The American signal crayfish escaped from farm sites

The American signal crayfish was introduced into the UK in the early 1980s for farming, but many escaped into the wild.

It colonises freshwater sites and burrows into riverbanks causing them to crumble, and excludes native crayfish.

Now people in Breckland are being asked to help trap the crayfish along a mile-long stretch of the River Lark.

The Lark Angling and Preservation Society have been monitoring the situation for the last four years and last year trapped more than 11,000 signal crayfish.

The Brecks Countryside Project is also involved in the scheme.

The signal crayfish is capable of walking overland to find a home and will rapidly colonise lakes and rivers.

It not only overwhelms native white-clawed crayfish, but also carries a fungal disease, the crayfish plague, to which the native crayfish have no defence.