Species
CARP (Cyprinus carpio)
Carp originated in China and spread throughout Asia and Europe as an ornamental and aquaculture species. Carp were released into the wild in Australia on a number of occasions in the 1800s and 1900s but did not become widespread until a release of 'Boolara' strain carp from a fish farm into the Murray River near Mildura in 1964. The spread of carp throughout the Murray-Darling Basin coincided with widespread flooding in the early 1970s, but carp were also introduced to new localities, possibly through their use as bait.
Introduced carp are now the most abundant large freshwater fish in the Murray-Darling Basin and are the dominant species in many fish communities in south-eastern Australia. A recent NSW Rivers Survey found that carp represent more than 90% of fish biomass in some rivers and have reached densities of up to one fish per square metre of water surface. They also occur in Western Australia and Tasmania and have the potential to spread through many more of Australia's water systems. Carp could eventually become widespread throughout the country. Carp have benefited from modification of river systems, including construction of dams and other barriers to fish movement, reduced river flows and inundation of floodplains, changes which conversely, have had major detrimental impacts on native fish.
Carp pose an economic threat by affecting industries that depend on pristine water quality and aquatic habitats. Such industries include domestic and irrigation water suppliers, agriculture, tourism and commercial and recreational fisheries. Carp cause significant damage to aquatic plants and increase water turbidity, negatively impacting native aquatic fauna, habitat and ecosystems. It is less clear what the impacts of carp are on native fish populations - many of which were in decline before carp became widespread. Carp carry a number of disease organisms. Some of these, such as the Asian fish tapeworm (Bothriocephalus acheilognathi) now occur in Australia, and may pose a serious risk to native fish.
The biology and ecology of carp are two of the major reasons why they are such an important and successful vertebrate pest in Australia. Carp have broad environmental tolerances and thrive in habitats disturbed and modified by humans such as where flows are altered, nutrients are enriched and streamside vegetation is cleared. Carp can migrate at any time of year and some individuals move up to 230 km.
Closely related to the goldfish, carp in Australia generally live between 15-20 years and can grow as large as 1.2 m and 60 kg. Fish up to 10 kg are relatively common in south-eastern Australia. Carp breed from 2-4 years of age and spawning usually occurs in late spring or early summer. Development is very rapid with eggs usually hatching within 2-6 days. A female carp may spawn several times in one season, producing up to 1.5 million eggs per year, depending on the size of the fish. Even though fertilisation is close to 100%, mortality of eggs and larvae is commonly high.
To date carp control has mainly consisted of commercial harvesting or poisoning. Whilst these options may reduce carp numbers, and poisoning may occasionally eradicate them from isolated areas, other options are being explored for more widespread control.
In collaboration with local communities and natural resource managers, scientists are investigating several ways that carp might be controlled in Australia. These are direct assaults on carp through fishing and biological control, and indirect assault through river restoration. However, it appears that the ultimate answer to carp will lie with an integrated approach. Environmental rehabilitation is seen as a way of improving habitat quality to favour native fish. Biological approaches to carp control are also being explored. Potential molecular approaches include immunocontraception to reduce carp fertility, ‘daughterless technology’ in which modification of a sex-determination gene results in production of male offspring only, and the introduction of a fatality gene to kill individuals at a later date.
