In the first place, the Environment Agency [or whichever government department it was at the time] and remember this is the department we anglers pay £25 million in licence fees to for the privilege of going fishing, should never have allowed their introduction from North America.
Trout fishery owners first applied to import them for the purpose of cleaning up dead trout lying on the bottom of their lakes, and for a ‘second’ crop. But of course it all got out of hand with the result that in a decade or two unless something is done to kill them off, they will have spread everywhere and pave the bottom of ALL our river systems.
Being more than twice the size and twice as aggressive than our native ‘white clawed crayfish, signal’s have all but wiped out the latter. Moreover, they consume vast quantities of invertebrate insect life which our river fish need to survive, and much of the spawn laid by our indigenous species. It really is a NO WIN situation in favour of this alien predator which can reach 8 ounces in weight.
They really do ‘pave’ the bottom in certain rivers, like sections of the River Lea, the Kennet, and particularly the upper Ouse just below Buckingham where the Claydon Brook joins the Ouse. For over a decade their existence produced multiple catches of huge perch to over 4lbs, plus the odd monster chub. All of which, once over a certain size grew to massive proportions upon the rich diet of crayfish. While the roach and dace and smaller fish lost out to cormorants.
It was however not going to last, because once the old monsters had died off, or started to fall foul of otters,[ which unfortunately is the case here] there was little allowed to come through by the colony of signal crayfish. Which more or less describes that particular part of the Ouse and Claydon Brook today. And while it is nice catching a whopper perch or chub every so often, a river supporting only a handful of big fish is an extremely fine balance.
They may be at the bottom of the pyramid, but signal crayfish hold part of the key to whether our rivers will ever be the same again..