They are the greatest threat to the wildlife in our rivers since the last ice age. They are as alien as the grey squirrel and much more dangerous to our environment. They are impossible to control and carry a fungus that is deadly to our native species. They are signal crayfish, or Pacifastacus leniusculus to give them their proper name. They have only one redeeming feature: they are superb to eat, a view shared by otters, which devour large quantities of them.

They were introduced in 1976 to be farmed for the gourmet market. It is the old story of a foreign species being allowed into this country and then escaping into the wild. There were, of course, dissenting voices who saw disaster looming, but they were shouted down in an orgy of greed and stupidity. Our native variety, the white-clawed crayfish (Austropotamobius pallipes), was to be found in almost all of our Southern waterways, provided they were unpolluted. It was particularly carefully monitored in the Dowles Brook in the Wyre Forest of Worcestershire where it was plentiful; the locals enjoyed crayfish feasts at harvest time. By 1988, after the brook had been overrun by the signal species, there was not a single native to be found. Later, in 1990, the River Camlad had a major killing, followed by the River Clun in 1991. The same is now true all over Britain. The indigenous crayfish has been almost completely wiped out to the extent that it is now a protected species.

The white-clawed crayfish really has no chance against the American invader. They compete for the same environment but the mature American can grow up to 15cm in length while ours is only 10cm at its longest. The former breed about a month earlier than the white-claws, and produce more eggs per female. Young signals are not only more numerous but can also establish themselves in favourable locations before the white-claws have even hatched. The signals maintain their advantage over the white-claws as they grow to adulthood. They have a head start in feeding, a faster growth rate, are more aggressive and are also more tolerant of pollution. Because the white-claw breeds later than the American, the young hatch from the eggs in June and July to find the baby signals already there. Many of the signals carry the fungal disease Aphanomyces astaci, which kills the native crayfish within 10 days. However, the signals are immune.

If our rivers and lakes were to be deprived of an old, indigenous inhabitant this would indeed be tragic but it would not be disastrous. However, the danger presented by the signal crayfish is far greater than this. They are omnivorous and voracious consumers of almost anything smaller than themselves. Fish spawn is high on the list of their favourite foods. Thus coarse fish, which breed between mid March and June, are particularly susceptible to their predations. Game fish do not suffer as badly as their spawning time is in the winter, when many signals are hibernating. Water plants and weeds, needed as sanctuary for fish, fly larvae and nymphs, are devoured in large quantities. For anglers they are a complete pest, removing any bait from the hook but being uncatchable on rod and line.

Their habitat is underwater holes and if they cannot find any already made by water rats or sand martins they will burrow for themselves, undermining and destroying the banks. During the cold weather they virtually hibernate in these holes. Last winter a keeper in Hampshire was working on his river, digging up a section of the bank. At least 6ft into it he was somewhat surprised to find a hibernating signal crayfish.

The signals are great survivors and very adaptable. Now that they are established in our waterways there is little that we can do to eradicate them. They are almost certainly impossible to remove from the British Isles. The only truly effective method of destroying them would be to poison them but this would, of course, affect fish and other species. It is therefore considered more important to protect the areas where native crayfish are most abundant rather than trying to eliminate the invaders altogether.

The most effective method of limiting the spread of the signal is by trapping. The traps are similar to small lobster-pots and work on the same principle. Dave Culley has been a keeper on the River Kennet for the past 40 years. He remembers the time when the river crawled with native white-clawed crayfish and the nearby Dundas Arms Inn had a crayfish feast every September.

“I have about 30 to 40 traps in the river from the end of March until about mid November. I have an average catch of 300 signals per day or 12,000 per year. I used to use plastic pots, which are cheaper, but when they have been in the water for six months I found it to be a false economy as the crayfish destroyed them with their claws,” says Dave.

KEEPING YOUR TRAPS SHUT

“Now I use metal traps, which are much more expensive but last a long time. As you can imagine,” he says, “I use a great deal of bait and therefore I try to find all of it on the estate as cheaply as possible. The best is a piece of venison because it smells gamey but almost anything will do. Road kill gives me a lot of free bait and I use any dead, unwanted fish such as pike as well. Towards the end of the summer, when the crayfish are coming up to their spawning season, female crayfish are very effective bait as they give off pheromones that attract the males. (You can tell the difference between the males and females because the males have bigger claws.) However, for the amateur who only has one or two pots and merely wants a few crayfish for his family, I’d recommend Kitekat.”

I ask Dave where the best places are to place the pots. “The crayfish seem to like shady places, so under willows and overhanging trees. They are most active at night and I check and rebait my traps almost every evening just before dusk.” I ask him what he does with his catch. “I sell most of them to local pubs and restaurants,” he said. “They give me £10 per kilo, which helps with the cost of the traps.”

It would seem from all this that it would help greatly if everyone bought traps and fished them in our waters. However, it is not quite as easy as that. A licence has to be obtained from the Environment Agency, which will want to ensure that the trap user knows the difference between the white-clawed crayfish and the American signal. It will supply tags that have to be attached to each trap. Also, the owner of the water must give his assent to the trapping.

There is one other fact which anyone considering trapping signal crayfish should know: they are masters at escaping. If this happens and they reach a river or lake which has so far not been invaded, and you release them, a very heavy fine could be imposed. So, please ensure that your crayfish are securely held until you can cook them.

COOKING CRAYFISH

If the crayfish have come from a muddy-bottomed river or lake it is best to put them into clean water and leave them for 24 hours so that they rid themselves of all their muck.

Allow 15 crayfish per person as a starter. Make a stock consisting of three teaspoons of salt to every two pints of water. Place a good handful of dill in the water together with a four-inch root of fresh ginger chopped very finely. Bring this to the boil.

After it has boiled for five minutes remove the dill as it will make the stock bitter if left in. Then put the crayfish into the boiling water and immediately remove the pot from the heat. Allow to cool completely, preferably overnight, so that the crayfish will become infused with the dill and ginger.

Remove the crayfish from the stock, drain and place them in a fridge before eating. Serve with crème fraîche, garlic mayonnaise or curry mayonnaise. It will be, as Izaac Walton said, a dish fit only for kings or very honest men.

CLAW AND ORDER

Environment Agency

Tel 08708 506506,

visit the website.

Collins Nets

Will sell in lots of 10 only. Goods Yard, Station Road

West Bay, Bridport, Dorset DT6 4EW, tel 01308 427352, visit the website.

Sharnbrook Tackle

Traps from £15 each.

12 Perring Close, Sharnbrook, Bedfordshire MK44 1JE, tel 01234 781902, visit the website.

Jarnn Fishing

39 Florian Avenue,

Sutton, Surrey SM1 3QH.

The Jarnn Fishing nets are particularly good for the optimistic amateur because they are collapsible.

Email John Clarke.

The Crayfishking website. is a website dedicated to all things crayfish, complete with a link to specialist auctioneers. When I last checked the site one of the lots was a crayfish poacher’s bag.