American Signal Crayfish
Ecology of the Red Clawed North American Crayfish (Pacifasticus leniusculus)
Habitat description
- The native habitat of the signal crayfish ranges from small streams to large rivers, and lakes from the coastal to the sub-alpine regions.
- Signal crayfish can also survive in brackish water
Reproduction and life cycle
- Breeds from the age of two (one in exceptional circumstances)
- Can hybridise with native crayfish, however offspring are sterile
- Females produce up to 500 eggs (some research has shown survival from egg to 2 years old is 10-52%)
- Mating and egg laying occurs during autumn, mainly in October.
- After egg laying the female carry the eggs under the tail until hatching.
- Hatching time varies greatly depending on temperature. But generally young hatch April to May
- The eggs hatch into miniature crayfish that stay with the mother for three stages (two moults). In the third stage the juvenile crayfish adopt a solitary life.
- Size at maturity is usually 6-9 cm total-length (from tip of head to edge of tail-fan) at an age of 2-3 years.
- Competition and cannibalism can greatly affect survival in dense populations.
- Maximum age and size are reported to be approx. 20 years and 16-18 cm, but such sizes are very seldom.
Dispersal and spread
- Humans are the overall most important vector for dispersal and spread as they are easily accessible as stocking material.
- Within the watercourse the signal crayfish can spread by own migration. Upstream migration rates of more than 1 km per year are reported
- Downstream spread can be faster.
- Where necessary the signal crayfish will walk across dry land to either migrate or to escape unfavorable conditions, making contamination from one water course to another easy.
Problems: 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, as they are more aggressive and they breed about a month earlier than the white-claws. In addition they produce more eggs per female. The Young signals are thus able to establish themselves 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, more tolerant of pollution, eat a wider range of food, and also they are carriers of the crayfish plague which they pass to white-claws to whom it is fatal.
Problems
- They are known to burrow into banks up to 1.2m which causes damage and affect the habitat of bank-dwelling species such as the threatened water voles. It also speeds up erosion causing changes in the bankside vegetation and increases the silt load in the water with knock on result.
- Signal Crayfish predate on fish eggs laid below the gravel, but also on live fish given the chance, and are of particular threats to bullheads and stoneloach. There are increasing fears of their impact on salmonids as their favorable habitat is the same as young salmonid fry and parr reducing the amount available to salmonids, or they will predate on them.
- They are also capable of outcompeting fish, through their consumption of plants and insects, and the loss of plants and insects has further knock on effects for both aquatic and terrestrial species.
- As well as preying on insects red-claws will also eat white-claws and their own juveniles (thus trapping must be done in an appropriate manner as the removal of larger specimens who self thin will increase the number of red-claws.
- Identification of red-claws and white-claws can be confusing, it is possible to tell them apart but this must be done by a trained eye so trapping must be conducted by a competent person (it also requires and EA licence)
Crayfish plague
The fungal spores of crayfish plague can survive for up to two weeks in water, but can be killed by drying or disinfecting. Its spores infiltrated the waterways, sometimes carried between sites on fishing equipment and even wellington boots. Introducing signal crayfish into water previously free of the disease can spread crayfish plague. It can also be spread on people’s wet footwear and equipment. Anglers have a key role in helping to reduce the risk of spreading the disease by:
- Drying, or disinfecting any boots or nets before moving between rivers. It is best to avoid fishing different rivers on the same day.
- Not using any crayfish as bait (this is illegal).
