Answer:
Dolphins are mammals, not fish. Unlike fish, who breathe through gills, dolphins breathe air using lungs.
They don't have gills, so they can't breathe underwater like fish. They breathe through a set of nostrils on top of their heads known as a blowhole. While swimming or resting under the water, they can take breathes by exposing only the tops of their heads to the air.
Like all mammals, dolphins have lungs and breathe air. They are the organ of the respiratory system which process the air inhaled through the spiracle.