The Salmon Reach Sexual Maturity
Young fish migrate through their natal streams to the ocean. There they grow, eat and reach sexual maturity in about three to five years, depending on the species of salmon. Once they are old enough, they return to the rivers in which they hatched. Pacific salmon will mark their lives' ends with this journey, while Atlantic salmon repeat the mating process over a number of years.
Salmon Travel to Their Natal Streams
In late winter or early spring, the ocean-dwelling salmon migrate to return to the streams and rivers in which they were born. Oftentimes these fish return to their natal rivers in groups called runs. The journey is a dangerous trek, but they can be seen jumping upstream in some places, just to return to the exact place where they first hatched. Predators---most notably bears---oftentimes await the returning salmon and prey on them as the fish navigate jumps and shallow places.
Salmon Battle Over the Best Spawning Grounds
When the female salmon has reached the spot she instinctively has selected for reproduction, she searches for a good place in the river bed to dig a nest---also known as a redd. She is in direct competition with other females and only those who emerge victorious from turf battles will dig their redds.
Female Salmon Dig Redds
The female salmon digs the redd in the riverbed with her dorsal fin. The nest measures about 30 square feet, sometimes more. Even though the nest is little more than a shallow depression, the salmon removes rocks and debris before she lays between 1,500 and 10,000 bright red fish eggs, or roe.
Male Salmon Deposit Milt over the Roe
The female salmon changes her color during this process and signals her readiness to mate with the male fish that has also migrated back to the stream or river. The males approach the female as she hovers over her redd and emit their sperm, also known as milt, over the roe. The female fish now uses her dorsal fin to spread some mud and gravel over the fertilized eggs.
The Process is Repeated up to Seven Times
Now the female salmon moves onto another area of the riverbed and repeats the process of digging a redd and depositing her roe. Once again she attracts male salmon to swim over the roe and fertilize it with their milt. Lastly she will repeat the covering of the redd with mud and gravel. She may continue to duplicate this process up to seven times or until she has no more roe.
Pacific Salmon Die
Male and female salmon may remain in the rivers for a couple more weeks, but in the case of the Pacific fish, they refrain from eating and eventually die of starvation. Atlantic salmon return to the ocean and regain their strength and also build up fat reserves for the next trek to their spawning grounds.