On the tidal and lower Thames it is the Chinese mitten crab that we are mainly concerned with, as to date I have not seen a crayfish in these waters. The locals joke that any signal crayfish that arrive in the section would soon be eaten by the Chinese mitten crabs.

Chinese mitten crabs are a fairly large freshwater crab that is native to China and Korea. We have seen them to nearly 5 inches across the back in the tidal Thames. They have eight fury legs with two large claws with what look like mittens on them - hence the name. The claws come out of the mittens. They are certainly fierce looking.

They are classed as a freshwater crab as they spend the majority of their lives in freshwater or water with a low salt content. They are only supposed to enter the sea to breed.

According to top fisheries biologist Alwynne Wheeler, they entered the Thames estuary from the ballast water from ships returning to the "Pool of London" or possibly lower down the river. Some of these ships will have come from countries where mitten are natural inhabitants and would have taken on ballast water containing the larvae of the mitten crab. When this ballast water is pumped out the larvae were released into the Thames. They were carried upstream by the tide where they hatched out into crabs. Naturally these crabs can walk upstream or across land to enter nearby stillwaters. Hence they are not only found in the Thames but also in local stillwaters.

From talking to local anglers and by using my own observations, it has become clear that these crabs are spreading and increasing in numbers at an alarming rate. I cannot understand how, for a species have to spawn in salt water, they have spread so far upstream. There are now reports of them appearing as far upstream as Staines. Just how far are these little luvvies prepared to walk?

The main problem with mitten crabs appears to be that they are terrific spawn eaters. This means that with mitten crabs devouring the spawn and our friendly cormorants finishing off any survivors there is little chance for fish stocks and the situation could turn critical. Possibly the cormorants could be eventually forced back to sea, as there is not enough food for them to survive in rivers!

Mitten crabs are also great burrowers and could destroy river banks and lock gates. When I went out with a small net and a bucket to see how many crabs I could collect in an hour from a section of the River Crane near where it entered the Thames, I was surprised how many I missed. As I tried to net them they disappeared into holes in the bottom and in the bankside. They disappeared in what is best described as a puff of light mud. I only netted 17 live mittens but should have had more than double that number.

Alwyne Wheeler recalls that in 1935 they removed nearly 12 tons of mitten crabs from Germany's River Weiser. That's a hell of a lot of crabs and the potential for a terrific amount of damage. Is the Thames going this way? I don't know, but I feel that we need to be alert to the problem and report to the Environmental Agency any catches of these creatures, particularly if they are coming from areas further upstream of the limit of tidal water at Teddington.

It is only by a concerted effort to inform the Agency that the magnitude of the problem can be recorded so that action can be taken.