Imported signal crayfish now plague UK rivers, threatening native species

British aquaculture firms imported North American signal crayfish in the 1970s hoping to develop a profitable freshwater fishery, but the species soon escaped into the wild. Their larger size, faster growth and aggressive behavior gave them a competitive edge over the native white-clawed crayfish, allowing them to proliferate throughout rivers in England, Wales and Scotland. The invaders also harbour crayfish plague, an oomycete pathogen that rarely harms them but is usually fatal to the native species, causing local extinctions.

In autumn 2024, roughly 100 white-clawed crayfish were found dead in the River Wansbeck, but investigations ruled out plague and signal crayfish presence, noting that older individuals suffered higher mortality. The UK government now classifies signal crayfish as a problematic non-native and mandates risk assessments for new introductions, alongside strict controls on moving live aquatic organisms. Conservation strategies include establishing isolated “ark sites” for the native crayfish and a public campaign to “Check, Clean, Dry” equipment to prevent disease transmission.