Anglers can help in the fight to rid a river of a crayfish interloper threatening to wipe out the native species.

The American signal crayfish -- twice as big as its British cousin -- have been found in a stretch of the River Aire near Silsden.

Alarmed Environment Agency experts have launched a campaign to try and snare the animal, which can gobble up the smaller native white clawed type.

So far they have caught about 40 of the beasts and now plan to set out a more intensive trapping programme later this month.

Meanwhile, anglers have been assured they are not breaking the law by taking them out on hook and line.

But they are barred from using a trap, which is not allowed under fisheries legislation.

Environment Agency biodiversity team leader Martin Christmas said trapping tended to snare mostly males, which were more useful in the river as predators on their own young. And special permission was also needed.

"Anglers can catch them by rod and line and there is no limit to the number they can take, so long as they have a fishing licence," he said.

Once caught the fish could be humanely killed by placing in the deep freeze.

His latest investigation had revealed that the signal crayfish had spread beyond where anglers were finding them at Silsden Bridge.

"The next step is to put out a number of small scale traps to check further distribution and see how far they have travelled and how near they are to the native population above Gargrave and to the feeder streams lower down," he added.

Anglers Steve Duncan, 41, of Kensington Street, Keighley, and his father, 72-year-old Jim, regularly catch the signal crayfish at Silsden Bridge.

Steve said: "They have been around for about 13 months. Every time we go down fishing for chub we get these crayfish which hold on to the line with their pincers.

"We need to get rid of them. They are spoiling our sport," said Steve, who has been fishing in the area for 30 years -- his father for 50.