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

In 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.

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