There are currently five species of fish in North America which are commonly referred to as Pacific salmon. These five species are chinook, coho, sockeye, chum, and pink salmon. Steelhead and sea-run cutthroat were recently added to the Pacific Salmon genus, but most people still call them trout. There are also two other species, masu and amago salmon, which occur only in Asia.

All Pacific salmon are anadromous. The word anadromous comes from the Greek word anadromos, which means "running up." Anadromous fish are fish that migrate up rivers from the sea to spawn in fresh water.

All Pacific salmon except steelhead and sea-run cutthroat are also semelparous. Fish that are semelparous are fish that die after spawning. Steelhead and sea-run cutthroat are iteroparous, which means that they are able spawn more than once during their lifetime.

Salmon Anatomy Diagram

Salmon Anatomy Diagram

Anal fin - Used for balance.
Caudal fin - Used to propel fish through the water with powerful sweeping movements.
Dorsal fins - Used for balance.
Gill Cover - Protects gills.
Gills - Aquatic respiration organs used to obtain oxygen from the water.
Lateral line - A line of sensory pores along the side of salmon by which currents, vibrations, pressure changes, and the movement of other bodies in the water are detected.
Pectoral fins - Used for steering.
Pelvic fins - Used for balance.

Salmon illustration in "Salmon Anatomy Diagram" courtesy of the U.S. Fish and Wildlife Service.