This thought occured to me recently. I think mermen might eat fish.
Merman are like fish and like humans, but they are not either. It's like humans eating monkeys or something, maybe close to cannibalism but not quite. However, I suppose we may not know enough about mermen in OP to be sure.
So, would a merman consider a fish close enough to himself to eat one? Then again, what do people who I assume live close to the sea eat? Also, there's Chopper's eating of meat, but that might be filler.