Crayfish populations are being researched in the South West

The white-clawed crayfish is under increasing pressure from a foreign species, the American signal crayfish.

The signal was first imported to Britain about 30 years ago to be farmed for food.

But they carry a dangerous fungal disease to which they are highly resistant. They are also able to leave a river to travel over land, and that has caused problems for the smaller native species.

There are only a small number of native crayfish populations in the South West, with two of the most important in Dorset's rivers, the Piddle and the Allen.

Cornwall has none. Somerset has hardly any.

In Devon, the Environment Agency is carrying out a survey of the River Creedy to see if there are any there.