Question

why are dolphins and whales considered as mammals although they live in water like fish

Answer 1

Mammals are those animal who give milk and give birth to babies instead of eggs

Whales and dolphins give milk and gave birth to babies. That is why they are considered as mammals

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Answer 2

Because they possess following mammillan characters:
1. Production of milk to feed their young ones.
2. Presence of hair on skin.
3. They’re warm blooded

Hence, whales and dolphins are mammals and not fish.

So we conclude that:
Not all aquatic animals are fishes.