What reward(s) can I get from fishing?
What reward can I get from fishing at the Fishing Hole? Is there treasure or equipment to be won from fishing, even if I catch the Hylian Loach?
You can earn a heart piece if you catch a fish as a kid, and the golden scale (which allows deeper, longer diving) as an adult.
As per Krazer, in the comments, there is a 50 Rupee reward from catching the Loach, and the owner will refuse to keep it in the fish tank (where your record catch is usually displayed).