According to the BBC, Scottish Environment Minister Mike Russell has declared war on the American signal crayfish
"Any angler who catches one is urged to kill it on sight, not to throw it back into the water or take it away alive and contact the Scottish Government, Fisheries Research Services, Scottish Natural Heritage or the Scottish Environment Protection Agency."
It seems the crayfish have been carrying off the young fish and flooding the countryside by boring holes into the stream and river banks.
Personally I recommend they have a wonderful Crawdad Étouffée
I'll even help with a starting point of a recipe.
I have no idea what they should do with the other two American species which, along with a Japanese seaweed, fill out the rest of the ministers top invaders. Those animals are vertebrates and I'm not sure I could recommend eating American mink and grey squirrel, though I know some as would.