Axe,
Pretty much as Cy said on the pest front and I have never come across a water where the white clawed cray is a 'problem' although the anglers at Brasside seem to think they are for some reason (reading too many comics probably). When i fished it they weren't a nuisance at all and just wandered around in the margins at night.

About 5 miles from me is a power station which has been used as a nest site by peregrines for the last 6 years. However there are pigeon lofts nearby so the pigeon racers don't like them and want them killed, even though they mainly feed on the skemmies and wood pigeons around Blyth. Are these a pest then? I would say no because they are only returning to areas they formerly occupied before they were almost driven to extinction by man. we have also provided them with a suitable habitat in the form of an artificial cliff (the building) and plenty of feral and woodpigeons because of the waste we produce.

Slightly different are grouse and hen harriers. Game keepers would regard hen harriers as a pest because they eat grouse, which is only their natural diet in the first place. The almost went extinct and are returning to moors. Game keepers will want to exterminate them if the reappear on 'their moor'. What the hell do they expect if they are managing a habitat that is the habitat favoured by the secuies in the first place? Instead of thinking 'oh no they'll cost me a few grand in grouse' they should be thinking 'blolody hell we have hen harriers back so we must be doing it right to support a population of them'. Besides there will only be as many hen harriers as the grouse population can support, therefore if you remove grouse by shooting you will only have a small population of hen harriers, which live at a low population density in the first place.

Things like rats and rabbits are a pest as they aren't native to Britain. Rabbits can decimate habitats if left to run wild and are out of hand, however they do provide some uses on habitats like sand dunes where they help to maintain the vegetation type through their grazing (a complex thing that i won't drone on about for ages like I have above )

Newt,
Yes I mean pollution free. Everywhere I have encountered the white clawed cray has been clear though.

As far as i know you can remove signals under licence/permission and you need this to try and prevent translocation into other waterbodies. The whole signal situation is being looked at at the moment by the EA and EN/JNCC.

Nice piece FA. Good information.

If your whites only like clear water, they are for sure a bit different than the US varieties you folks are having problems with. "Mudbugs" is a fitting name for them.

One tiny quibble. You wrote " ... Rabbits can decimate habitats ... " Since decimate means to kill/destroy 1/10, I'm not sure that is the term you intended there.

Newt, decimate as in destroy, through their digging and grazing. Have a look for some picuters of what they have done in Australia for the best example of how destructive they can be.
They are becoming a big problem in my local country Park as they are preventing the natural woodland flora from growing through digging (burrows and to get roots) and grazing seedlings and new coppice shoots.

With rabbits the solution is fairly simple. They are tasty and if hunted, the population will stay at a nice low, tolerable level. Also if the pred population is reasonable, they will stay under control. Foxes and hawks - to name just a couple - are very fond of rabbit and very good at catching them.

I've seen some of the OZ things. OTOH, they don't allow guns over there at all so IMHO they have asked for part of their problem. And I seem to remember they totally lack any pred that will take a rabbit other than the occasional croc if the rabbit is stupid enough to get too near the water. Bad situation altogether and I'm not really sure how (or if) they are going to get it under control.