Carp crusades: Steve Davidson gives the European carp a fair trial, and outlines a potential genetic route to its long-term management.

Abstract: European carp are widespread in the Murray-Darling Basin and occur in all states and the ACT. They are blamed for many aspects of river and wetland degradation, it is unlikely that carp are responsible for declines in native fish or riverbank erosion, but they do cause turbidity, reduce aquatic vegetation and probably increase the occurrence of algal blooms. Successful carp management will require both control techniques and river rehabilitation. A potential long-term control method, daughterless carp technology, entails altering the genetic make-up of carp so that fewer and fewer females are produced from one generation to the next. The technique may be ready in about seven years.

Keywords: European carp, introduced species, feral animals, pest control, environmental degradation, Daughterless Carp Project, genetic manipulation.

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Cast a line into almost any Australian river, stream, lake or billabong and you're more likely to haul in a plump European carp than a native fish.

Admittedly, the European carp is one of the most abundant and widespread large, freshwater fish on Earth, but how has an introduced species become so dominant in Australia? What impact is this rabbit-of-the-river having on the health of our precious waterways? And what is being done about it?

The carp (Cyprinus carpio) actually originates from central Asia, spreading from there to China, Japan, southern Europe, and thereafter to every continent.

Although the carp was introduced to Australia in the 1850s as an ornamental fish, it had little impact until the 1960s when the Boolarra strain was released from a fish farm into the Murray River, near Mildura, Victoria. This invasive strain took a liking to conditions here and the carp population exploded, aided by floods in 1974 and 1975.

Carp are now widespread in the Murray-Darling system and are present in all states and the ACT. They continue to colonise new waterways, often helped by humans, including fishermen using carp as live-bait.

Key to the success of feral carp is their great fecundity (each female carp can lay a million or more eggs a year) and their ecological flexibility (see story opposite).

They are most at home in warm, still waters, such as those of inland southeastern Australia, but can adapt to a broad range of environments, even saline waters. A 1997 survey of rivers in New South Wales recorded carp virtually everywhere except unregulated (free-flowing) coastal rivers and high mountain streams.

Carp now comprise at least 80%, and often more than 90%, of fish biomass in the Murray-Darling Basin. In the lower reaches of the basin's Bogan River, scientists found one carp for every square metre of water. Not surprisingly, carp are generally despised, and they are blamed for many aspects of river and wetland degradation.

Marshalling forces

The scale of the problem is immense. Carp are now a national issue. With the Murray-Darling Basin being the epicentre of the carp invasion, early responses have come from the Murray-Darling Basin Commission (MDBC) and CSIKO, as well as from local government and the Murray Darling Association. In 1996, the association sponsored the formation of the National Carp Task Force, a community forum for addressing carp management and control.

A Carp Control Coordination Group, managed through the MDBC, has developed a National Management Strategy for Carp Control, a strategic research plan and a Guide for Carp Management Groups. It also promotes liaison between the many groups involved in carp control.

The Bureau of Rural Sciences has produced a publication, Managing the Impacts of Carp, that provides the scientific basis of the National Management Strategy. CSIRO is working on control techniques.

Carp crimes

Most people loathe carp. Their abundance and rapid spread causes great concern about their effects on water quality and native fish.

While it seems obvious that feral carp, being the predominant fish in many waterways, must have deleterious impacts on aquatic ecosystems, it is also likely that they are also blamed for some environmental problems in fact caused by humans. They are possibly the first scapegoat to have fins and scales.

Research confirms that carp:

* increase water turbidity, or cloudiness, due to their manner of feeding;

* reduce submerged vegetation by uprooting aquatic plants or, indirectly, by stirring up sediments, reducing light available to plants; and

* influence the frequency of algal blooms by increasing nutrients in water. These are significant crimes.

Carp are also blamed for everything from riverbank erosion to spreading disease to declines in native fish numbers. Is this really the case?

By feeding at low levels of the food chain, carp could theoretically act as an energy trap, preventing the flow of energy to populations of other fish species. But there is little evidence to support accusations that carp have caused declines in native fish, spread fish diseases or caused riverbank erosion.

Sole cause or collaborator?

