Life cycle

Eggs in different stages of development. In some only a few cells grow on top of the yolk, in the lower right the blood vessels surround the yolk and in the upper left the black eyes are visible, even the little lens

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 latitutes. The eggs hatch into alevin or sac fry. The fry quickly develop into parr with camouflaging vertical stripes. The parr stay for one 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.

Species

The various species of salmon have many names, and varying behaviors.

Atlantic Ocean species

Atlantic salmon

Atlantic ocean species belong to the genus Salmo. They include,

Atlantic salmon or Salmon (Salmo salar), was the first salmon to be classified.

Pacific Ocean species

Pacific species belong to the genus Oncorhynchus, some examples include;

Cherry salmon (Oncorhynchus masu or O. masou) is found only in the western Pacific Ocean in Japan, Korea and Russia and also landlocked in central Taiwan’s Chi Chia Wan Stream.

Chinook salmon (Oncorhynchus tshawytscha) is also known in the USA as King or Blackmouth Salmon, and as Spring Salmon in British Columbia. Chinook are the largest of all Pacific salmon, frequently exceeding 30 lb (14 kg). The name Tyee is used in British Columbia to refer to Chinook over 30 pounds. Chinook salmon are known to range as far north as the Mackenzie River and Kugluktuk in the central Canadian arctic.

Chum salmon (Oncorhynchus keta) is known as Dog, Keta, or Calico salmon in some parts of the USA. This species has the widest geographic range of the Pacific species: south to the Sacramento River in California in the eastern Pacific and the island of Kysh in the Sea of Japan in the western Pacific; north to the Mackenzie River in Canada in the east and to the Lena River in Siberia in the west.

Male ocean phase Coho salmon

Coho salmon (Oncorhynchus kisutch) is also known in the USA as Silver salmon. This species is found throughout the coastal waters of Alaska and British Columbia and up most clear-running streams and rivers. It is also now known to occur, albeit infrequently, in the Mackenzie River.

Pink salmon (Oncorhynchus gorbuscha), known as humpies in south east and south west Alaska, are found from northern California and Korea, throughout the northern Pacific, and from the Mackenzie River in Canada to the Lena River in Siberia, usually in shorter coastal streams. It is the smallest of the Pacific species, with an average weight of 3.5 lb (1.6 kg) to 4 lb (1.8 kg).

Sockeye salmon (Oncorhynchus nerka) is also known in the USA as Red salmon. This lake-rearing species is found south as far as the Klamath River in California in the eastern Pacific and northern Hokkaid Island in Japan in the western Pacific and as far north as Bathurst Inlet in the Canadian Arctic in the east and the Anadyr River in Siberia in the west. Although most adult Pacific salmon feed on small fish, shrimp and squid; sockeye feed on plankton that they filter through gill rakers.

True salmon

Rainbow trout

Male ocean phase steelhead salmon

Steelhead are true salmon belonging to the taxonomic family Salmonidae; all modern texts list it being as such. There is much confusion on this, and many books do not state this clearly.

Rainbow trout or Steelhead trout (Oncorhychus mykiss) are river spawners, usually found in the same rivers that produce chinook, especially the Columbia, Snake, Skeena, and other large rivers on the Pacific Coast of North America. Steelhead have also been introduced into some rivers surrounding the Laurentian Great Lakes.

Other species

Land-locked salmon (Salmo salar sebago) live in a number of lakes in eastern North America. This subspecies of Atlantic Salmon is non-migratory, even when access to the sea is not barred. Another kind of landlocked salmon exists in the Qijiawan Stream in Taiwan.

Kokanee salmon is a land-locked form of sockeye salmon.

Huchen or Danube salmon (Hucho hucho), the largest permanent fresh water salmonid

Salmon fisheries

Spawning sockeye salmon in Becharof Creek, Becharof Wilderness, Alaska

The salmon has long been at the heart of the culture and livelihood of coastal dwellers. Many people of the Northern Pacific shore had a ceremony to honor the first return of the year. For many centuries, people caught salmon as they swam upriver to spawn. A famous spearfishing site on the Columbia River at Celilo Falls was inundated after great dams were built on the river. The Ainu, of northern Japan, taught dogs how to catch salmon as they returned to their breeding grounds en masse. Now, salmon are caught in bays and near shore.

