They are becoming as big a pest in Britain as the grey squirrel or Japanese knotweed, and seemingly impossible to control. But the answer to dealing with Chinese mitten crabs, the invasive species infesting the Thames and other English rivers with damaging results, may be simple: eat them.
The large and aggressive Asian crabs with their hairy mitten-like claws are damaging native wildlife and river embankments as they spread across the country. Yet diners in China, Japan and Singapore consider them a tremendous delicacy, and will pay the equivalent of £24 for a single mitten crab in the right condition. It is a famous ingredient of Shanghai cuisine, and the roe is especially prized.
There has been no check, natural or otherwise, on the species' expansion. But Paul Clark, a marine biologist at the Natural History Museum, believes human consumption may be the answer. "It is a huge pest problem," he said. "It burrows into river banks and causes them to collapse, and is very damaging to native wildlife." In the Thames, it is now present in such numbers that there is also a risk that the crabs may clog the water intakes of power stations and other industrial facilities along the river. It is steadily spreading across Britain and it is "only a matter of time" before it reaches Scotland, he added.
Dr Clark is organising a conference in London in March to explore whether the abundant mitten crabs of the Thames – and there may be millions of them – can be harvested commercially, as a means of controlling their numbers. The possibility of setting up a mitten crab fishery in the Thames has been mooted thanks to a recent study which concluded that mitten crabs from the river were fit for human consumption, and that the population was large enough to be exploited.
Trials have shown that the best way to catch them is by the use of fyke nets, long bag-shaped nets which are held open by hoops. The fishery would have to be located in the lower Thames, between Greenwich and Erith, because it is to here, in the more saline water, that all the mature mitten crabs of the river migrate in the autumn to spawn – which is when they are their most edible.
Dr Clark said the conference will also be looking at the risks posed by a commercial fishery. The trials showed that the fyke nets used to catch the crabs attracted a substantial bycatch of eels, which are increasingly considered a threatened species in Europe. Furthermore, he thinks there is a risk that if a fishery took off and was commercially successful, people might intentionally disperse the crabs into other UK rivers, with the intention of making money.
"We are damned if we do and damned if we don't. "Mitten crabs have few natural enemies capable of reducing their numbers, but the establishment of a fishery would certainly carry risks."
The mittened mini-monster was first recorded in Britain in the Thames in 1935 having almost certainly arrived as larvae in the ballast water of ships from the Far East. It is found as far upstream as Windsor, and has also spread to other watercourses as far north as the Tyne. With a body the size of a human palm, and legs double the width of that, Eriocheir sinensis has spread around most of Europe during the last century, and has also arrived in the US, where it is considered a major pest.
Not welcome in Britain: Troublesome aliens
Japanese knotweed (Fallopia japonica) is the UK's most invasive non-native plant. The Victorians introduced it as an ornamental plant, but fertile British lands lacking in biologicalenemies have enabled it to flourish.
Grey squirrels (Sciurus carolinensis)are to blame for the decline of the UK’s native red squirrels, of which there are estimated to be only 140,000 remaining. Greys were brought fromAmerica in the late 19th century,and red squirrels are usually displaced within 15 years of theirarrival locally.
Harlequin ladybirds (Harmonia axyridis) are a recent addition to Britain and are the most invasive ladybirds on earth. They arrived in the UKin 2004, but were introduced to North America in 1988 and have since become the most widespread ladybird species on the continent.
The Rose-ringed parakeet (Psittacula krameri) has already caused serious damage to crops in Britain. It is found mainly in London and southern England, but how it wasfirst introduced is unclear. It is noted for its loud, screeching calls.