Why can salmon stay underwater all the time but a dolphin cannot?

This is a great question that highlights the differences in how fish and mammals breathe! Here's the breakdown:

* Salmon are fish: They have gills, which are specialized organs that extract oxygen from the water. They can stay underwater indefinitely as long as there's enough oxygen dissolved in the water.

* Dolphins are mammals: They breathe air through lungs just like we do. They need to come to the surface to take in fresh air.

While dolphins are incredibly adapted to life in the ocean, they can't breathe underwater like fish. They have to hold their breath and can only stay submerged for a certain amount of time before needing to surface.