Salmon population levels are of concern in the Atlantic and in some parts of the Pacific but in Alaska stocks are still abundant. Fish farming of Pacific salmon is outlawed in the United States Exclusive Economic Zone, however, there is a substantial network of publicly funded hatcheries, and the State of Alaska’s fisheries management system is viewed as a leader in the management of wild fish stocks. Some of the most important Alaskan salmon sustainable wild fisheries are located near the Kenai River, Copper River, and in Bristol Bay. In Canada, returning Skeena River wild salmon support commercial, subsistence and recreational fisheries, as well as the area’s diverse wildlife on the coast and around communities hundreds of miles inland in the watershed. The status of wild salmon in Washington is mixed. Out of 435 wild stocks of salmon and steelhead, only 187 of them were classified as healthy; 113 had an unknown status, 1 was extinct, 12 were in critical condition and 122 were experiencing depressed populations. The Columbia River salmon population is now less than 3% of what it was when Lewis and Clark arrived at the river. The commercial salmon fisheries in California have been either severely curtailed or closed completely in recent years, due to critically low returns on the Klamath and or Sacramento Rivers, causing millions of dollars in losses to commercial fishermen. Both Atlantic and Pacific salmon are popular sportfish.

Aquaculture

Salmon farm in the archipelago of Finland.

Main article: Salmon in aquaculture

Salmon aquaculture is the major economic contributor to the world production of farmed fin-fish, representing over billion US annually. Other commonly cultured fish species include: tilapia, catfish, sea bass, carp, bream, and trout. Salmon farming is very big in Chile, Norway, Scotland, Canada and the Faroe Islands, and is the source for most salmon consumed in America and Europe. Atlantic salmon are also, in very small volumes, farmed in Russia and the island of Tasmania, Australia.

Salmon are carnivorous and are currently fed a meal produced from catching other wild fish and other marine organisms. Salmon farming leads to a high demand for wild forage fish. Salmon require large nutritional intakes of protein, and consequently, farmed salmon consume more fish than they generate as a final product. To produce one pound of farmed salmon, products from several pounds of wild fish are fed to them. As the salmon farming industry expands, it requires more wild forage fish for feed, at a time when seventy five percent of the worlds monitored fisheries are already near to or have exceeded their maximum sustainable yield. The industrial scale extraction of wild forage fish for salmon farming then impacts the survivability of the wild predator fish who rely on them for food.

Work continues on substituting vegetable proteins for animal proteins in the salmon diet. Unfortunately though, this substitution results in lower levels of the highly valued Omega-3 content in the farmed product.

Intensive salmon farming now uses open-net cages which have low production costs but have the drawback of allowing disease and sea lice to spread to local wild salmon stocks.

On a dry-dry basis, it takes 24 kg of wild caught fish to produce one kg of salmon.

Artificially-incubated chum salmon

Another form of salmon production, which is safer but less controllable, is to raise salmon in hatcheries until they are old enough to become independent. They are then released into rivers, often in an attempt to increase the salmon population. This practice was very common in countries like Sweden before the Norwegians developed salmon farming, but is seldom done by private companies, as anyone may catch the salmon when they return to spawn, limiting a company’s chances of benefiting financially from their investment. Because of this, the method has mainly been used by various public authorities and non profit groups like the Cook Inlet Aquaculture Association as a way of artificially increasing salmon populations in situations where they have declined due to overharvest, construction of dams, and habitat destruction or fragmentation. Unfortunately, there can be negative consequences to this sort of population manipulation, including genetic “dilution” of the wild stocks, and many jurisdictions are now beginning to discourage supplemental fish planting in favour of harvest controls and habitat improvement and protection. A variant method of fish stocking, called ocean ranching, is under development in Alaska. There, the young salmon are released into the ocean far from any wild salmon streams. When it is time for them to spawn, they return to where they were released where fishermen can then catch them.