Scientists have found that native fish populations were heading downhill years before carp numbers exploded.

Commercial catches of Murray cod, silver perch and golden perch in the Murray-Darling Basin declined in the early 1960s, well before the rapid expansion of carp between about 1968-69 and 1974-75.

This dispels the notion that carp are the main cause of native fish decline in Australia. Some native fish actually feed on small carp, so it's a complex picture.

It seems likely that carp have gained a competitive advantage and benefited from changes made to the character of most inland rivers.

We have regulated the flow of rivers so that natural cycles of drought and flood are less prevalent, salinity has increased due to clearing of vegetation, and sediment loads and nutrient levels have increased as a result of pastoralism and agriculture. Dams limit the movement of fish.

On balance, these modifications seem to have favoured carp over native fish and have caused much of the environmental degradation for which carp are held responsible.

Illustrating this, scientists have found that in four different catchments in the Murray-Darling Basin, interference with the natural flow regime in increasingly regulated catchments was associated with lowered fish species diversity and greater carp abundance.

For example, the free-flowing Paroo River had much greater diversity and fewer carp than the highly-regulated Murray River. The Darling and Murrumbidgee Rivers were in-between.

The stable conditions produced by regulation of river flows for irrigation definitely seem to favour carp.

A recent experiment, in which researchers manipulated carp biomass by dividing billabongs into halves with high and low carp abundance, showed that carp had a significant impact on turbidity and intensity of algal blooms. Factors other than carp, however, usually contributed to most of the variation in water quality.

Carp also had a negative impact on the beneficial biofilms that developed on wood placed in the billabongs, but the responsible mechanism varied between billabongs.

These qualified results highlight the difficulty in teasing out the impacts of carp on wetlands.

Because the carp population explosion coincided with marked degradation of our rivers, it is easy to conclude that carp are the cause of it, but they could just as easily be a symptom.

The Carp Control Coordinating Group suggests that, generally speaking, carp are probably both a cause and an effect of environmental degradation of our inland waterways. Whether predominantly a villain or scapegoat, carp are certainly a part of the problem and need to be managed together with other impacts. What, then, are the options for control?

Carp management

Scientists are unanimous that, with current technology, nation-wide eradication of carp is little more than a dream and the emphasis has shifted from simply killing carp to `strategic management of carp and their habitats to minimise the damage carp cause'.

This is not to say that small, localised populations of carp cannot be eliminated. In Tasmania, 20 small populations were eliminated in the 1970s, but carp were reintroduced to the state two decades later. They are still there.

Potential techniques for carp control include: commercial harvesting and poisoning (the main methods used to date), genetic manipulation, biological control with a species-specific virus, immuno-contraception, and environmental restoration.

The National Management Strategy states that successful carp management will involve a combination of control techniques aimed directly at the fish itself and integration of these techniques with other measures for river rehabilitation.

Commercial harvesting, at first glance, seems an ideal control measure. Lots of carp are killed and fishers generate a saleable product and make a living.

However, for established carp populations close to carrying capacity, such as those of the Murray-Darling river system, harvesting is likely to have little effect on the impacts of carp. Models indicate that as carp densities decline, survivors have more resources available to them. Once fishing stops, the population rebounds.

Dr Ronald Thresher, of CSIRO Marine Research, says that on available evidence `physical removal, by commercial harvesting, recreational fishing or scientific harvesting, will in most instances have only a small-to-moderate impact on carp numbers'. He points out that heavy fishing pressure could also put less abundant native fish at risk.

A dilemma also arises because, when it comes to fish management commercial fishermen, and others who see carp as a valuable sustainable resource, may not see eye-to-eye with those who see carp as a repulsive pest and `the fewer the better'.

River rehabilitation

An approach gaining increasing favour is that of environmental rehabilitation. This involves indirect carp control through restoration of ecological processes in our disturbed and degraded rivers and wetlands.

The idea is to bring back the natural resilience of ecosystems and limit the success of the resourceful carp.

This environmental rehabilitation of our river systems or catchments can be a classic win-win activity because, while many of our rivers are now unsuitable for native plants and animals, but happily inhabited by carp, habitat improvement should make them less suitable for carp and more favourable to native species.

