In the Lake District in Britain, populations of native vendace, whitefish, and Arctic charr, are threatened after anglers who, actively breaking criminal law, stocked ruffe and roach.
The alien fish compete against the local fish, and prey upon them. Sickeningly, the anglers put in the fish so that they could be used as live bait in the "sport" of pike fishing.
STOCKING INTRODUCES PARASITES AND DISEASES
"Stocking waters with fish carries the risk of introducing parasites and diseases that may harm the fish, and developing fish stocks that could damage the environment, either by changing habitats or competing with other species."
"In the Lake District where native vendace, whitefish, and Arctic charr populations are threatened by ruffe and roach introduced illegally. These fish, again probably introduced by anglers live-baiting for pike, threaten native species by competing with them and preying on them. As it is practically impossible to remove them, they can now only be managed by stopping any more being introduced."