Freshwater salmon and saltwater salmon are the same species. In North America, these include Atlantic salmon, of which there is just one species, and five Pacific salmon species: coho, chinook, pink, sockeye, and chum. There are, however, many differences in behavior and physiology between freshwater salmon and saltwater salmon. Some grow larger in saltwater, some don’t eat when they are in freshwater rivers, some die after spawning in freshwater, and so forth.
All salmon are classified as anadromous, a term derived from Greek words that mean upward (ana) and running (dromos). This refers to fish that spend part of their lives in the ocean and move into freshwater rivers or streams to spawn. Thus, anadromous fish are born in freshwater, move to saltwater to grow to adulthood or sexual maturity, and then return to freshwater to reproduce. When to fish for salmon is dictated by when they appear in these different locations.
Atlantic, coho, and chinook salmon are the best-known anadromous fish. Others include steelhead, sturgeon, striped bass, herring, and shad. Around the world there are approximately 100 species of anadromous fish.
Some Live Entirely in Freshwater
Complicating an understanding of anadromy is the fact that some anadromous species have adapted, either naturally or by introduction, to a complete life in freshwater environments. These species, which include freshwater salmon, striped bass, and steelhead, make spawning migrations from lakes, where they live most of their lives, into rivers to spawn. In such instances, these fish are originally saltwater in origin. They remain anadromous when moved into purely freshwater, although they use the lake as they would the ocean.
Fish that originate in saltwater but have freshwater forms are often called "landlocked," whether or not they have a clear path to and from the sea. Sometimes these fish are physically blocked from reaching the ocean. Fish in a reservoir or lake may be unable to leave. Fish in some streams, like those in high-mountain areas, have a clear passageway to the sea but no means of returning because of obstructions, particularly waterfalls.
Sometimes the terminology used for the freshwater forms of saltwater fish is confusing. Atlantic salmon that exist in freshwater lakes without access to the sea are popularly called landlocked salmon or landlocked Atlantic salmon to distinguish them from the saltwater version. Some folks may refer to them simply as freshwater salmon. Sockeye salmon that exist in freshwater lakes without access to the sea are often called Kokanee salmon. However, coho and chinook salmon (as well as striped bass and Arctic char) are called by the same name in saltwater or freshwater, although chinook salmon are sometimes called “king” salmon in freshwater and saltwater.
Regardless of what you call them, remember that if you fish for any salmon in saltwater or in tidal rivers, you need a saltwater fishing license; if you fish for salmon in freshwater environs you need a freshwater fishing license.