Black bullheads are a type of fish.
Like all types of fish, each black bullhead gives one fish fillet when caught.
Catching black bullheads
Black bullheads can be caught in small circles and therefore are harder to catch.
See Fishing List for more detailed information.
Black bullheads in the fishing book
The black bullhead is on page 20 of the fishing book:
This night owl loves to sleep in - wake him up, he'll be sour!