Originally from the USA, signal crayfish were introduced to the British Isles in the 1970s and were cultivated in farm ponds for sale to the food industry. Escaping from captivity, they have now spread to ponds, lakes, streams and river systems across the British Isles.
The invasion of signal crayfish has been detrimental to the native white-clawed crayfish (Austropotamobius pallipes, Lereboullet 1858), whose population was already threatened in the British Isles by habitat alteration and pollution.
Signal vs white-clawed crayfish
Signal crayfish have similar habitat requirements to white-clawed crayfish, which means that when signals invade, the two species often compete for the same space. In this competition, the signals have the advantage from the outset. They breed about a month earlier than the white-claws, and produce more eggs per female. Young signals are not only more numerous, but can also establish themselves in favourable locations (including shelters) before the white-claws have even hatched. The signals maintain their advantage over the white-claws as they grow to adulthood. They have a head start in feeding, a faster growth rate, they are more aggressive and also more tolerant of pollution.
A final blow to the white-claws is that some signals may also carry a fungus (Aphanomyces astaci Schikora 1903) that causes crayfish plague, the signals are usually immune, but the plague kills virtually all infected white-clawed crayfish.
Effects on the wider community
It is not just white-claws that suffer from the invasion of the signals. Once they have taken over, signal crayfish can cause problems for the whole aquatic system. They can live at higher densities than the native species, which means more crayfish per metre and potentially more effects on other species.
Signals eat plants, other invertebrates including native white-clawed crayfish, and both fish and fish eggs. They also compete with other species such as fish for shelter. The burrowing of signals can cause erosion of riverbanks and grazing of aquatic plants can cause change the structure of the habitat. The loss of plants means that there are fewer places for insect larvae to hide or for fish to lay their eggs.
The signals are now probably impossible to remove from the British Isles. Without resorting to drastic measures such as applying poisons, which could affect other species, or using intensive, disruptive and expensive methods, such as hand removal, it seems unlikely that signals will ever be entirely removed. Work with crayfish has focused on protecting the areas where native crayfish are most abundant rather than trying to eliminate the invaders altogether.
Trapping crayfish usually has limited success, as only a small proportion of the signals are attracted to traps, and these tend to be large males. Studies of the reproductive biology of the signals showed that the females produce a chemical, known as a pheromone, to attract male signal crayfish. Pheromones are highly specific and non-toxic. Putting female signal crayfish pheromones in traps could help to catch more male signals and is currently being trialled in the British Isles as a method of control.