My thoughts exactly Ross...these things have been introduced to numerous waters deliberately all over the country...and as usual profit is the at the root of things. An example of this, which typically grew from tiny beginnings to become something of a disaster was one I witnessed first hand....because it happened within a few hundred yards of my front door.
When I first moved to Chesham in 1971/2, there were a couple of very small fields close to my home, separated by a public footpath, the larger of these was used used as grazing for horses. At the end furthest from the footpath in that larger field, a small test dig to discover the potential for gravel excavation had been carried out at some stage (in the late fifties/early sixties I believe)...but not taken further at that time. The result was a small pond which was soon inhabited by roach, tench and a very few small carp, fish introduced by local anglers to create a tiny fishery. By the time I arrived, it was very lightly fished, but the roach had now grown on...beautiful mint fish to well over a pound, possibly approaching the magical two pound mark. The tench were numerous, but not quite the specimen size achieved by the roach...I certainly never caught anything above five pounds. The carp too were hardly monsters, a double being a good fish....but fun on light tackle.
My short term memory loss prevents me from recalling things exactly, especially dates, but my best guess is that it was in the late eighties that a small local-ish private company bought the mineral excavation rights, and gravel extraction began in earnest (G.E can put me right on any muddled facts here...he is nowhere near as senile as me

)
Several years later, a small lake of somewhere between one and two acres (guessing here again) existed in the larger field....and a pond of barely half an acre in the smaller field. Extraction was done and dusted, and the owner started to run the lake/s as a day ticket fishery. Then came the bad bit....his final fling was to add a seeding batch of the odious foreign crays to dredge (excuse the pun) the last ounce of profit from the water, by trapping and selling these to London restaurants. Sickeningly, to my knowledge this 'master plan' was never seriously pursued....probably too much hard work....but the damage had been done.
Unfortunately, the tiny historic chalk stream which gives the name to our town, the Chess, runs within perhaps thirty yards of these ponds, with one small branch of the river actually being fed by seepage from the smaller pond. The result was of course inevitable, the river now being infested with the damned things, and the native crays virtually (possibly by now completely) wiped out....certainly locally. Fishing the lakes now has the obvious drawbacks, though big carp are now present (one reaching 40+ at one stage). The roach population went down hill after the lake was enlarged, no doubt due to dual predation by the crays and the cormorants that visit....and the same for the tench, though a few larger samples do exist.
All in all, a prime example of profit being at the root of another piece of the jigsaw that is the ecological disaster these damned things have become...and having seen the futile attempts at eliminating them by trapping, I see no end to the problem any time soon...if ever. Damned shame...ecological vandalism in the name of profit. Why is it that that....and killing each other...seem to be two of the things that we are spectacularly good at?
Cheers, Dave.