Salmon

Salmon are various species of medium-sized, fusiform (a vertically compressed, torpedo shape) fish with small scales. Their fins are arranged like those of most freshwater fish. On the underside are two pectoral fins, a pair of pelvic fins, an anal fin, and a caudal (or tail) fin. On the back are a dorsal fin and a smaller adipose fin located in front of the tail. The mouth is wide and has numerous strong teeth. The coloring ranges from silvery, to green, brown, gold, or red, and changes with environmental conditions and stage of life. At sea, the muscle of most salmon becomes pink-colored as they accumulate fat; in freshwater, most species become somewhat paler-green. Salmon are native to the Northern Hemisphere, but some species have been introduced to the Southern Hemisphere. The lifestyles of the various species are broadly similar; they lay their eggs in freshwater, are born and spend their early juvenile life there, then migrate to ocean to feed, and return as adults to their natal river to spawn.

Salmon belong to the family (Salmonidae), in the suborder Salmonoidei of the order Salmoniformes. The salmon family is broken down into three subfamilies, containing species of salmon, whitefish, and grayling. Within the subfamily of salmon, there are five genera: Salmo (Salmon, also containing trout), Oncorhynchus (Pacific Salmon), Hucho, Salvelinus (Charrs), and Brachymystax.

Life cycle of Atlantic salmon

The life-cycle of the Atlantic salmon is typical of other species. While some populations live their entire lives in Sockeye salmon (Oncorhynchus nerka). Photograph by Kennan Ward. The Stock Market. Reproduced by permission. inland waters, most leave the river where they were born, going out to sea to feed and grow. At sea, Atlantic salmon feed voraciously on smaller species of fish. When they become sexually mature they return to their natal freshwater habitat to spawn. Individuals may enter the rivers at different times of the year, but spawning always takes place in the wintertime, from about October to January.

When preparing to spawn, the female digs a shallow nest, called a redd, by pushing pebbles on the river floor out of the way with her tail. The redd is generally 6-12 in (15-30 cm) deep, and a few stones are usually present on the bottom. In a crouching position, the female then lays her eggs; at the same time, the male, also crouching, fertilizes them with his milt. While this is occurring, young males who have never been to sea may dart in and out of the nest, spreading their own sperm. This behavior ensures that most of the eggs will be fertilized.

The female repeats this nesting procedure several times in separate locations, moving upstream each time. She covers her older nests with the pebbles from the newer ones, thus protecting her eggs. Overall, spawning lasts about two weeks, during which time the salmon lose about 35% of their body weight. In this depleted condition, they are known as kelts. They return downstream, and in their weakened physical state, many of them die of disease or are taken by predators. Unlike Pacific salmon, Atlantic salmon are capable of spawning more than once during their life. Typically, about 5-10% of the kelts return to spawn the following year.

The eggs stay in the nest all winter and hatch in the springtime. During their incubation, it is important that they have a steady supply of clean freshwater and oxygen. When they hatch, they are said to be in the alevin stage, and they feed on the remainder of their yolk sac. When the yolk runs out of nutrients, the young, now called fry, come out of the gravel and feed on aquatic invertebrates. As they grow, they become parrs, and are camouflaged by dark splotches on their body. The young salmon spend 1-6 years in their natal river. When they grow to 4-7.5 in (10-19 cm) long, they lose their splotches, becoming completely silver, and migrate out to sea. At this point they are called smolts.

The smolts remain at sea for one to five years, feeding on fish and growing and building up a large store of fat. Then they return to freshwater to breed, usually to the river where they were born. They swim energetically up streams and rivers, going through rapids, and even leaping up small waterfalls. They do not feed during this migration. They may travel hundreds of miles inland during this trip. During their journey, they change color and physical appearance. Originally silver, they turn brown or green, and males develop a hooked lower jaw, called a kype. Males use their kypes for fighting other males while defending their breeding territory.