We got more crabs than Cromer in the Fens

Rings appear around the blob of orange sitting on a flat calm drain. When it falls flat, I pick the rod up, wind down and connect with fresh air.

I use big baits and from time to time, I fail to connect with what's on the other end. Usually, this means a small fish has picked it up by the head, avoiding the hooks.

I recast to the same spot and the same thing happens. This time, I leave it a few seconds longer, but bend into nothing again.

Checking the bait, it's clearly been damaged but the slashes in its flank don't look like teeth marks from a pike. A zander's front fangs weren't responsible either, because it was only marked on one side.

I've seen similar on the Ouse and Middle Level, which have both been colonised by Chinese mitten crabs. I am a long way from both and even further from the sea, but suspect the crabs have now made it to this part of the system.

Something tells me this isn't good news. These things are capable of travelling a considerable distance upriver from the sea as larvae, living in freshwater until they migrate back downstream to breed.

First found at Denver Sluice, in around 2005, they are becoming more numerous. Matchmen have caught them when their claws have become entangled in their feeders. Some now count their fingers after mixing groundbait.

The as-yet unanswered question is what impact they're going to have on the ecology of our rivers, where coarse fish already have cormorants, increasing numbers of other fish-eating birds and otters to contend with.

Crabs eat fish spawn. Some believe they are partly responsible for the decline in schoolie zander on the Middle Level and lower reaches of the Ouse. I have no idea whether this is true, but one thing's certain. Looking at how efforts to eradicate the American signal crayfish have failed, these things are probably here to stay.