American signal crayfish are an invasive species introduced by the government in the 70's as a potential export to the Scandinavian market, but these crayfish are carriers of a plague that is thought to kill our native species - the White-clawed crayfish. They also tend to dominate the smaller native for food and river habitat. The native White-clawed crayfish is now an endangered and highly protected species. The American signal crayfish is found in huge numbers on some chalk streams with their burrowing riddling the banks to the point of making them unstable. Trying to eradicate the crayfish by licensed trapping works to a point. But if only the larger adults are removed then this method tends not to be successful as apparently the adults eat juveniles so the result is more juveniles survive thereby increasing the population yet further!
River keepers try to keep the numbers down and the large numbers trapped are popular with restaurants! Vicious things though with their claws. The creature in this image is clearly cross and would gladly have grabbed my finger and squeezed harder with its teeth lined claws.