One important aspect of this hypothetical river is what other rivers are nearby and how they are managed. The impact and fishing pressure on this river X has a lot to do with what's available in the next river down the road. The health of the wild fish in the next river down the road has a lot to do with how this river should be managed too. I think we've reached the point that we need to set aside certain rivers for the well-being of the fish: likely those with the best habitat, and set aside other rivers for the fishermen.

Without fishermen, who really cares about the fish?