Spawning

Salmon that survive to maturity in the oceans return to the freshwater river of their origin to spawn, although some may stray to other streams. Scientists cannot fully explain why salmon are able to return to their native rivers to spawn, but it is known that in the final stages of their migration, the fish are guided largely by their sense of smell.

The migration to the spawning grounds begins at the same time for all the salmon in any one population. Depending on the particular population, the migration may begin any time from May to December, and spawning takes place from about June to January. The trip may be 200 miles (320 km) or more upstream. At the start of the run the fish are in prime condition; during the migration they eat nothing.

Once the spawning grounds are reached, the female, by using her tail, makes a nest (called a redd)in the gravel. She deposits her eggs, and the male releases sperm to fertilize the eggs. Salmon lay several thousand eggs. After spawning, the adults of the Pacific species die. Adult Atlantic salmon slowly make their way downstream to reenter the sea. The Atlantic salmon may spawn several times before dying.

The eggs hatch the following spring. Attached to the underside of a baby salmon (called an alevin or fry)is a yolk sac that will nourish the young fish for a few weeks. After the food material in the yolk sac is used up, the fish is called a parr or fingerling. At this time the young fish may immediately make its way to saltwater, or it may remain in freshwater for up to four years, depending on the habits of its population. At all times, salmon are prey to other animals, and only a small percentage of them live to reach the ocean. The salmon remain in the ocean from two to eight years before they are ready to spawn.

Where Are Baby Salmon Born?

Most adult salmon live in the salty ocean. In the summer and fall, however, they swim up a freshwater stream to spawn, or mate. That’s where the females lay their eggs.

Soon after the adult salmon arrive, the female digs a redd, or nest. She does this by slapping her tail hard on the gravel bottom. When the redd is finished, the female lays her round, pink eggs. As the eggs are laid, a male fertilizes them.

After the redd is filled with fertilized eggs, the female swims a little farther and digs a new redd. As she digs, she stirs up gravel with her tail. The current carries the gravel over the old redd. There the gravel settles and protects the eggs. Altogether, a female salmon may lay between 2,000 and 17,000 eggs, depending on her size.

How Do Baby Salmon Grow?

Baby salmon first grow inside eggs. They use the yellow-orange yolks in the eggs as food. After two to four months, the babies hatch out of the eggs.

After hatching, the baby salmon are called alevins (AL uh vihnz). They stay in their nests for several weeks or months. During this time, these tiny salmon still have yolk sacs attached to their bodies that supply them with food.

While in the nest, an alevin smells the water around it. Specific rocks, plants, and chemicals in the stream give the water its smell. And no other stream has the exact same smell. An alevin remembers this smell and never forgets it. When it is an adult, a salmon uses the smell to return to the exact same stream to spawn.

What Is a Salmon Fry?

A salmon fry sounds like a fish dinner, but it’s actually the name for a young salmon. By the spring, a baby salmon is big enough to leave its nest. When it is ready to do this, it is called a fry. A fry is about 1 inch (2.5 centimeters) long.

The yolk sac that had fed the alevin until this point is gone. Now the fry must start to look for food. It eats insects and small water organisms called plankton. Fry must be careful because trout and other big fish eat them. Hungry ducks, herons, kingfishers, and sea gulls do, too. The fry starts to grow scales on its skin to protect its body. Most salmon fry also develop spots called parr marks. These spots help fry hide in the rocky stream. Even so, many of them are caught and eaten.

The fry of some kinds of salmon stay in fresh water for only a few days. Then they start their long swim to the salty ocean. The fry of other kinds of salmon stay in fresh water much longer before heading to the ocean.

When Is a Salmon a Smolt?

A salmon becomes a smolt when it swims from its freshwater stream and heads to the ocean.

A smolt’s body goes through many changes as it makes this journey. It loses its parr marks. It turns dark on top and silvery-white on its sides and belly. Its new colors help the smolt hide in the ocean. From above, a smolt’s dark top will be hard to see against the deep blue waters of the ocean. And from below, a smolt’s silvery-white belly will be hard to see against light shining down through the water.

Changes also take place inside the young salmon. Its kidneys and gills become very good at getting rid of large amounts of salt. This will come in handy in the smolt’s new ocean home. Many smolt actually wait at or near the mouths of streams or rivers for several months. This water is less salty than water farther out in the ocean. Waiting here gives a smolt’s body time to get used to the change.

How Do Salmon Return Home?

When adult salmon are ready to spawn, they return to their home streams. Salmon use their sense of smell to help guide them back to the same stream where they hatched.

On their journey home, adult salmon make many changes—much as they did on their first trip to the ocean. Most salmon change color to attract a mate. Adult chinook salmon turn from silvery-white to brownish-red. Sockeye salmon turn from silver to bright red. Most salmon also stop eating and focus on their journey.

The trip home is very hard. It can take several weeks, and it can cover 2,000 miles (3,200 kilometers). The salmon must swim upstream, against the water current. They leap high over big rocks and waterfalls. Some become weak or get injured and die along the way. For most kinds of salmon, even those that survive the trip die after they spawn.