Elizabeth A Clements – Deputy Director of Conservation, Head of Natural Environment
American Signal crayfish (Pacifastacus leniusculus), are an incredibly successful crustacean – but unfortunately this very success is bad for our aquatic habitats and native species.
Reports of the Invasive Non Native (INNS) Species American Signal crayfish had been getting closer to the North York Moors for a while. But recently the species have been identified within the National Park. Sadly they have been recorded in the River Rye catchment and also at Scaling Dam Reservoir which is connected to Staithes Beck and is close to the River Esk catchment.
There are currently up to six species of non-native crayfish in England, but the ones here now are the American Signal crayfish species from North America. There are a relatively recent arrival. They were imported into England/Wales in the 1970s to kick start crayfish farming ventures. But no one had quite realized what an entrepreneurial creature they would be in their own right, given new territory. They were soon out of control and have continued to be so ever since, marching menancingly on and spreading throughout the country. They can move up and down stream, between water bodies and watercourses, and over land for short distances crossing physical barriers. They can even survive out of water for a few days.
Signal crayfish out compete other species and disrupt the interconnected biodiversity chain. They are particularly fertile producing up to 500 eggs in one go, and can live up to 20 years.
They eat fish and amphibian eggs, tadpoles and juvenile fish as well as aquatic vegetation. It’s not that they don’t have any predators, they do – otters, salmon, trout and eel can eat them – but they can reproduce in such large numbers that predation has little impact on the growing populations. Another adverse impact on the ecosystem is their habit of burrowing into banks to hibernate in the winter – banks are therefore weakened and more prone to erosion, increasing sedimentation and flood risk, and decreasing water quality.
In the National Park we are very lucky to still have a population of Britain’s only native freshwater crayfish the White-clawed crayfish (Austropotamobius pallipes). White-clawed Crayfish are declining throughout its range and are therefore protected by European and UK legislation. These native crayfish are declining due to competition, predation and disease. The American Signal crayfish are not only more aggressive and can tolerate poorer water quality and habitat conditions, but they also carry ‘crayfish plague’ (Aphanomyces astaci), a fungal disease which has devastating impact on our native species. Remaining populations of White-clawed crayfish are at risk of being wiped out once the Signal crayfish turn up.
Researchers have been striving for years to find a way of successfully controlling and eradicating non-native crayfish but as of yet nothing has been successful so far. That research continues but out native crayfish are under threat in the meantime. It’s not possible to safely exterminate a whole population in a connected watercourse or a large waterbody but many alternatives have been tried. Trapping and removing larger adult crayfish only allows the smaller younger population to thrive. Trials of chemical treatments have not yet been a success and in the aquatic environment have been particularly tricky. There have been attempts to remove adult crayfish, castrate them, and return them to the population in the hope they would control the population but that has not worked out yet either. Attempts have been made to erect physical barriers to prevent their free movement but only with very limited success. There is also a persistent rumour that people purposefully release Signal crayfish presumably to resuscitate the failed 1970s vision of crayfish farming.
It’s illegal to introduce Signal crayfish, that includes using crayfish as fishing bait – either dead or alive. A licence is needed in England to purposefully trap any species of freshwater crayfish, in an effort to assert some control over the situation.
The best thing we can all do at the present time is follow very clear biosecurity guidelines when we are in and around water.
It’s very important to note that people can unintentionally spread crayfish plague as well as the actual Signal crayfish but following the Check, Clean, Dry campaign is good practice and should help people avoid spreading the plague.
We can all do our bit to help protect, conserve and enhance our native species populations in the North York Moors and beyond.