American signal crayfish are aggressive predators in the wild

The pest in Suffolk rivers is the signal crayfish, a North America native introduced to Britain in the 1970s.

It has driven out native white-clawed crayfish because the intruder carries a disease which is lethal to them.

Trappers trying to eliminate the pest are catching hundreds every night, but say the problem is going to take years to eradicate.

Native species driven out

Trapping red signal crayfish can only be done under licence and it is illegal to release them into waterways.

Trapper Adrian Murfitt said: "We very often catch up to a 100 in each trap and this week in one 130m (142yd) stretch of the River Lark near Bury St Edmunds we found over 600.

"As well as driving out the native species about five years ago, the American pest has a big environmental impact.

"They clear out crustaceous creatures, fish eggs and weed but now they are tunnelling up to eight feet into the river banks making them unstable."

Fish merchants collect the trapped fish as they were introduced into the UK as a delicacy and were kept in ponds.