Description
Carp can grow as large as 1.2 metres and 60 kilograms. Fish up to 10 kilograms are relatively common in south-eastern Australia. Carp can tolerate: water temperatures as low as 4 C and as high as 35 C; salinity levels of 14 % (seawater is 35%); relatively extreme pH levels; and polluted and poorly oxygenated water. Carp produce up to 1.5 million eggs per year.

History
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 (see attached map), but carp were also introduced to new localities, possibly through their use as bait. 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.

Perceived and actual impacts of carp
There is clear evidence that carp can increase water turbidity and damage aquatic plants. There is also some evidence that carp increase water nutrient levels. These impacts can alter ecological functions and affect tourism and recreational values of otherwise scenic wetlands.

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 may make aquatic habitat less suitable for native fish breeding and survival, but there is little evidence of carp feeding directly on native fish whereas small carp provide a food source for a number of fish and bird species.

There is no clear evidence that carp cause bank erosion and in any case it would be difficult to isolate the effects of carp from other influences such as high flows, excessive water extraction, lack of riparian vegetation and livestock access.

Commercial use of carp
The largest carp harvester/processor in Australia (Gippsland Lakes, Victoria) processed 900 tonnes of carp in 1999 for domestic and export markets, with a target of 1000 tonnes for the export market alone in 2000. Retail prices for whole carp are up to $7 per kilogram in some Sydney shops. There is some potential to increase the value of carp with further processing (smoked and canned fish etc). However, currently much of the commercial catch is used for low-value products such as fertiliser (15 cents per kilogram) and crayfish bait (50 cents per kilogram).

Managing the carp "problem"
All of the actual and perceived impacts of carp need to be placed in the context of overall waterway management and the management of surrounding land. Provision of more natural water flows for example, can have direct benefits for native fish, as well as making conditions less favourable for carp and thereby reducing their impact. Therefore carp management should not just focus on physical removal of carp.

There is a widespread belief that commercial use of carp can solve the carp "problem". The reality is that carp are currently a low-value product, which limits commercial offtake to areas that have high carp densities and good access to waterways and markets. Nevertheless, there is some potential to increase the recreational and commercial removal of carp by promoting carp as a target fish and increasing the value of carp products.

Poisoning may be used to eradicate carp from small isolated areas (e.g. farm dams) or when water levels in large public water storages are very low. The possibility of incorporating poison into pellets which could be placed to target the sediment-feeding action of carp is currently being investigated. This would enable poison to be distributed more safely and economically over large areas of open waterway.

Environmental rehabilitation is seen as a way of changing the environmental variables to favour native fish. By potentially increasing native fish numbers, particularly larger predators, predation pressure on carp will be increased.

The use of viral agents for biological control, such as the Spring Viraemia Carp Virus is considered to be unreliable for technical, commercial, conservation and logistic reasons and some sectors of the public have expressed concerns about the use of viral control agents.

Potential genetic manipulation approaches to carp control need to be explored. Potential molecular approaches include immunocontraception to reduce carp fertility and the introduction of a fatality gene into the carp population which can then be triggered chemically or by some other means.

However, there are currently no biological or contraceptive control agents suitable for use against carp, and gene technology is not yet at a stage where it can be used for carp control. Therefore carp management in the immediate future will rely on environmental rehabilitation, physical removal, poisoning where appropriate, and most importantly, reducing their spread.

In addition to becoming involved in habitat restoration and carp removal, the community has a valuable role to play in monitoring of water quality and other aquatic ecosystem indicators and early detection and reporting of the establishment of carp populations in new areas. As part of the latter, local communities could erect signs indicating carp-free areas, with a carp diagram and contact phone number to allow positive identification and reporting of carp sightings. Such signs would increase awareness and community ownership of the issue.

For further information please contact Quentin Hart:

Within Australia Ph: 02 6272 3801
International Ph: +61 2 6272 3801

or email quentin.hart@brs.gov.au

Web site: http://www.affa.gov.au/content/output.cfm?ObjectID=4895611E-6AF1-444B-8FEC2BF6E44EB30B