Here's why:
* They breathe underwater using gills: Fish have gills, which allow them to extract oxygen from the water.
* They have scales: Salmon have scales covering their bodies.
* They lay eggs: Salmon reproduce by laying eggs, which hatch into larvae.
* They are cold-blooded: Fish are cold-blooded, meaning their body temperature changes with the environment.
Mammals, on the other hand, have lungs, fur or hair, give birth to live young, and nurse their offspring with milk.