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Australian Society for Limnology Carp Policy Document

ASL Policy on Carp in Australia

Carp in Australia – the present situation

Huge numbers of young carp are produced
in years of good rainfall, and they migrate
into new habitats.

Carp, Cyprinus carpio, are one of the most significant of the animal species that have been introduced to Australia’s waters. They are also among the most widespread aquatic species; Cyprinus carpio now occurs in every State and is the dominant fish species in rivers of the Murray-Darling Basin, especially below about 500 metres altitude. Carp have become the focus of intense debates, initially about their environmental impacts and, more recently, about potential control methods. In rural communities especially, there is widespread support for the view that carp invasion of Australia’s fresh waters is a national environmental problem. It is a noxious species in a number of States.

Environmental effects

During the rapid spread of the ‘Boolara’ strain of carp first introduced to Victoria in 1961, it interbred with other carp strains that had been in Australia for up to a century. The great numbers that appeared in some waterways, and concerns over their potential impacts, prompted research to clarify the environmental effects of the carp invasion. Many studies have been completed and others are continuing. Following is a brief synthesis of the current results.

The main environmental effect of carp results from their bottom-feeding behaviour, which generally occurs when insufficient preferred foods are readily available in the water column. The fish suck in mouth-fulls of sediment, sifting through it for material that they can digest and expelling the remainder into the water column. Under some conditions this behaviour increases turbidity, releases sediment nutrients, uproots submerged aquatic plants, and prevents plant regeneration.

In inland lakes of the Darling system, fish-kills caused by droughts
show the immense biomass of the dominant carp populations.

The great abundance of carp, especially in the warm, still habitats they prefer, combined with their generally large body size, can result in very high carp biomass. Population densities up to one carp per square metre of water surface area have been recorded. Such dense populations not only have an impact on food-web structure, but also excrete large quantities of wastes into the water. Secondary effects that can follow these environmental modifications by carp include altered aquatic animal communities, changed habitat for various plants and animals, loss of aquatic vascular plants and promotion of nuisance algal and cyanobacterial growth. Direct effects, such as faunal-community modification through predation on the eggs and young of native fish or macroinvertebrates, have been suggested but not yet demonstrated.

The environmental impacts of carp in Australia have occurred alongside other large-scale ecological disturbances such as water diversion, damming, flow regulation, riparian damage, erosion, various forms of pollution including cold water releases from dams, and the impacts of other alien biota. Many of these disturbances interact. For instance, carp succeed better in habitats that are more disturbed by agriculture and river regulation. In this complex situation it is difficult to ascribe general changes to individual causes. As an example, carp are undoubtedly implicated to some degree in the decline of native fish, but they are only one of many causes.

Disease effects

Carp act as vectors for Lernaea, a parasitic copepod that causes
ulceration, weight loss and sometimes mortality amongst a range of other
native and alien species.  They are also vectors for the pathogenic
tapeworm, Bothriocephalus, and other disease agents.

A recent survey in New South Wales showed that the incidence of externally visible abnormalities of fish was correlated with the density of carp. Carp were probably the vector responsible for the introduction and spread of the damaging parasites ‘anchor worm’ (Lernae a) and a virulent tapeworm, Bothriocephalus.

Beneficial values

The great abundance of carp supports a commercial fishery in some areas and provides some recreational-fishing value. Small amounts of fresh or canned carp are used for human consumption, other catch is directed to fertiliser production, or used as crayfish bait. An export market is beginning to develop.

Potential for further spread

The colourful Koi strain, widely promoted as an ornamental fish,
commonly escapes into the environment where it quickly reverts to the wild
type of carp, causing the usual environmental disruption.  Koi were parent
stock for carp invasions in Lake Burley Griffin (ACT), Lake Crescent
(Tasmania), the Murrumbidgee River (NSW) and systems including the
Shoalhaven, Richmond, Hawkesbury, Hunter and other coastal rivers.

As the world’s most widely distributed freshwater fish, carp have clearly shown their adaptability and tolerance of varied conditions. They survive in an extremely broad range of temperatures and water qualities, and tolerate low dissolved oxygen and saline conditions. Carp’s spread in Australia has been facilitated by floods, and inhibited by natural and artificial barriers. But they have continued to progress steadily into higher altitudes through their powerful migratory responses in floods and, regrettably, through deliberate (and illegal) human transportation. Furthermore, both accidental and deliberate transportation by people such as anglers and koi carp breeders have frequently moved them across catchment boundaries. The ornamental koi strain of carp is strongly represented among wild populations and its continuing culture in ponds is a serious source of new infestations. Carp do not survive long in full seawater and therefore cannot migrate extensively along coasts.

