Classification by Fat Content

Fatty, or pelagic, fish (from the Greek pelagos, meaning “ocean”) such as salmon, tuna, and eel swim over large distances, up and down currents. Constantly on the move, they require a consistent supply of fuel, which is provided by the oil stored in their muscle and tissue.

Lean fish, also called whitefish or demersal (from the Latin mergere, meaning “plunge” or “dive”), live just above the seabed, where they are less affected by ocean currents. These fish swim over much shorter distances, or in short bursts as needed to find food or escape predators. Demersal fish such as cod store most of their natural oils in the liver.