In the 1970s, Britain imported North American signal crayfish for farms; they escaped, bred in huge numbers and now infest rivers across England, Wales and Scotland, carrying a plague deadly to native crayfish

• Representative imageIn the 1970s, British aquaculture businesses imported North American signal crayfish to stock farms, hoping to build a profitable new freshwater fishery.
• But the plan backfired: the hardy invaders escaped into rivers, multiplied rapidly and spread across England, Wales and Scotland.
• Today they dominate many waterways and carry crayfish plague, a disease that is usually fatal to the UK’s only native crayfish, the white-clawed species.