Sorry but i'm going to disagree with John, well a litle anyway
It's a fair comment about the overfishing of the sea's in britain forcing them inland, although i was under the impression that the one's inland are a subspecies?? and it's also fair to say that stocking fish is not a natural way for lakes to survive.
But, it's not just these lakes they have been a problem on, they have caused major problems in river systems that aren't stocked, virtually wiping out rivers
now if this was a
Mink killing
Water Voles or Grey Squirrel passing on disease people would be saying that we have to do something about it. But it seems that just because it's fish it doesn't matter
Artificial lakes can survive because of the stocking of fish, but many of the places they inhabit aren't these, they are natural lakes and rivers which are much quieter that commercial fisheries and therefore the birds are happier being there. These type of places need protection from cormorants IMO