PA,

I would still argue containment or "privatized conditions". I'm not sure about PA, but in NY and CT landowners own the land under the river, just not the water. Therefore, they can entice and to some extent control the fish's environment, habitat and food resources. I know of a few people who actually downed large trees or built stone dams to create pools on their section of water, which created havens for larger trout and bass. The DEP/DEC doesn't really mind as long as water still flows and the areas downstream (or other landowner's properties) are not ill-affected by flooding, etc..

Deer can only be contained with a fence...but either in a contained lake/pond or river, that fish isn't leaving the water, only traveling up or downstream in a river.

Still a funny little tidbit, though...and these are just my best guesses.