Since salmon do not feed once they leave the ocean, some will die on the way because they lack enough stored body fat to make the trip. Many will be caught in fishermen's nets. Those that evade the nets may have to swim through polluted waters near cities. Many must make their way over power dams, leaping up from one tiny pool to the next along cement stairstep cascades called fish ladders. In the tributary streams, waterfalls and rapids are steep and swift enough to eliminate all but the strongest. Otters, eagles, and bears stalk the salmon in shallow riffles. Once on the spawning grounds, the fish battle each other: females against females for places to nest, males against males for available females.

The female builds her nest, called a redd, by agitating the bottom gravel with her fins and tail, and bending her body into a U shape first one way, then the other. As soon as she has excavated a depression, she settles onto it and deposits her first batch of eggs, or roe. The male then moves alongside and deposits his sperm, called milt, over the roe. The female rakes her tail back and forth to cover the redd with loose gravel. She then excavates her next redd a short distance upstream.

The process continues until all the roe and milt have been deposited. One pair of salmon may have as many as seven redds, with four or five being the average. The salmon die within a few days of spawning.

The translucent salmon eggs range in color from pale yellowish-orange to dark reddish-orange. The color varies both by species and within species and is determined by water temperature, sediment composition, age, and other factors. The eggs vary in size from the tiny Sockeye roe (average 1/4 inch across) to the large Chum roe (average almost 1/2 inch).

Incubation time ranges from 5 to 10 weeks. The newly hatched fish are called fry, or sometimes “sac fry” because they continue to feed on the yolk sac attached to their bellies. Depending on water temperature, species, and other factors, they may stay in the gravel for several weeks before swimming up into the open water of the stream, where they feed on plankton and other tiny aquatic organisms. By the time they reach fingerling size (approximately 3 inches), most Pink and Chum salmon have begun the long journey downstream and out to sea. Sockeye, Coho, and most Chinook spend from 1 to 2 years in fresh water before heading out to sea.

Stream currents carry the young salmon tail-first to the ocean, where they will spend the majority of their lives. Gradually, their bodies undergo physical and chemical changes to enable them to survive in salt water.

Most adult Pacific salmon species feed on aquatic insects and small fish. However, Sockeye are filter feeders. They take in water full of plankton, and as the water flows back out of their mouths, specialized organs called gill rakers act like strainers, holding the plankton in to be swallowed.

Some species of Pacific salmon stay within a few hundred miles of their home river, while others disperse north, south, west, or in the case of salmon originating in Russian and Japanese rivers and streams, east, into feeding grounds in the Aleutian Islands and other areas of the north Pacific. Chinook Salmon may travel as far as 2,500 miles from their home stream and stay out at sea 4 to 7 years. Pink Salmon, on the other hand, seldom range more than 150 miles from the mouth of their home river or stream where they hatch in the fall, and turn homeward in the spring, sometimes traveling 45 miles per day to reach their spawning grounds.

Pacific salmon encounter increasing human-caused hazards in their migrations to and from spawning grounds. All salmonids require pure, well-oxygenated cold water and are one of the first species to suffer when water quality is degraded. Many salmon stocks are seriously threatened by what are called the "four H's": Habitat destruction, Hydroelectric dams on migratory rivers, over-Harvest of rare stocks, and competition with Hatchery fish. Some stocks are so severely reduced that they have been listed as endangered or threatened species under the Endangered Species Act. “Endangered” means they are likely to become extinct. “Threatened” means they are likely to become endangered in the near future—Coho found along Oregon’s coast were listed as threatened in 2011.

A 1991 report by the American Fisheries Society indicated that 214 of about 400 stocks of salmon, steelhead, and sea-run Cutthroat Trout in the Northwest and California are at risk of extinction. The report also indicated that 106 are already extinct.

Primary listing and recovery responsibilities for Pacific salmon belong to the Department of Commerce's National Marine Fisheries Service. The U.S. Fish and Wildlife Service and other federal and state agencies also have recovery responsibilities.

The largest of the Pacific salmon, Chinook (Oncorhynchus tshawytscha) average about 24 pounds when they return to their natal river to spawn, most after 2 or 3 years at sea. The Chinook is the least abundant of the Pacific salmon.

Coho Salmon (Oncorhynchus kisutch), fourth in Pacific fishery abundance, is the number one sport fish. It spends only one winter at sea, returning the next fall to spawn. It averages about 10 pounds when full grown.

Sockeye Salmon (Oncorhynchus nerka) make up about 25 percent of the West Coast catch, and chum salmon (Oncorhynchus keta) make up about 13 percent. Both follow similar migration paths in the Pacific and reach a common weight of about 12 pounds before returning to their natal river to spawn.

Pink Salmon (Oncorhynchus gorbuscha), the smallest of the Pacific species, average only about 3 to 5 pounds. However, they make up more than half the total West Coast commercial catch. Pink Salmon seldom travel more than 150 miles from the mouth of their natal river.