Wednesday 24th May, 2017
As a result of our past hard work and commitment on the Grizzled Skipper Project we have been asked to take part in further conservation work in the form of a signal crayfish eradication programme with the Nottinghamshire Biodiversity Action Group and the Environment Agency.
It’s been long known that the American signal crayfish, an alien species, has posed a massive threat to populations of our native crayfish, the whiteclawed crayfish, since its introduction to Northern Europe and later central Europe in the sixties and seventies respectively. Initially imported as a food supplement in Sweden, sought after and distributed by collectors and finally being used by fishfarms and collectors alike in the UK. The fish farm bubble burst in the late 70’s, early eighties and signal crayfish were left unchecked and unmanaged. This allowed their dispersal into nearby water courses. Their bigger size, up to three times the size of our native crayfish, aggressive nature, and their taste for our native crayfish, soon put native populations under threat. The signal is also a carrier of a virulent form of crayfish plague, which can wipe out whole populations of native crayfish within months. These factors, coupled with its ability to burrow up to a metre into river banks, its omnivorous appetite, and its taste for fish, eggs and fry, label the signal crayfish a major threat to our inland waterways and ponds.
The eradication programme involves the trapping of signals in 50 nets placed in different locations around the pond. Any males caught with a carapace length >40mm will be sterilised by snipping off their copulatory stylets which are found on the underside. These males are then marked with nail varnish and released back into the pond. Any males with a smaller carapace length of 40mm and all females will be removed and humanely dispatched. The rationale behind this is once all the breeding females are removed the larger males will predate any remaining smaller males and so the population should fall.
After a mornings training at Bulwell Hall fish ponds, a date was allocated to us, a 12 strong group of volunteers set about emptying the traps and checking carapace lengths. In just under two hours we had checked 43 traps, containing a total of 196 signal crayfish, of which 37 were sterilised, marked and released. 5 males were retrapped and released again.
In just under 2 months, about 1300 crayfish have been trapped and processed by all volunteer groups. The mark and recapture figures show a huge population of signals, possibly as many as 12000 in the one pond.