While I don't want to get involved in the domestic politics of another
country it does seem that our situation could be unwittingly used in
your argument.
 
There are a large number (exactly how many is not known) of feral
ferrets in New Zealand.  They have been feral for over 100 years and,
initially at least, their existence was due to deliberate pest-control
releases into the wild of "ordinary" ferrets from Britain. From 1882
thousands of ferrets were introduced from Australia and Britain and
thousands more were bred locally by the Department of Agriculture until
1897.  These releases were accompanied by legal protection until 1903 to
allow their numbers to build to match the rabbit explosion at the time.
It is also possible that their numbers were "refreshed" in the late 1970s
when breeding fitches were released as the fur market disappeared.
 
Their feral existence is a bit of problem but the fact is they had
quite a helping hand to get where they are today.  Consequetly there is
no establishment comparison between here and where you folk are. The
only thing that could be drawn from here is that once established ferrets
appear capable of maintaining a feral population.
 
Possibly a more useful comparison is with domestic cats & the extent to
which they can establish feral populations.
 
Good luck
 
Lawrence Roberts
[Posted in FML issue 1184]