Fishing pays its experience when a fish is landed, weighted by the catch. Its curve is cheaper per level than channelling's — what is being levelled is patience rather than throttling.
What a level buys
Reach, in both directions: each level adds to the chance per second that something takes the lure, and raises the top of the size roll — you do not stop catching small fish, the big ones simply become possible. See Fishing for the whole fight.