"It would be fair to call signal crayfish the grey squirrels of Britain's waterways," says Mary Gibson, a freshwater ecologist at English Nature, the body that advises the government on conservation issues.
"It's bigger and more aggressive than the native, white-clawed, crayfish so it out-competes them as well as preying on young white-claws. Worse, the signals carry a plague which is devastating the natives, but to which they seem immune."
How did this situation arise? Following the successful introduction of rainbow trout in the Sixties, the Ministry of Agriculture, Fisheries and Food (MAFF) was looking for another aquaculture crop and spotted imported crayfish on the menus of exclusive restaurants. There was clearly a market for this lobster substitute. The American species (Pacifastacus leniusculus) seemed perfect for the job because it grows faster and bigger than our native species.
"They could be farmed easily, had high margins and there were ready markets both at home and abroad," explains Ms Gibson. In 1976 the first specimens were imported by a Dorset farmer. Unfortunately, little research had been done on the potential damage that could arise if these crayfish established themselves in the wild. And this didn't take long, as crayfish are at home both in and out of water. In no time dozens of farm crayfish clambered out of their enclosures to trundle off in search of pastures new.
Reports of escapees were soon coming from across southern England, but at first there was no indication of the devastation they would wreak. In 1981, however, an angler near Bristol noticed native crayfish were missing from his favourite fishing spot. On investigation, the river bed was found to be littered with dead white-claws and within months this was being repeated across the country. The killer turned out to be a fungus which had been known on the Continent since the 1860s, but until then was unknown in Britain. It transpired that the main link between these outbreaks of plague was the proximity of fish farms where signal crayfish were being bred: signals could carry the disease, but were immune to its effects.
The plague is still scything through British waters and the once-common native species is now a rarity in most English rivers and streams. The situation is particularly acute south of a line running from Bristol to the Wash, but in just 20 years the invaders have spread rapidly from their original release sites and now the first reports are coming in from the vast Trent and Severn catchment areas. In contrast, once established signals can reach very high densities indeed. On parts of the Thames and Kennet anglers find fishing almost impossible because signals are so quick to steal the bait.
The scale of the disaster is particularly alarming as native crayfish are reliant on clean water and were already suffering from agricultural pollution. Today they are one of our most protected water creatures: their capture is illegal under the Wildlife and Countryside Act and they are listed as one of 116 priority species across Europe. In contrast, the pollution-resistant invaders are recognised pests and since this spring, moving them to new sites has become illegal. The authorities are particularly anxious to prevent their arrival in Wales and Scotland where so far they are largely absent.
Although much of the colonisation has been under the signals' own steam, all too often, they move with human help - either accidentally in fish consignments or as deliberate introductions. At prices reaching pounds 4 a pound, there is money to be made from ranching wild stocks, avoiding the costs of farming.
"It is outrageous that they can still be bought in pet shops as 'freshwater lobsters' to be put in garden ponds," comments Ms Gibson.
English Nature is concerned by the proliferation of hunters. "Adult signals are much bigger than native crayfish, but are not always easy to identify," says the ecologist. She is worried that some isolated, surviving pockets of white-claws could be wiped out in a case of mistaken identity. Despite the efforts at stemming the invading tide, it looks as if yet again we are trying to shut the door long after the horse has bolted. If the aliens really are here to stay, it could signal - all too literally - the end for our native species.
A Guide to Identifying Freshwater Crayfish in Britain and Ireland is available from the Environment Agency's enquiry line 0645-333111Reuse content