Most waterbirds that eat fish, swallow it whole: Great Blue Heron, Common Merganser, Horned Grebe, Pied-billed Grebe, Common Loon, Belted Kingfisher. These birds lack the ability to hold a fish with claws, tear it apart, and eat it piece by piece, as would an Osprey or a Bald Eagle.
So, how does it happen that the female Common Merganser, below, is downing only a piece of a fish? The answer is simple, she was hunting in a spawning creek which contained not only complete fish, but also fragments that had been torn apart by other birds and mammals.
A female Common Merganser is about to down a fish fragment.