Are dolphins and whales a type of fish?

Dolphins and whales are not fish. Fish is any cold-blooded (poikilothermic) vertebrate animal that lives in water. A fish typically has jaws, fins, scales, a slender body, a two-chambered heart, and gills for providing oxygen to the blood. Fish is capable of breathing under water without making frequent trips to the surface to breathe air like marine mammals. This is due to the fact that fishes have a breathing organ known as gills. This is made up of thin feathery sheets of tissue membrane containing many blood vessels through which oxygen passes allowing fishes to breathe. In other words, fish use their gills to extract oxygen from the surrounding water in which they live.

Dolphins and whales on the other hand, are mammals just like humans and breathe air using lungs, they give birth to live young (rather than laying eggs), and feed their young with their milk. They do not have gills and get the oxygen they need by breathing air, using their lungs. This explains why they come to the surface of the ocean. They have a hole (their nose) in the flat part of the middle of their heads called blow holes. This closes when the whale or dolphin is under water and it opens when they are at the surface and need to breathe.

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