Roaches are a type of fish.
Like all types of fish, each roach gives one fish fillet when caught.
Roaches can be caught with green lures in any fishing spot, but their favorite spot is #1 (the shallow area in front of the fisherman's hut).
Roaches can be caught in large circles and therefore are easier to catch.
See Fishing List for more detailed information.
Roaches in the fishing book
The roach is on page 3 of the fishing book:
Common and easy to catch, this sporting fish still enjoys the thrill of the chase.