i think the issue arises from the legality of "his" pond.When a person starts asking about “legalities” in scenarios like this, I have to believe it’s because common courtesy and being neighborly isn’t working out for him.
I would say if the neighbor doesn’t want you fishing his pond that it would be a good idea to stay on your side of the fence no matter what the legality is.
where i'm from, no one owns the water lol
i'm assuming if i owned property and someone made a pond that extended onto mine i would most likely say that is not "their" pond.