The approach also tics in with programs to save our rivers. Carp control is but one of a suite of actions needed to rehabilitate Australia's once magnificent river systems.

Daughterless carp

Perhaps at the other extreme of control options is the prospect of interfering with the carp's awesome capacity to reproduce.

Scientists at CSIRO Marine Research in Hobart have developed a way to sabotage carp populations by manipulating their genes. The technique, developed using zebra fish as a `model' animal, has been dubbed `daughterless carp'.

Thresher and colleague Dr Nic Bax say the idea is to modify the embryonic development of carp so that all offspring are males.

In fish, as in many other lower vertebrates, at an early stage of embryonic development of females a certain gene `switches on', triggering the release of a hormone that causes the embryo to henceforth develop as a functional female.

By altering the genetic make-up of carp, the CSIRO scientists introduced a mechanism for blocking or `switching off' this gene so that genetically female fish are steered down the (default) male developmental pathway. Instead of developing into females, they become pseudo-males or, in scientific jargon, `neomales'. Hence the term daughterless carp.

Neomales are otherwise quite normal. When they reach sexual maturity and breed, as males, because they carry copies of the genetic blocker they also produce only male offspring, and so on, from one generation to the next.

So in the carp population as a whole, fewer and fewer breeding females are produced with every generation until the carp population consists mostly of lonely males. This, at least, is the theory.

`We have demonstrated that the sex-biasing method works in the laboratory with zebra fish and we have isolated the gene in carp, but we have a long way to go,' says Thresher.

`We believe that this technology has the potential to greatly reduce carp numbers in the Murray-Darling Basin within 20-30 years of releasing carriers of the daughterless carp gene.'

Partner in the project, Jim Barrett of the MDBC, which supports the project, says that, before release, they need to refine the method in carp, assess the risks, seek regulatory approval, consult communities and obtain public and industry feedback.

CSIRO will continue to meet regulatory requirements for development of the technique and will work closely with the MDBC in responding to risks and concerns. The technique does not involve the use of foreign genes from other species and it will not affect fish other than carp.

If all goes well, field releases of carp will begin in about seven years time, using carp carrying altered gene sequences. It seems to offer hope for long-lasting and widespread control of carp when integrated with other methods of carp, management, such as fishing, habitat rehabilitation and more natural river flows.

More about carp

Roberts J and Tilzey Reds (1997) Controlling carp: exploring the options for Australia. Proceedings of a Workshop, 22-24 October, 1996, CSIRO Land and Water.

Koehn J Brumley A and Gehrke P (2000) Managing the Impacts of Carp. Bureau of Rural Sciences, Canberra.

Carp Control Coordinating Group (2000) National Management Strategy for Carp Control. Murray-Darling Basin Commission, Canberra.

Braysher M and Barrett J (2000) Ranking Areas for Action: A Guide for Carp Management Groups, Murray-Darling Basin Commission, Canberra.

Carp Control Coordinating Group (2000) Future Directions for Research into Carp 2000-2005.

RELATED ARTICLE: A piscatorial profile.

CARP ARE not only a big problem, they are often just plain big. They can reach a length of 120 centimetres and weigh in at 60 kilograms. Total biomass can be upwards of 1500 kilograms per hectare and they have colonised many inland waterways.

This success of carp in Australia is largely attributable to their adaptable biology and ecology. They can tolerate a broad range of environmental conditions in both still and flowing water. They thrive in degraded habitats and can even tolerate water that is up to half as salty as seawater.

Young carp eat plankton, switching to larger items as they grow. Adults are generalist feeders, eating plant material, crustaceans and aquatic insects. Carp feed by filtering small particles from the water or sieving food from the bottom sediments.

The fish mature in two to five years and females lay numerous eggs on plant material, in spring to early summer, when water temperatures exceed 16[degrees]C. Young carp are preyed upon by birds and other fish, including Murray cod. Up to 98% of young fail to survive the first year of life. Survivors though can live for decades.

Carp get around. Tagged carp have moved more than 200 kilometres in a few months and they can migrate at any time of year. One could characterise carp as opportunistic fish that are good at feeding, breeding and spreading.