Fishing lines: The crays: rise of an evil empire

Berkshire's River Kennet is a rare place. You can fish all day, catch nothing, and still go home with enough protein to feed the whole family. Trouble is, this supper is aggressive, mean-tempered - and shouldn't be in the river in the first place.

Berkshire's River Kennet is a rare place. You can fish all day, catch nothing, and still go home with enough protein to feed the whole family. Trouble is, this supper is aggressive, mean-tempered - and shouldn't be in the river in the first place.

The North American signal crayfish came to Britain in the 1970s. A prolific little breeder, it was established in fish farms to provide food for restaurants. But the crayfish had other ideas. It preferred eating to being eaten, and escaped from captivity (crays can walk overland). Within a couple of years, it had colonised rivers throughout South-east England. Now, I'm told, it is invading the North and Scotland. It's a perfect example of our stupidity with alien species.

Good news, you might think, for those who enjoy a dish of crayfish tails, a delicacy that can cost more than £10 a pound. But the crays are well named. Like their East End namesakes, they have built an empire of underwater terror. Not content with driving out our smaller, native white-clawed crayfish, they have infected it with a deadly disease that has eradicated it in many rivers.

Once established, signal crayfish are devils to drive out. A friend of mine, Ed, who has a stretch of the Kennet, reckons he has trapped several thousand, but says his Dalek policy appears to have affected the population not one jot. They are not picky about what they eat, either. Ed catches them in nets baited with lumps of potato, but you can also tempt them with a dead crayfish, for the species is cannibalistic.

Crays are bad news for anglers too. Hook on a maggot, lump of luncheon meat or a worm, and within minutes the crayfish have eaten it. They often use their sharp pincers to cut through the line, and if they are feeling really bloody-minded, they will hang on to the bait and argue it out with you on the bank. It's like getting into a scrap with a baby lobster.

Small fish are a rarity on the Kennet nowadays. There are so many crayfish that they gobble up much of the spawn, and attack baby fish too. It is now a big-fish river, the surviving species being those such as barbel and chub that are partial to a crayfish. Fishing the Kennet last week, I caught nothing. Not a tiddler. But my companion Mark caught 10 chub: the smallest 4lb, the biggest over 6lb. I met an old friend on the same day. A few months ago, he caught a 16lb 3oz barbel. A few years ago, that would have been nearly 2lb over the British record.

Environment Agency attempts to curb the crays' advance have been signally, as it were, unsuccessful. They have been experimenting with underwater baskets laced with female crayfish pheromones. But Ed's lumps of potato work just as well.

Neither method appears to dent their numbers. As was said of the Americans during the Second World War, they're over-aggressive, oversexed and over here - and we're being overrun. Look out for them moving into a river near you.