An environmental watchdog has asked for the public to help to halt the invasion of Scotland by the signal crayfish, which has unexpectedly spread across the River Clyde catchment.

The crayfish, introduced to Britain from North America by fish farmers, escaped and caused havoc on rivers across England, stripping them of other aquatic life by eating fish eggs and pushing out native species. So far they have been limited to a small percentage of sites in Scotland, but conservationists have noted their slow but steady spread across the south of the country.

Signal crayfish can grow as big as a man’s hand, and have been found in five sites in the Clyde and its tributaries, posing a threat to recovering salmon and trout populations.