An alternative method to hatcheries is to use spawning channels. These are artificial streams, usually parallel to an existing stream with concrete or rip-rap sides and gravel bottoms. Water from the adjacent stream is piped into the top of the channel, sometimes via a header pond to settle out sediment. Spawning success is often much better in channels than in adjacent streams due to the control of floods which in some years can wash out the natural redds. Because of the lack of floods, spawning channels must sometimes be cleaned out to remove accumulated sediment. The same floods which destroy natural redds also clean them out. Spawning channels preserve the natural selection of natural streams as there is no temptation, as in hatcheries, to use prophylactic chemicals to control diseases.

Farm raised salmon are fed the carotenoids astaxanthin and canthaxanthin, so that their flesh color matches wild salmon.

Diseases and parasites

Henneguya salminicola, a protozoan parasite commonly found in the flesh of salmonids on the West Coast of Canada. Coho salmon

See also: Fish diseases and parasites

According to Canadian biologist Dorothy Kieser, protozoan parasite Henneguya salminicola is commonly found in the flesh of salmonids. It has been recorded in the field samples of salmon returning to the Queen Charlotte Islands. The fish responds by walling off the parasitic infection into a number of cysts that contain milky fluid. This fluid is an accumulation of a large number of parasites.

Henneguya and other parasites in the myxosporean group have a complex lifecycle where the salmon is one of two hosts. The fish releases the spores after spawning. In the Henneguya case, the spores enter a second host, most likely an invertebrate, in the spawning stream. When juvenile salmon out-migrate to the Pacific Ocean, the second host releases a stage infective to salmon. The parasite is then carried in the salmon until the next spawning cycle. The myxosporean parasite that causes whirling disease in trout, has a similar lifecycle. However, as opposed to whirling disease, the Henneguya infestation does not appear to cause disease in the host salmon  even heavily infected fish tend to return to spawn successfully.

According to Dr. Kieser, a lot of work on Henneguya salminicola was done by scientists at the Pacific Biological Station in Nanaimo in the mid-1980s, in particular, an overview report which states that “the fish that have the longest fresh water residence time as juveniles have the most noticeable infections. Hence in order of prevalence coho are most infected followed by sockeye, chinook, chum and pink.” As well, the report says that, at the time the studies were conducted, stocks from the middle and upper reaches of large river systems in British Columbia such as Fraser, Skeena, Nass and from mainland coastal streams in the southern half of B.C. “are more likely to have a low prevalence of infection.” The report also states “It should be stressed that Henneguya, economically deleterious though it is, is harmless from the view of public health. It is strictly a fish parasite that cannot live in or affect warm blooded animals, including man”.

Sample of pink salmon infected with Henneguya salminicola, caught off the Queen Charlotte Islands, Western Canada in 2009

According to Klaus Schallie, Molluscan Shellfish Program Specialist with the Canadian Food Inspection Agency, “Henneguya salminicola is found in southern B.C. also and in all species of salmon. I have previously examined smoked chum salmon sides that were riddled with cysts and some sockeye runs in Barkley Sound (southern B.C., west coast of Vancouver Island) are noted for their high incidence of infestation.”

Sea lice, particularly Lepeophtheirus salmonis and various Caligus species, including Caligus clemensi and Caligus rogercresseyi, can cause deadly infestations of both farm-grown and wild salmon. Sea lice are ectoparasites which feed on mucous, blood, skin, and muscle tissue, and normally latch onto the skin of wild salmon in the open ocean during free-swimming, planktonic naupli and copepodid larval stages, which can persist for several days. Large numbers of highly populated, open-net salmon farms can create exceptionally large concentrations of sea lice; when exposed in river estuaries containing large numbers of open-net farms, many young wild salmon are infected, and do not survive as a result. Adult salmon may survive otherwise critical numbers of sea lice, but small, thin-skinned juvenile salmon migrating to sea are highly vulnerable. On the Pacific coast of Canada, the louse-induced mortality of pink salmon in such affected areas is commonly over 80%.

Environmental pressures

All species of Pacific Salmon die shortly after spawning. This one was photographed at a spawning site along Eagle Creek in Oregon.

The population of wild salmon declined markedly in recent decades, especially north Atlantic populations which spawn in the waters of western Europe and eastern Canada, and wi