I'm not sure where you get that from. I would assume you fish where you expect to find something, and my interpretation of the many places "fishing" is used (or even "phishing" to a lesser degree), is that it's done where you expect some payoff. Maybe you're thinking of the phrase "he's just fishing", which while often used by people to describe someone searching for something, often fruitlessly, generally the person searching is doing it with the expectation they will find something.