Killers on the loose

Despite its sweet appearance, the domestic goldfish poses a serious threat to our only native carp species. Simon Hadlington reveals the dangers of setting free your pets

Meet the crucian carp - the hard nut of Britain's freshwater fish. The Carassius carassius, to give it its proper name, is Britain's only native carp species. It may not grow as quickly or as large as the non-native common carp, and may not be as prized by specimen-hunting anglers, but it is much, much tougher. It can survive a vast range of temperatures, from an almost slow-cooking 38C down to 0C. It has even been known to hibernate in mud while the water above it has been frozen. It can tolerate highly acidic water, and it can survive in water where there is virtually no oxygen - conditions that would be fatal for almost all other freshwater fish in this country.

But the hardy crucian is under serious threat, and one of the unlikely villains of the piece is the domestic goldfish. Detective work by genetics experts has revealed that goldfish released into the wild can breed with crucian carp to produce hybrid fish, and that these, in turn, can breed among themselves, or back-cross with pure crucians. Such contamination of the gene pool could have disastrous consequences for the crucian carp, and experts are drawing up plans to protect the species.

Philip Bolton, a fisheries officer with the Environment Agency, has been investigating the threats to the crucian carp. "The first problem is loss of habitat," he says. "Crucians prefer small ponds - the sort that used to be found on farms everywhere - but many have silted up or been filled in."

The other main problem is competition from non-native fish species that have been introduced into waters, usually to improve the fishing. The main culprit is the common carp, an aggressive, voracious feeder that uproots plants at the bottom of lakes and ponds, disturbing the silt and making the water cloudy, which in turn makes it difficult for plants that are part of the crucian carp's diet, to re-establish themselves.

The crucian is also susceptible to non-native parasites, including a tapeworm that is thought to have been introduced by another popular non-native species, the grass carp. "As far as we know, the crucian carp has no natural tapeworms, so it is ill-equipped to deal with them," says Mr Bolton. "It impacts on the fish very heavily." Recently, a nematode worm has also been identified that can infest the crucian, and it is believed to have come from goldfish.

But perhaps the most insidious threat is from hybridisation - native, pure crucian carp cross-breeding with non-native species in the wild, creating aggressive hybrids that might in turn continue to breed and cross-breed and gradually contaminate the finely tuned set of genes that have made the crucian carp such a hardy fish.

To discover if this is a genuine threat, scientists need to know for certain whether crucians can interbreed with other species in the wild - in particular, with goldfish. It has been suspected for some time that goldfish do cross-breed with crucians, but not confirmed, and hybrids are difficult to distinguish purely by visual inspection.

The key fact is that only a relatively small number of goldfish are bright orange. In a brood of thousands, most will be brown, and will, superficially, resemble crucian carp. Commercial goldfish breeders often sell these brown goldfish to fishing clubs to stock their waters. Also, domestic goldfish are often released into the wild when people no longer want them as pets.

To discover whether interbreeding does occur, the Environment Agency sought the help of gene detectives at the Molecular Ecology and Fisheries Genetics Laboratory at the University of Hull. The scientist who carried out the investigation was Dr Bernd Hänfling. "The aim of the project was to find a way to unequivocally differentiate between the three species of carp - crucian, common and goldfish - and to find out if there has been any cross-breeding," he says. "We approached this via genetic fingerprinting."

The basis of genetic fingerprinting is to identify a short stretch of genetic material, DNA, which has a common pattern in each of the species under investigation, but that differs in its length. These segments of DNA are termed microsatellite markers. So, for example, a particular marker might be on average 100 units long in the goldfish; 85 in the common carp; and 65 in the crucian carp. A pure-bred goldfish would have markers only of 100 units, and a crucian carp only of 65 units. However, if a pure-bred goldfish successfully crossed with a pure crucian carp, one parent would contribute a marker 100 units long, and the other a marker of 65 units. In this case, analysis of the DNA would show both markers present - indicating a hybrid.

"Isolating these markers is technically extremely challenging," says the Hull laboratory's Professor Gary Carvalho. "However, Bernd succeeded in identifying 10 such markers that were common to each of the three species, and produced a specific signal within each species."

So, when a sample of tissue from a fish was sent to the laboratory, the team could extract its DNA, identify the markers, and confidently assign a genetic history to the sample. By using this technique, Dr Hänfling confirmed that the samples he analysed included hybrids between goldfish and crucian carp, and hybrids between crucian and common carp. This was clear evidence that interbreeding does indeed occur, as the fisheries scientists suspected.

But of more concern was the fact that the genetic analysis showed that a small number of hybrid individuals had interbred or "back-crossed" with pure stock. "Of 250 samples I analysed, two were back-crosses," says Dr Hänfling. "This isn't a lot, but it indicates that it can happen."

The phenomenon of hybrids interbreeding or breeding with pure stock is called introgression. Professor Carvalho explains why it is an important issue. "The key is to protect the gene pool of the native species," he says. "Hybridisation can result in the degradation of the ability of the fish to survive in its native habitat. Once you start to get introgression, you start to get contamination of the gene pool, which can be detrimental to the survival of the species. This has been seen where escaped farmed salmon breed with wild salmon, resulting in hybridisation and introgression. The resulting offspring are less fit than their pure-bred cousins to withstand the rigours of the environment, and have lower resistance to diseases and have confused migrating behaviour, for example."

For Philip Bolton, the Hull research provides the hard evidence that scientists need in order to devise appropriate conservation strategies for the crucian carp. One fear is that the hybrids might become more aggressive than the pure species, and out-compete them. "I was impressed with the power of the DNA technology," says Bolton. "This work will give us solid scientific support for any policies we introduce to protect the native crucian population, and will help us to produce our updated field guide to crucian carp, which will in turn help in their identification. The issue of introgression is interesting - and disturbing. We have anecdotal evidence that in waters where it is suspected that hybrids exist, there has been a dwindling of the native population."

Bolton hopes to organise a nationwide audit of pure crucian stocks in the country so that these populations can be protected from contamination by goldfish and common carp. "There is no doubt that the crucian-carp population is under serious threat, and this has been further confirmed by this study," he says. "As things stand now, it is not likely that the crucian will be wiped out, but there is an outside potential for that to happen if we don't act now."

In the meantime, Bolton has advice for anyone who can no longer look after their pet goldfish. "Releasing fish into the wild is illegal without consent from the Environment Agency," he says. "Take the goldfish back to the person who sold it to you, or give it to a neighbour with a pond. If neither of these are possible, contact your local Environment Agency office for advice."