Will we soon be tucking into mitten crabs fresh from the Thames?

Chinese mitten crabs are infesting Britain's waterways – yet in south-east Asia, they're a delicacy. So shouldn't we be eating them? Richard Sharp goes fishing

I didn't believe that the Thames had the power to cause hallucinations until one day its banks started to wriggle and writhe in front of me, as if taking on an undulating life of their own. From out of nowhere, eight hairy legs crawled along the mud banks and drunkenly staggered sideways towards the water. Yet more crabs followed. Feeling a little like the lead actor in an improbable 1950s B-movie (Killer Crabs Eat Tower Bridge!), I ran off along the embankment to warn friends and family of the invasion.

Then a friend explained that what I had most likely seen were some of the Chinese mitten crabs that have colonised the Thames for the last 100 years in their tens of thousands. They get their mitten name from the furry cuffs that surround each claw and while their name is cute and conjures up images of grannies knitting booties, they are environmental hooligans, destroying local freshwater species such as native crayfish and collapsing natural river banks by digging deep burrows into the mud.

The crabs were first spotted in the early part of the last century and have increased dramatically in numbers. The Natural History Museum investigated how to reduce the population of mitten crabs in the Thames and became aware that this species is a popular culinary delicacy among south-east Asian populations. This offered the possibility of harvesting the crabs to supply specialist shops and restaurants in London and even, potentially, markets in Asia. But first, the London Port Health Authority required evidence that the crabs were suitable for human consumption, which included gathering information about concentrations of toxic metals and organic chemical contaminants. Analysis of numerous samples revealed that levels of metals and hydrocarbons were too low to cause concern – though researchers also found that levels of organochlorines (PCBs, dioxins and dibenzofurans) in the brown meat, which is the preferred part, were relatively high. In fact, the levels were found to be higher in crabs from the Netherlands, which is where most London food businesses source their mitten crabs. The report concluded that, in the light of these findings, harvesting Chinese mitten crabs from the Thames for culinary use could go ahead as a commercially viable way of controlling the species.

I was inspired by the idea that the Thames was a potential source of free food and set myself a challenge to catch and cook the mitten crabs. To find out if they were good to eat, I spoke to Karl Ng, commercial manager from the Royal China Group Restaurant. Mitten crabs, he explained, are quite a delicacy. "September is the time to eat the female crab for the extra roe that makes the crab so tasty. The extra roe is the reason why the mitten crabs are so different from ordinary crabs, and this is why they are prized as a delicacy." The best way to eat the crab, he says, is to steam it for 20 minutes and serve with fresh ginger and Jinjiang vinegar. "The crab should be eaten by taking apart the stomach cover and eating the roe and then the meat. The meat is often dipped in vinegar to give the taste a lift. Afterwards it is recommended to drink some hot ginger tea to balance the energy in the body, as according to traditional Chinese medicine, crab is cold or 'yin'."

However, Lee Bennett, head chef at the seafood specialists Le Pont De La Tour restaurant, had never heard of mitten crabs. He explained: "When I heard that you had caught some crabs in the Thames I did some research to find out more about them as I'd never heard of them. I contacted two of my fish suppliers and neither had they. I had heard anecdotally that the best way to catch them is to leave a sheep's head in the water for 12 hours and you will be guaranteed to catch a great number, who will be clinging to it. That, however, is a pretty extreme, old-school method of catching them. If I was to use them for cooking, I think the best thing to do would be to make a tasty soup with them."

Claudine Fontana from the Natural History Museum advised: "You could eat the meat from the mitten crabs at any time of the year, but it is actually the gonads of the Chinese mitten crabs that are considered a delicacy and these can only be eaten when they become sexually active in the Autumn months." Realising that this information might cause my crab campaign to lose momentum, I decided to go crab-hunting earlier in the season, with the hope that smaller crabs could still make a good meal. I was advised that the crabs can be found all along the Thames and have been found along the Chelsea stretch of the Thames as well as at Chiswick.

I headed for the Thames, ready with bucket, spade and one of those rectangular orange plastic holiday crab-fishing lines. At Wapping, I hopefully deposited some bacon into a mesh bag that normally contains washing machine cleaning tablets and dropped it in the water. Half a day passed without so much as a nibble and so I moved up to the Grand Union Canal, where large mitten crabs had been spotted in the water near Limehouse basin. After further fruitless hours, I looked dejectedly into the water and suddenly spotted the unmistakable sight of a large mitten crab scuttling about at the bottom of the canal. When it managed to swiftly evade my clutches and retreated to deeper water, I figured that there must be an easier way to catch an inner-city crab.

