The lakes are public but the land around them is not.
The issue will be whether there is an easement on behalf of the fishermen. Blocking access to someone who has an easement is itself a trespass upon that right of easement and can create a cause of action for civil suit.
Also, the private ranch stocks the public lake with rainbow trout. Who owns the fish? See It's Not About the Fox: The Untold History of Pierson v. Post
On the flip side, what if patented [RoundUp Ready] seeds end up on public land, and grow, giving rise to more seeds; is there an act of infringement? What if the seeds end up on private land?