Salmon is the common name for several species of of the family Salmonidae. Several other fish in the family are called trout; the difference is often said to be that salmon migrate and trout are resident, a distinction that holds true for the Salmo genus. Salmon live in both the Atlantic (one migratory species Salmo salar) and Pacific Oceans, as well as the Great Lakes (approximately a dozen species of the genus Oncorhynchus).
Typically, salmon are anadromous: they are born in fresh water, migrate to the ocean, then return to fresh water to reproduce. However, there are rare species that can only survive in fresh water. Folklore has it that the fish return to the exact spot where they were born to spawn; tracking studies have shown this to be true but the nature of how this memory works has long been debated.
Salmon fry hatching — the baby has grown around the remains of the yolk — visible are the arteries spinning around the yolk and little oildrops, also the gut, the spine, the main caudal blood vessel, the bladder and the arcs of the gills
Salmon eggs are laid in freshwater streams typically at high latitudes. The eggs hatch into alevin or sac fry. The fry quickly develop into parr with camouflaging vertical stripes. The parr stay for 6 months to three years in their natal stream before becoming smolts, which are distinguished by their bright silvery colour with scales that are easily rubbed off. It is estimated that only 10% of all salmon eggs survive to this stage. The smolt body chemistry changes, allowing them to live in salt water. Smolts spend a portion of their out-migration time in brackish water, where their body chemistry becomes accustomed to osmoregulation in the ocean.
The salmon spend about one to five years (depending on the species) in the open ocean where they become sexually mature. The adult salmon return primarily to their natal stream to spawn. In Alaska, the crossing-over to other streams allows salmon to populate new streams, such as those that emerge as a glacier retreats. The precise method salmon use to navigate has not been established, though their keen sense of smell is involved. Atlantic salmon spend between one and four years at sea. (When a fish returns after just one year’s sea feeding it is called a grilse in the UK and Ireland.) Prior to spawning, depending on the species, salmon undergo changes. They may grow a hump, develop canine teeth, develop a kype (a pronounced curvature of the jaws in male salmon). All will change from the silvery blue of a fresh run fish from the sea to a darker color. Salmon can make amazing journeys, sometimes moving hundreds of miles upstream against strong currents and rapids to reproduce. Chinook and sockeye salmon from central Idaho, for example, travel over 900 miles (1,400 km) and climb nearly 7,000 feet (2,100 m) from the Pacific ocean as they return to spawn. Condition tends to deteriorate the longer the fish remain in fresh water, and they then deteriorate further after they spawn, when they are known as kelts. In all species of Pacific salmon, the mature individuals die within a few days or weeks of spawning, a trait known as semelparity. Between 2% and 4% of Atlantic salmon kelts survive to spawn again, all females. However, even in those species of salmon that may survive to spawn more than once (iteroparity), post-spawning mortality is quite high (perhaps as high as 40 to 50%.)
To lay her roe, the female salmon uses her tail (caudal fin), to create a low-pressure zone, lifting gravel to be swept downstream, excavating a shallow depression, called a redd. The redd may sometimes contain 5,000 eggs covering 30 square feet (2.8 m2). The eggs usually range from orange to red. One or more males will approach the female in her redd, depositing his sperm, or milt, over the roe. The female then covers the eggs by disturbing the gravel at the upstream edge of the depression before moving on to make another redd. The female will make as many as 7 redds before her supply of eggs is exhausted.
male ocean phase Chinook
male freshwater phase Chinook
Each year, the fish experiences a period of rapid growth, often in summer, and one of slower growth, normally in winter. This results in rings (annuli) analogous to the growth rings visible in a tree trunk. Freshwater growth shows as densely crowded rings, sea growth as widely spaced rings; spawning is marked by significant erosion as body mass is converted into eggs and milt.
Freshwater streams and estuaries provide important habitat for many salmon species. They feed on terrestrial and aquatic insects, amphipods, and other crustaceans while young, and primarily on other fish when older. Eggs are laid in deeper water with larger gravel, and need cool water and good water flow (to supply oxygen) to the developing embryos. Mortality of salmon in the early life stages is usually high due to natural predation and human-induced changes in habitat, such as siltation, high water temperatures, low oxygen concentration, loss of stream cover, and reductions in river flow. Estuaries and their associations wetlands provide vital nursery areas for the salmon prior to their departure to the open ocean. Wetlands not only help buffer the estuary from silt and pollutants, but also provide important feeding and hiding areas.