The fish in every pond were either stocked or came from some where upstream. Like esshup said unless you tagged every fish you stocked and every single fish you ever caught. Even then, not every fish would be tagged.
I think it comes down to personal morals.
For me, jludwig nailed it. We can discuss who has the most resources, and where the burden of proof lies, until those cows come home. Fact is, if my pond is the only one around with Black Crappie in it, and my pond's unscreened drainage is at a higher elevation than my neighbor's pond, and he suddenly finds BC in HIS pond, I'm gonna' be pretty sure where those fish came from.
So, should I make an effort, as a responsible pondmeister, to try and limit the chances of my fish escaping? Or should I do nothing, and hope he either can't, or is unwilling to front the resources necessary to pursue any legal action that might be available to him?
Remember, this can also go the other way just as easily. My ponds are on two different elevations, with my property line behind the upper pond, at a higher elevation yet! If my neighbor should build a pond, there is a very real possibility that his overflow could find it's way into my pond.
Should I expect him to at least make an effort to restrict fish travel through his overflow? Or resign myself to the fact that he subscribes to the "whatever happens, happens" philosophy?
Would there be legal recourse available, or would I be forced to hope that his moral integrity would preclude such an event in the first place?
It seems to me, that if I profess to be a responsible, ecologically minded pondmeister, then I am morally bound to do all in my power to try and lessen the chances of my fish escaping into private, or public waters, irregardless of whether I think any damage might be caused by their introduction.
Anything else would strike me as hypocritical behaviour on my part.
