If "natural" is the key word Peter would it make a difference if a
"natural" disaster like a flood was the mechanism allowing the new species
into the waterway rather than deliberate introduction by a (shall we say)
Fish-stocking organisation.:-)
If we take a broader view perhaps we should consider all these mechanisms
(natural and ?synthetic) to be all part of the evolutionary process as are
our attempts ( futile as they may ultimately be) to keep the species
untainted.
Standing back a bit
Bruce Hansen