Here's why:
* They breathe air: Blue whales have lungs and must come to the surface to breathe. Fish have gills and breathe underwater.
* They give birth to live young: Blue whale calves are born alive and are nursed by their mothers. Fish lay eggs.
* They are warm-blooded: Blue whales maintain a constant body temperature, unlike fish which are cold-blooded.
* They have hair: Though very sparse, blue whales have hair, which is another characteristic of mammals.
So, while they live in the ocean, blue whales are actually mammals, not fish.