The main regions particularly threatened by further expansion of the range of carp include:
• the many coastal river catchments of south-eastern Australia that are still free of them,
• the large coastal catchments of central and southern Queensland,
• inland drainage systems such the Lake Eyre tributaries,
• rivers in south-west Western Australia,
• Tasmanian waters,
• headwaters of streams that currently have carp, and
• waterbodies near population centres where koi are kept

Potential controls

Increasing community concern has driven the search for ways to control carp and a series of groups representing State and Commonwealth governments (particularly the Carp Control Co-ordinating Group and the National Carp Task Force), research bodies and community organisations have been established to pursue effective controls. Sound ecological management would ideally seek to eradicate the species but this is not achievable with existing technology and in the short term. Potential control methods can be classified as targeted fishing or physical removal, chemical and physical control, biological control or environmental rehabilitation.

Powerful electrofishing boats are a potent means of catching carp and
reducing their abundance, at least temporarily, in small-to-medium sized
enclosed waterbodies.

Targeted fishing involves applying recreational or commercial fishing gear to selectively remove carp, mainly the larger fish. This simple approach produces visible results, and has achieved considerable public support. It is limited by the need to continue efforts indefinitely, the relatively small number of areas where it can be applied and the marginal economic viability of commercial projects. It can be used in small, enclosed waterbodies. Large fyke nets (> 2m mouth opening) have recently been shown to be effective gear in such waters.

Fish are drawn to the electrofisher's electodes 
and immobilised, so they can be dipnetted from 
the water.

An important problem with targeted fishing is that there is preliminary evidence suggesting that carp populations may be self-limited by a feedback mechanism, whereby adult fish in dense, highly competitive populations eat their own eggs and larvae. If adult numbers are reduced, the survival of young carp may be greatly enhanced, so that the total numbers of carp actually increase and a strong new generation emerges. Another problem is that, if carp-based industries become established, the introduction of biological control methods will inevitably be resisted.

Chemical control using ichthyocides such as rotenone have often been used in the past to prevent carp spreading in new catchments, but these are limited-scale projects with significant impacts on other aquatic biota. A new project by NSW Fisheries is developing a carp-specific poison-baiting method to avoid this problem and to improve efficiency. Work by the Cooperative Research Centre for Freshwater Ecology is examining pool draw-downs to desiccate eggs after spawning as a way of limiting population recruitment.

Initial studies have been done by Victorian Fisheries and CSIRO to evaluate a virus pathogen – Spring Viraemia Virus of Carp (SCVC) - as a biological control agent. However, the unmodified virus is inactive over much of the seasonal temperature range of Australian lowland rivers, and it is not totally specific to carp. A current project in CSIRO is assessing the feasibility of genetic manipulations to restrict breeding or induce early mortality. Biological control through re-introduction of hatchery-reared predatory native fish has been attempted, but apparently with little success.

Environmental rehabilitation as a carp-control method is based on the knowledge that carp are more successful in damaged habitats. Carp are increasingly seen as a symptom of degradation, as much as a cause. Rehabilitating damaged freshwater ecosystems is predicted to increase their resilience and resistance to invasion by alien species like carp. Plans for rehabilitating ecosystems must include restoring fish passage past dams and weirs, providing environmental flows, rehabilitating riparian vegetation, re-establishing more-natural water temperature regimes below dams, and reducing salt levels.

Of the various approaches being investigated, environmental rehabilitation is presently considered to be the most likely to produce significant, widespread reductions in carp numbers for the foreseeable future.

Knowledge gaps

Gaps in knowledge about carp in Australia continue to drive research. Studies to evaluate control methods remain among the highest-priority national environmental projects. These should especially include biological methods that have potential for basin-scale control. The interactions of carp with their habitats and other biota continue to be an important gap in ecological and management knowledge. Bioturbation of sediments is a key process to be addressed, together with its consequences for nutrient release and implications for algal growth. There is an urgent need to improve understanding of carp’s vector role in fish diseases. Modelling studies of carp’s significance for nutrient dynamics and problems with cyanobacteria are likely to provide important insights for water managers and to drive efforts for better control. Greatly improved knowledge of carp population dynamics, including recruitment success, population limitation and movement patterns, is needed to develop models to support carp-control programs. A comprehensive understanding of the economic implications of carp in Australia is also needed.

The ASL goal for carp is: