fishing increments xp Level

Catch anything (any fish or item) with a fishing rod and your experience level is incremented by +1 rather than by experience points.

I'm pretty sure fishing isn't meant to make someone with level 50, gain level 51 by catching a single fish. Easy to reproduce, just fish.
I've gone from level 10 to level 134 by catching 124 fish and miscellaneous items/ armor/ weapons/ saddles.