Carp have already changed the entire face of fishing in this country, not to mention a lot of waters. With them gaining a foothold in several river systems, it remains to be seen what's going to happen.
Beavers are far fussier and there are few habitats suited to them. They certainly aren't going to spread like the mink, because they are a highly specialised animal, which requires the right blend of surroundings.
As the Welsh experiment a decade or so ago shows, they can be stocked into seemingly ideal habitat and still disappear.
They're also vegetarians, so it's hard to see what damage they could do to fish stocks.
Otters are making a comeback and there are far more food sources available to them than there were in the 1960s and 70s - before they went into sharp decline - because of the number of gravel pits which have been dug, most near to rivers.
For example John Wilson's first Go Fishing book, published in 1973, contained around 40 stillwaters. The new edition, out a week or two back, catalogues 300-odd.
These days most pits contain large fish, which are semi-dormant in the winter like bream, carp and tench.
This is precisely the time when otters find it harder to feed, because river water is coloured-up and they hunt by sight.
Most pits which are no longer worked tend to be clear in winter, so they're an easy source of food.