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Grizzly bear fishes for a salmon during a salmon run. The salmon run is the time when salmon, which have migrated from the ocean, swim to the upper reaches of rivers where they spawn on gravel beds. Salmon spend their early life in rivers, and then swim out to sea where they live their adult lives and gain most of their body mass.
When they have matured, they return to the rivers to spawn. Usually they return with uncanny precision to the natal river where they were born, and even to the very spawning ground of their birth. In northwest America, salmon is a keystone species, which means the impact they have on other life is greater than would be expected in relation to their biomass. Most salmon are anadromous, a term which comes from the Greek anadromos, meaning “running upward”. Anadromous fish grow up mostly in the saltwater in oceans.
Anadromous salmon are Northern Hemisphere fish that spend their ocean phase in either the Atlantic Ocean or the Pacific Ocean. They do not thrive in warm water. There is only one species of salmon found in the Atlantic, commonly called the Atlantic salmon. These salmon run up rivers on both sides of the ocean.
The life cycle of an anadromous salmon begins and, if it survives the full course of its natural life, usually ends in a gravel bed in the upper reaches of a stream or river. As they approach the time when they are ready to migrate out to the sea the parr lose their camouflage bars and undergo a process of physiological changes which allows them to survive the shift from freshwater to saltwater. At this point salmon are called smolt. Then, in one of the animal kingdom’s more extreme migrations, the salmon return from the saltwater ocean back to a freshwater river to spawn afresh. After several years wandering huge distances in the ocean, most surviving salmon return to the same natal rivers where they were spawned.