Why worry? If misguided anglers stock your river with a new species we will simply accept that they're there, join a society to promote and protect them, and even forego the future of the resident indigenous species in favour of the new species. You may even be lucky enough to have local anglers so concerned enough about perceived low stocks of said new species to join your local angling club and commence talks on further stocking. Some people will even try to convince you that a drop in numbers of indigenous species is a fantastic opportunity for a new species to 'fill the gap'.