Rainbow smelt are a type of fish.
Like all types of fish, each rainbow smelt gives one fish fillet when caught.
Catching rainbow smelt
Rainbow smelt can only be caught with blue lures in fishing spot #3 (the reedy area under the waterfall).
Rainbow smelt can be caught in small circles and therefore are harder to catch.
See Fishing List for more detailed information.
Rainbow smelt in the fishing book
The rainbow smelt is on page 9 of the fishing book:
Shimmering, sleek and beautiful, this fish really loves to show off its scales.