Unwanted visitors may track dirty footprints across your new carpet, leave dishes unwashed and toilet seats up, but at least they tend to leave your home still standing.
The Chinese mitten crab is a house guest of entirely different order. A UK resident for at least three-quarters of a century - its first sighting in the Thames was recorded in 1935 - this non-indigenous invasive species nests in the soft mud of riverbanks, burrowing into them in such numbers that they become destabilised, and eventually collapse.
Native to China and South Korea, mitten crabs are thought to have been carried here in ships' ballast water, and have certainly made themselves comfortable, infiltrating river systems and estuaries including the Thames, Tyne, Humber, Medway, Wharfe and Ouse.
And while its name suggests a certain cuddliness (for a crab) its eponymous bristle-covered claws are those of an aggressive crustacean with a voracious appetite, a fondness for fish spawn and no natural predators. It is also supremely adaptable, as comfortable in saltwater as in fresh, and elsewhere in the world has been known to cross miles of land in order to colonise new waterways.
Über crab
With carapaces up to 4in wide, mittens spend their lives in fresh or brackish water but reproduce in seawater, and in China migrate up to 1,000 miles along certain rivers in order to breed. For evidence of how good they are at that, consider that nearly 12 tonnes of mitten crabs were removed from Germany's River Weiser in 1935.
Dr Paul Clark of London's Natural History Museum has been looking into the spread of crabs in the Thames and estimates a population in the millions. 'Numbers increased dramatically around the late 1980s, probably due to a combination of shipping ballast water containing larvae or juvenile crabs being dumped in the Thames, and extremely low river flows due to drought season and/or water abstraction.'
Others speculate that burgeoning numbers owe something to cleaner rivers: populations were effectively being kept in check by pollution before the introduction of new European environmental laws in the 1990s.
All you can eat?
Prized in the far east, where the crab's gonads are considered a delicacy worth paying high prices for, many wonder why the UK isn't dining its way out of trouble - not least chef Gordon Ramsay, who featured mittens in an episode of Channel 4's 'The F Word' last year, pronouncing the flesh sweeter and more intense in flavour than typical crabmeat.
So why aren't we taking advantage of this unexpected bounty?Primarily, says Heidi Stone, fisheries manager for the Environment Agency, because the crabs are currently still too 'niche' and not yet widely known about beyond the Chinese community. 'Usually our phones ring off the hook with people wanting to try the latest celebrity chef wild food craze,' says Stone. 'We steeled ourselves for a big response after 'The F Word', but were surprised by the lack of interest.'
Mittens are currently classified as bycatch, typically of eels, which in their former Thames stronghold have suffered a near-terminal decline over the past few years. The EA says an increase in demand would oblige it to facilitate the catching of mittens - their limited ability to spread would be 'a factor in their favour' - but would not support any commercial endeavours that would interfere with eels and other native species.
Commercial opportunities
No legislation exists that allows mittens to be fished commercially, but the Environment Agency has a duty to regulate and improve fishing for certain species, including mittens, since the introduction of the Marine and Coastal Access Bill, which came into force in January.
In addition, a licensing and authorisation process is currently underway that from January 2011 will require anyone wanting to catch the crabs to obtain EA permission to do so. There are other area-specific agencies involved, however, including the Port of London Authority, which owns the Thames riverbed.
In terms of how useful it would be to control mitten crab numbers by eating them, Stone says this would produce a reduction in a particular area as part of a controlled management programme. But, she adds, the mitten is not currently a high EA priority, certainly not compared to another invasive aquatic species, such as the American signal crayfish, which has practically wiped out Britain's native crayfish. Often tarred with the same brush, adult mitten crabs need saltwater to reproduce, and as a result are less successful at colonising waterways than the signal. Once they reach a critical mass, however, an established population is able to breed in large numbers.
Crab contamination
The London Port Health Authority has already conducted a two-year study into the possibility of harvesting the crabs for domestic consumption. Working alongside the Natural History Museum, Food Standards Agency and others, levels of toxic metals, dioxins, PCBs and organochlorides were found to be mostly below European regulatory limits in white meat, though levels in brown meat - including the gonads - occasionally exceeded them.
'The adverse effects of exposure to dioxins and PCBs were chronic, not acute, however,' says Paul Jones, who is leading research into mittens in the UK. 'Also, since the crabs are ripe for harvest only in autumn, from September to November, that reduces the risk even further. Any commercial fishing would be limited to those three months of the year, timed to catch the annual downstream migration.'
Coals to Newcastle...
Mittens currently being caught as bycatch are being sold on, says Heidi Stone, and anyone wanting to source the crabs should contact river fishermen direct. She adds that the EA hopes to be working with Paul Clark later this year, trialling crab-catching equipment from Holland that will allow endangered eels to slip through the net.
If UK demand for mitten meat is low at the moment, one conservation and commercial solution that could prove lucrative would be to ship them back east. Mitten crabs are at a premium in Shanghai, where domestic stocks are ironically suffering at the claws of an even more voracious invasive species: the Japanese crab. There have been proposals to export mittens to replenish stocks, but harvesting them here and selling them there would not only save our waterways, but could also raise much-needed funds to protect the indigenous species living in them.
- To find out more about licensing for mittens, contact the Environment Agency
- Learn more about the crabs and the Natural History Museum's work with them here
Eifion Rees is a freelance journalist
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