Rural: A sting in the tale of the crayfish

When thousands of crayfish were found dead in the river Avon, sheep dip was immediately suspected.

Early on the morning of Thursday, 17 April, Alan Jones went down, as he always does, to inspect the river Avon where it runs past his cottage in the village of Pinkney, Gloucestershire. To his consternation, he saw that the bottom of the stream was covered with dead and dying crayfish; many were upside down, and those still the right way up were barely moving.

Alan - a carpenter and lecturer in construction techniques - has lived there, and fancied the crayfish, for most of his life. Indeed, when he was a boy his family frequently ate them; but since they became a protected species he has been an observer only. It was immediately clear to him that some ecological disaster had taken place, for until then the Sherston Avon had been one of the few remaining habitats in which native crayfish survive. He therefore rang the Environment Agency's emergency line, and within 15 minutes an expert arrived.

At first biologists feared that the mortality was due to the plague imported by American crayfish early in the Eighties - a fungal disease, carried by spores, against which the English species has no resistance. So deadly is it that when it got into the Hampshire Avon it killed the entire population -millions - in the space of two weeks.

Plague, however, was ruled out by analysis of bodies recovered from the river near Sherston, which showed that the killer agent had been the insecticide cypermethrine, used in sheep dips and sprays. Today, nearly a month after the incident, the hunt is still on for the source of the poison, which has wiped out not only the crayfish but also most other invertebrates along at least three miles of the waterway.

Initial suspicion fell on recently-dipped sheep upstream at Badminton, where drainage ditches - known as winter-born streams - wind through lush watermeadows and run into the river. The chemicals used in dips are so toxic that even if a sheep merely strays, or falls into the water, it can wipe out all invertebrate life for 100 yards or more. In the past few days, however, the agency's investigating officers have been concentrating on other possibilities.

Compared with the catastrophe of the burst dam in the Coto Donana, this is a disaster in miniature. Nevertheless, it is intensely disappointing for British conservationists, because an enormous effort has gone into preserving the integrity of the Sherston Avon, and several batches of healthy crayfish have been taken from there to restock less fortunate stretches of the river.

Not that all the work has pleased local people; many have objected to the fences recently built along the river to preserve the banks and keep farm animals out of the stream. Wooden posts and barbed wire, they say, spoil the appearance of the gentle valley. Perhaps they do - but they also prevent encroachment by cattle, reduce erosion and improve the habitat for many river creatures, not least crayfish and water voles. Feelings about the fences run so high that hints have been dropped about possible sabotage; could someone have deliberately dumped something in the river to settle scores with the agency?

That seems pretty far-fetched. Yet certain it is that the people who began bringing crayfish in from America have a lot to answer for, because the importations produced a classic illustration of the folly of introducing alien species into an environment that lacks the controls and balances to keep them in check. Like the grey squirrel, the mink and the rainbow trout, the American signal crayfish is by no means a welcome immigrant. Being larger than our own, more aggressive, and omnivorous, it has spread relentlessly through our river systems, eradicating the natives by eating them, driving them off their own territory, and giving them a fatal disease.

At one stage signals were widely advertised as a fine commercial proposition. Anyone who owned a pond was incited to farm them: seed the pond with nippers, you were told, and in a couple of years, at no further cost, you could haul out lobsters. What nobody realised was that signals are amphibious; on wet nights they take to the fields and crawl overland into streams. So they invaded our rivers, with disastrous effect.

Some of them are resistant to the plague, and may survive indefinitely. Why, then, should they not eventually replace our native stocks? "Because our freshwater fauna have evolved over thousands of years, since the last ice age, to live in harmony with each other," says Dr Nick Giles, a freshwater fisheries consultant. "Nobody knows whether, in the long run, signals will fit in." As a black mark against them he cites their habit of clipping off weed near the bed of the stream - something English crayfish do not do - and so clearing whole patches of habitat.

No one is yet sure what the long-term effects of the pollution at Sherston may be. Alan Jones noticed that the brown trout, which he feeds every morning, showed no interest in his offerings for the next three days. Then they appeared to recover their appetites - but what is going to happen to their offspring? The insecticide also wiped out mayfly larvae: Martin Frayling, the agency officer first on the scene, saw "hundreds of thousands of them dead on the river bed". The main food supply for trout fry and other small fish has gone.

Whatever the outcome, all river managers remain acutely aware that farmers are regularly using substances so poisonous that even extremely low concentrations are lethal.