tell you what though, them cats do grow big lol, not letting my dog swim in there.
ive got river monsters book and man has actually prevented 'invasive species' entering our rivers through locks and dams etc not to mention stopping eels and salmon etc from breeding, so one could argue that man is the invasive species, in some of the world bull sharks have been found 250 miles inland up rivers changing from salt to freshwater, one could argue that man is preventing these sort of acts of nature happening. Afterall I dont think carp are native to the nene or any other river but are escapees from flooding, from pits stocked by man and flooded due to building on flood plains.
can we really fight a war against our own actions or just let nature get on with it? Sorry for droning on