Carp: a case study

During the nineteenth century, the common carp was introduced to some south-eastern Australian waterways. Cyprinus carpio, often referred to as the European carp, had a disastrous effect on these ecosystems.

A single carp

Pest Animal Control Cooperative Research Centre

Carp are native to many temperate regions of Asia, and are closely related to goldfish. At least three strains of carp have been introduced to Australia, including an ornamental strain near Sydney (1850–60), a Singaporean strain in the Murrumbidgee (1876), and a hybrid ‘Boolara' strain in Victoria (1961). All have dominated the waterways and become major pests.

Carp compete with native fish for habitat and food resources, and contribute to the degradation of waterways. They uproot aquatic plants and sift through sediment during feeding, which increases water turbidity. This means that less light can penetrate, which stunts surrounding plant growth. In turn, this can lead to erosion and subsidence of river and lake banks, which further contributes to water turbidity.

Traditional methods of control

Carp are extremely adaptable animals and can thrive in environments that would kill other freshwater fish. It is thought to be impossible to totally eradicate the existing population, and so the most realistic approach is to control carp numbers.

A pile of carp

Pest Animal control Cooperative Research Centre

A number of attempts to control the carp population in south-eastern Australia have been made. Simple measures rely on the cooperation of anglers and hobbyists: if you catch a carp in Victorian waters, it is illegal to throw it back. It is also illegal in Victoria to keep live carp, including Koi carp, desired by some people as ornamental fish. Other control methods include commercial harvesting and poisoning.

While these measures may reduce carp numbers in some areas, other options are being explored for more widespread control.

Biological control – the Daughterless Carp Program

‘Daughterless' technology aims to control carp numbers by creating a fish population containing many more males than females. By blocking a specific gene involved in female carp development, only male fish are produced.

The project, which began in 2003, is a collaborative project between the Murray-Darling Basin Commission, CSIRO Marine Research and the Pest Animal Control Cooperative Research Centre (now known as the Invasive Animals Cooperative Research Centre).

The researchers are targeting a gene called aromatase. Aromatase encodes an enzyme that converts testosterone to oestrogens in the developing carp embryo. Without aromatase, the embryo cannot make the hormones required to develop into a normal female fish.

By having fewer females in the population, in conjunction with other management practices, researchers hope that carp numbers in the Murray-Darling Basin will plummet within 20 to 30 years of release.

The scientists have made a ‘daughterless carp' modified gene and have tested it using Japanese rice fish, which have a much shorter life cycle than carp. Experiments have shown that the gene is only partially inherited, and so further modifications to the gene are being made and will require further testing to see if inheritance has improved. The next step will be to breed large numbers of carp containing the modified gene in a safe facility.

Once the modified gene is shown to work, it will take at least another five years before any daughterless carp are ready to be released into rivers, after strict field trials. Because carp have a very long generation time, the approach would need to operate over 40–50 years, and be used in conjunction with other short-term control techniques, to have a lasting effect.

Although the program involves relatively high-risk research, the possible benefits have warranted its further development.

Any new technology involves risks that need to be identified and assessed. The researchers are assessing and reviewing the risks from the beginning of the project to identify any risks associated with the technology and to take the steps to ensure they are addressed.

The technology involves manipulating only carp-specific genes, minimising the risk to non-target species. The release of any genetically modified organism into the environment is subject to legislation which considers any economic, social and ethical concerns related to the technology. All research is being conducted in secure laboratory facilities and has been approved under the strict guidelines of the Office of the Gene Technology Regulator (http://www.ogtr.gov.au).