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You could in theory trap every single crayfish and record the numbers but that would take too long and cost far too much. So instead you have to rely on some form of sampling.

I've managed to track down a scientific paper in the public domain that discusses the various ways of doing this and the sort of population estimates and errors they generate. You can read it .

In the case of in Nuneaton the team caught a sample of native crayfish, placed identifiable marks on them and then released them.

They then returned at a later date to catch another sample. Of course some of those crayfish will have been caught and marked the first time around. And knowing that number allows you to make an estimate of the population in the pool as a whole.

This is slightly easier in this case as Ensor Pool is a closed system unlike a stream where crayfish might wander in and out of the stretch you are interested in.

There's some discussion in the paper I linked to about the problems of using the same technique to catch the crayfish. The suggestion is they might actually learn to avoid being caught if you use the same method twice and that would skew the result.

In the end the estimate of the population of Ensor Pool is between 50,000 to 100,000 native crayfish. Which is more than enough to help repopulate similar pools and help stop the native crayfish being wiped out by infected interlopers.

Finally there has been a bit of a revival of interest in eating crayfish in recent years and it's even featured as a storyline in "The Archers". But as Josh Archer discovered there are rules and regulations in place that you need to pay attention to. You need a licence for one thing and you have to be certain not to catch the rarer native crayfish only interlopers like the American signal crayfish. You can read more and apply for a licence .