What is the difference between fish and whales?

Whales are considered to be mammals, while fish are simply fish, a different class of animal, and are not mammals. Whales are considered mammals because they are warm blooded and breathe air through lungs. Additionally, they give birth to live young and feed that young with milk.

Whales are not the only marine mammals in existence. Dolphins, walruses, porpoises and other marine mammals all have things in common to separate them from fish species:

• While fish lay eggs to reproduce, mammals mate and give birth to live young after gestation.
• Fish do not feed their young with milk, while mammals do.
• Fish have scales, while marine mammals have skin and fur, although the fur may be sparse.
• While whales do live underwater, they still breathe oxygen through lungs. Whales have blow holes above their heads that allow them to come to the surface to inhale air. Whales can stay under the surface of the water for up to 120 minutes, making them the most adapted to the ocean out of other marine mammals.
• The whale has a close biological resemblance to the hippopotamus and even has evidence of "hind legs" in their skeletal structure, although they are not visible from the outside.
• A whale's horizontal fin has an up and down motion, as opposed to the side-to-side motion of a fish's tail.