Why are seals and sea lions not considered fish?

Seals and sea lions are not considered fish because they are mammals, not fish. They possess characteristics typical of mammals, such as breathing air through lungs, giving live birth, and nursing their young with milk. Additionally, seals and sea lions are warm-blooded and have a more complex nervous system compared to fish. These differences place them in the class Mammalia, while fish belong to various classes within the phylum Chordata.