Here's why:
* Mammals are warm-blooded animals that give birth to live young, nurse their offspring with milk, have fur or hair, and breathe air.
* Fish are cold-blooded animals that live in water, breathe through gills, and lay eggs.
Walruses clearly fit the description of mammals: they are warm-blooded, give birth to live young, nurse their pups, have thick blubber and fur, and breathe air.