First off, Wii or GC? Seccond, Bobber fishing, or the fishing mini-game (where you're in the boat).
Guessing you're in the boat, fish near the arch shaped rock, north of the arch shaped rock, or south of the arch shaped rock. This is where I found most of my fish.
If your bobber fishing, follow what tbug2007 said.
Hope I helped!
I'm Baaaaack. :P