I don't think its a natural migration of the species at all when the Russians have been actively transplanting them in Atlantic rivers for quite some time. They didn't get there from the Pacific which would be a natural migration and highly unlikely considering the distance. Its an invasive species in Ireland and the UK, Norway, Finland, Sweden or anywhere else they may be entering rivers in that part of the world , their concern is warranted as introductions of invasive species rarely have no other consequences on the ecosystems they come into. Hopefully they are unable to reproduce successfully.