Why Do Salmon Swim Upstream? The Spawning Migration Explained

Why do salmon breed in rivers but feed at sea?

Salmon are anadromous, which means they are born in fresh water, grow up in the sea and come back to fresh water to breed. Each half of the journey gives them something the other cannot.

The ocean is where the food is. Feeding grounds in the North Atlantic and North Pacific are rich in krill, shrimp-like crustaceans and small fish such as herring and capelin. An Atlantic salmon may leave its river weighing well under 100 g (3.5 oz) and come back one to four years later weighing 2–9 kg (4.5–20 lb) or more. No river could feed a fish that grows so fast.

Rivers, on the other hand, are safer nurseries. Salmon eggs need cold, clean, oxygen-rich water flowing through loose gravel, and shallow headwater streams have far fewer egg-eating fish than the sea. Bigger females can lay more eggs, so growing large at sea and breeding in rivers lets salmon get the best of both worlds.

What happens to salmon on the journey upstream?

The upstream run is one of the hardest journeys any fish makes. Adults stop eating once they enter fresh water and must live on the fat and muscle stored at sea. Their bodies also change for breeding: their eggs and sperm ripen, their silver skin turns bronze, red or dark, and males grow a hooked jaw called a kype.

Distances vary from river to river. Many Atlantic salmon spawn within tens to a few hundred kilometers of the sea. Some Pacific salmon go much farther: Chinook and sockeye that spawn in central Idaho travel about 1,400 km (900 mi) up the Columbia and Snake rivers and climb about 2,100 m (7,000 ft) above sea level.

Atlantic salmon often enter rivers months before spawning. They wait in deep pools through the summer and move upstream in bursts, especially when rain raises the water level. Higher, faster flows, called spates, help them swim over shallow riffles and low weirs.

How do salmon leap waterfalls?

When salmon meet a waterfall or weir, they gather in the pool below and look for the best place to jump. They use the standing wave at the base of the fall, where falling water bounces back up, as a launch pad. A few strong tail beats from powerful white muscle, the type used for short bursts of speed, drive them up and out of the water.

Salmon have been recorded jumping as high as about 3.65 m (12 ft), but most leaps are much lower. A fish may fail many times, fall back, rest and try again. Waterfalls can delay a run for days when the water is too low or too high, and tall natural falls mark the upper limit of salmon in many rivers.

Leaping salmon also attract predators. In Alaska, brown bears gather at waterfalls such as Brooks Falls in Katmai National Park to catch sockeye in mid-air, and salmon are a key food for many bears, as explained in What do brown bears eat?

Fish ladders and dams

Dams, mill weirs and other barriers block salmon from many miles of river. A dam that is too high to jump can wipe out the salmon above it within a few generations, because the fish cannot reach their spawning gravel.

Fish passes, also called fish ladders, give salmon a way around. The most common type is a series of small pools that rise in steps, so the fish can swim or hop from one pool to the next and rest between them. One of the best known in Europe is the ladder at the Pitlochry dam on the River Tummel in Scotland, about 310 m (1,000 ft) long and completed in 1951, where visitors can watch salmon through an underwater window.

Removing old dams often works even better than building passes. On the Penobscot River in Maine, the removal of the Great Works and Veazie dams in 2012 and 2013 reopened much of the river to Atlantic salmon, shad and river herring.

When do salmon runs happen?

Atlantic salmon can be found coming into rivers in almost every month, but each river has its own pattern. In Britain and Ireland, large “spring salmon” that have spent two or more winters at sea often arrive early in the year, smaller grilse that have spent one winter at sea run mainly in summer, and many more fish arrive in autumn. Spawning happens from October to January, most often in November and December.

In eastern Canada and Maine, most salmon return between June and October and spawn in October and November. Pacific salmon runs vary by species and river, from spring to winter. In British Columbia, the sockeye of the Adams River peak in October, with an especially large run every four years.

What sets the timing is a mix of age, day length, water temperature and river flow. Salmon tend to stop moving when the water is too warm or too low, and set off again after rain.

Do salmon swim upstream to die?

Pacific salmon do die soon after spawning, but that is the end of their journey, not its purpose. Atlantic salmon are different: many die, but a minority survive as thin, dark fish called kelts and return to sea to feed and spawn again. To learn why, see Why do salmon die after spawning?

Finding the right river is a feat of its own. Salmon use Earth’s magnetic field at sea and their sense of smell near the coast, as explained in How do salmon find their way home?