How about a free or very inexpensive ($5.00 for the year) Park Fishing License? In Yellowstone, they do it that way (I forget the charge but it wasn't that high).
In the Smoky's a Tennessee / NC license and a yearly Park Fishing Pass.
I'd gladly pay it, especially if it improved enforcement and fisheries work.
Of course, administering such a thing could cost more than the revenue generated and it could even lead to harrassment by rangers (I could see getting checked multiple times a day by every ranger who drove by).
Doubt if there is one simple answer to the poaching question but perhaps a lot of little things (more volunteers, more involvement by us when we see something, more money through fees for enforcement, more awareness in local businesses, better sinage in the park,