After frenzied enquiries and subjected to some ridicule at several angling shops across South London, I got a call from Andy Hawkins, who coordinates the Thames Explorer Trust, an educational charity concerned with the future of the Thames. Andy had been finding crabs as part of his beach-combing trips and invited me to join him on a crab hunt. The following day I found myself up to my knees in the shallows of the Thames, happily overturning rocks, feeling as carefree as a five-year-old. Then the moment I had waited for as Andy shouted "found one" and from underneath an overturned rock I finally came face to face with the elusive Chinese mitten crab.

As the beach-combed Chiswick crabs were on the small side, I needed some larger ones if my Thames supper was going to be more substantial. Thankfully, I got a call from Gary Hiller, an eel fisherman who presented me with an impressive selection of various shaped and sized mitten crabs that he had caught earlier at Tower Bridge. I expectantly eye up the crabs and I wasn't disappointed, as some of their carapaces measured about 3in across and so I reasoned we were in for a decent meal. Gary explained: "The crabs end up in my eel nets and I usually throw them back. You have to be really careful of their claws as they can give you a nasty nip."

Clutching my bounty, I set off to hook up with Ricardo Gibbs, who runs Rick's Restaurant near my home in Tooting. This was the first time he had worked with the crabs and he set to work knocking up Thames crab soup, which involved boiling and smashing up the crab shells with a rolling pin to release both flavour and meat from the claws and shell, and adding onions, garlic, lemon juice, celery, carrot, rice, tomato puree, fish stock and herbs and spices. Ricardo said: "I boiled the crabs all in the same pot and then simmered the ingredients for an hour and finally liquidised and sieved the crab. The crab soup turned out brilliantly and I was really pleased with it as it tasted amazing. A soup recipe is ideal for smaller crabs and this can easily be undertaken at home as it's basically a one-pot recipe."

Ricardo set down a bowl of the terracotta-coloured crab soup and looked at me expectantly. I closed my eyes and I was staggered at the incredible taste and kick of the soup, which left me with a warming feeling that I hoped wasn't anything to do with any heavy metals that the crabs might have ingested. The vegetables and rice gave the soup a delicate richness and the soup still retained a distinctive and subtle flavour of crab. Ricardo had converted one kilo of crab into three litres of soup and it was thrilling that, possibly for the first time in modern London culinary history, the mitten crabs had been created into a fantastic Thames soup, served up in Tooting, with a garlic rouille and washed down with a glass of chilled wine.

Chinese restaurants have to resort to importing their mitten crabs from Holland, where they are farmed, as they are not commercially available in the UK. Perhaps an effective method of pest control for the Thames mitten crab may be both supplying the domestic food market and exporting the crabs to China, where demand is out stripping supply. Karl Ng says: "Our restaurant would definitely use the London mitten crab if it was clean and safe to eat and it would be really interesting to see how the two compare."

So, forget lobster or langoustine – just head down to the Thames and try turning over a rock or two. You never know what you may find.

Mitten Crab Soup by Ricardo Gibbs

Ingredients

1 kilo live mitten crabs
1 small onion peeled and chopped roughly
8 cloves of garlic, cut in half
1 medium potato, sliced
1 pint rice
1 small chilli (optional)
1 bay leaf
3 litres good fish stock
1 tablespoon tomato puree
1 pinch saffron strands
1 glass white wine
Half a head fennel, chopped
2 glugs of olive oil and a splash of Thai fish sauce

Wash the crabs thoroughly. Turn the crabs on their backs, pierce quickly in the middle and cut in half with a heavy cleaver or knife. (The crabs can also be put to sleep first in the freezer before boiling).

Put a large steel pot on the stove with the olive oil, onions, garlic and fennel. Sweat off slowly on a low heat for a couple of minutes and add the crabs, tomato puree, and cook for another couple of minutes. Bash the crabs for a minute with a rolling pin, or heavy wooden spoon, breaking up the legs and claws as you go. This will help to release the flavour and the meat.

Now add the rest of the ingredients and bring to the boil. Skim off any froth or foam that may rise to the surface and lower the heat to a simmer for one hour. Keep skimming the impurities off the soup and stirring occasionally. Blend for 30 seconds and pass through a sieve, not too fine or the soup will be too thin.

Season with Thai fish sauce (if needed). If using stock cubes, don't forget the crabs are salty already. Finish with pepper and a splash of brandy if you like. Serve with a garlic rouille and some crusty bread.