Monbiot cooks up revenge on invasive signal crayfish

Step one: Get permission to go crayfishing from the Environment Agency: it's trying to ensure that no one accidentally traps the white-clawed crayfish.

Go to a bike shop and ask for some old scrap wheels. If the wheel comes with a rubber rim tape, remove it and put it to one side. If you're very patient, unscrew all the spokes. If not, cut them off with boltcroppers or heavy-duty wire cutters. Cut close to the hub so that they're easier to bundle up and recycle Photograph: George Monbiot/Guardian

Step two: Lay out a sheet of garden netting on the ground. Any kind will do. Place the wheel rim on top of it and cut out a circle about 40cm wider than the rim.

Cut a 4m length of string, cord or twine (preferably rot-proof). Poke it through the valve hole and tie it to the wheel rim, leaving a tail of about 10cm.

Then fold the edge of the netting over the wheel rim, so that it overlaps the netting below by about 2cm. Pass the string over the net and through the first spoke hole in the rim. Keeping the string tight, carry on until you're a quarter of the way round the rim Photograph: George Monbiot/Guardian

Step three: Fold the netting over itself to make a pleat about 5cm wide. This ensures that you end up with a smoothly curving basket, rather than an uneven drop and a clump of loose netting when you get back to where you started. Put in a pleat at every quarter.

When you get back round to the valve hole, pass the string through it twice then tie the end to the tail you left when you started.

You should end up with a net about 30cm deep. Any less than this and the crayfish might scoot off when you are raising it; any more and it could get trapped in obstructions on the river bed.

If the wheel came with a rim tape, put it back on now, over the netting. This takes some of the pressure off the netting where it crosses the rim Photograph: George Monbiot/Guardian

Step four: Take a length of strong cord (about 1.8m) and tie it to one side of the rim with a reef knot, avoiding the pleats. Tie the other end to the opposite side of the rim.

Tie on another length of cord, at one of the two remaining compass points. Wind it twice round the opposite point, then pull or loosen it until the two loops are of equal length and the net hangs horizontally. Tie it off.

Hang the net off your finger with both loops in order to find the point of balance, then bring the two loops of cord together with an underhand knot, leaving a fixed loop about 10cm high.

You've made your net. If you have time, make several of them Photograph: George Monbiot/Guardian

Step five: Get some scraps from your local butcher or fishmonger. Almost anything will do, but the best bait is an oily fish head or skeleton. A chicken carcass is OK, and easy to tie on. Meat bones, fat and bacon are fine: you can use any carrion which can be tied onto a piece of string.

Pack the bait, the nets, a penknife, some cloth bags (or better still a hessian sack), some string and some stout cord or rope (or old washing line or telephone cable). Get to the river an hour before dusk. Peak fishing is an hour either side.

Tie a short piece of string to each side of the rim, then use the two lengths to tie on the bait, which should be positioned in the middle of the net. Tie the rope to the fixed loop with a bowline, sheepshank or half-bloodknot, and lower the net into the water beside the bank (vertically if possible).

If you've made more than one net, drop them a few metres apart. If you're clever and are next to a pub, grab yourself a pint Photograph: George Monbiot/Guardian

Step six:. By the time you've got your pint, the first net will be ready to lift (they need be left for only five-10 minutes).

Pull it up gently and smoothly and swing it onto the bank. If you're in a good spot it should contain half a dozen or more; sometimes I've pulled up 20 or 30 crayfish at a time.

You should have made sure the spot you've chosen is not one of the few where the white-clawed crayfish still lives Photograph: George Monbiot/Guardian

Step seven: Wet a cloth bag or sack, then pick the crayfish up by gripping them behind the thorax (the swollen part of the shell). If you hold them anywhere else, they will nip you. Their claws are very sharp and strong: they can cut straight through the ball of your thumb. The smaller the crayfish are, the more flexible they are and the better able to get you.

If any cling onto the net, don't try to pull them off: they will just cling on tighter. Hold the net upside down until they fall off. Drop them into the sack and fold the top over, otherwise they'll climb out. Spare none of them, however small: in this case all the usual ecological rules are reversed.

Red signal crayfish are distinguished by the scarlet underside of their claws Photograph: George Monbiot/Guardian

Hard work, this crayfishing ... Photograph: George Monbiot/Guardian

Step eight: When you've caught as many as you want, catch some more for your friends and neighbours – preferably for the whole street.

Wrap them up carefully in the wet sack or bags. As long as the fabric remains damp, they'll live this way for up to three days; they would die very quickly in a bucket of water.

When you're ready to eat them, wash them in the sink and boil up a large pot of salted water. Drop just a few in. This way the water temperature doesn't fall very far, so they die within 10 seconds. Cook them for no more than five minutes: generally for only as long as it takes for the water to return to the boil. Scoop them out and drop in the next batch Photograph: George Monbiot/Guardian

Step nine: Break off the tail, then break the scutes sticking out at the sides along one edge. Pull the flesh out Photograph: George Monbiot/Guardian

Step 10: With your thumbnail (or a knife if you don't have any nails) open up the flesh along a line in the middle of the back (it peels open naturally). You'll expose the 'black vein' – really the gut. Pull it out and discard it.

If the crayfish has big claws, crack them open with an empty wine bottle, then pull the smaller mandible off. If the flesh doesn't come out easily, use the small mandible to winkle it out Photograph: George Monbiot/Guardian

Step 11: Pull up the thorax to see if it contains any roe: the firm red eggs are good to eat.

You can eat all this just as it is. The flesh has a sweeter, more delicate flavour than either lobster or prawns. Or you can eat it with mayonnaise, or make a bisque (a thick soup) or a crayfish salad. But my favourite dish is … Photograph: George Monbiot/Guardian

… crayfish paella. Bon appetit, and death to the usurper. Photograph: George Monbiot/Guardian