Our South Cornwall team made an unusual find this week while carrying out some fish surveys on the rivers of South Cornwall. Matt Healey, Field Officer on the South Cornwall River Improvement Project, said, “We were out doing some electro-fishing when we unexpectedly came across two signal crayfish, a rare sight here in Cornwall.”
Signal crayfish were originally brought over from the United States and farmed as a commercial species, before they escaped and started colonising British rivers and streams. They pose a threat to our native white-clawed crayfish because they out-compete the natives for food resources. Signal crayfish also carry a potentially lethal disease which has spread throughout many native populations and caused a rapid decline in white-clawed crayfish population numbers.
Matt continued, “What makes this sighting unusual is that crayfish records are virtually unheard of in Cornwall. There are no records of white-clawed crayfish anywhere in the county at all. This is because of the predominantly acidic nature of the soils and underlying geology, making for unsuitable aquatic conditions for crayfish, as their exoskeleton is partly made up of calcium salts.”
Although finding signal crayfish in a Cornish stream can in general be considered bad news, the presence of crayfish at all is certainly an interesting find. Sadly, the pair caught this week, who were nicknamed Ronnie and Reggie (after the infamous Cray brothers), had reached the end of their time in this particular stream, as the law prohibits re-release of this invasive non-native species once caught.
There’s lots more useful information about crayfish in Britain on the Buglife website here: