieties of fish to the extent that scientists are planning GM warfare to kill off the invaders. The problem has become so bad that now 90% of the fish stocks in Australia's Murray-Darling river system are European Carp. The plan is to use gene technology to stop the carp from producing any female offspring, causing the population to plummet. They intend to do this by making some genetically modified male fish in the lab which contain multiple copies of a gene called daughterless which makes carp produce only male offspring. (daughterless is an altered version of the 'aromatase' gene involved in metabolism of the female hormone oestrogen). By introducing these GM fish into the wild periodically, slowly the population will all become male and so the population will collapse. But the scientists must first demonstrate that their plan is environmentally safe and will not create more problems than it solves and so they are currently testing the idea in the lab on small, fast-growing zebrafish.