Plague wipes out the majority of native crayfish on key Yorkshire Dales river

The deadly plague, which is usually carried by American Signal crayfish, has killed thousands of white-clawed natives since it was first detected on the river Ure five years ago, upstream of Aysgarth Falls in the Dales.

The UK's native crayfish, a vital part of the freshwater ecosystem, is important because it's an indicator of healthy water quality, a food source for various animals, and an "ecosystem engineer" that modifies habitats.

Ironically man-made weirs and waterfalls create barriers which break up the native populations have prevented the plague from spreading and the majority of the Ure's big tributaries remain disease-free.

Crayfish plague, caused by fungal-like Aphanomyces astaci, is a highly contagious disease that causes massive mortality in susceptible crayfish species.

No American Signal crayfish have been detected above the waterfalls, and it is not known how the outbreak spread.

The Environment Agency (EA) is urging visitors to take precautions as damp boots and equipment can harbor and transport spores to new water bodies.

Crayfish expert Tim Selway from the EA said: “We are talking thousands, if not tens of thousands, of crayfish dying in recent years. There's still some right at the top of the catchment and we still have thousands in the tributaries which we are desperate to protect.

"Crayfish are teetering on the brink of being critically endangered. I would urge people to think before they go into a watercourse – are their boots cleaned from previous walks or have they cleaned their watersports or fishing equipment, for example? All of this really matters."