Bitterlings are a type of fish.
Like all types of fish, each bitterling gives one fish fillet when caught.
Bitterlings can only be caught with purple lures in fishing spot #6 (the deep water area with a sunken ship).
Bitterlings can be caught in small circles and therefore are harder to catch.
See Fishing List for more detailed information.
Bitterlings in the fishing book
The bitterling is on page 6 of the fishing book:
This minnow makes friends across species. Its babysitter is a mollusc!