Salmon, along with carp, are the two most important fish groups in aquaculture. In 2007, the aquaculture of salmon and salmon trout was worth US$10.7 billion. The most commonly farmed salmon is the Atlantic salmon. Other commonly farmed fish groups include tilapia, catfish, sea bass, bream and trout.
Salmon aquaculture production grew over ten-fold during the 25 years from 1982 to 2007. Leading producers of farmed salmon are Norway with 33 percent, Chile with 31 percent, and other European producers with 19 percent.[1]
There is currently much controversy about the ecological and health impacts of intensive salmon aquaculture. There are particular concerns about the impacts on wild salmon and other marine life. Some of this controversy is part of a major commercial competitive fight for market share and price between Alaska commercial salmon fishermen and the rapidly evolving salmon aquaculture industry.[2]
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The aquaculture of salmon is the farming and harvesting of salmon under controlled conditions. Farmed salmon can be contrasted with wild salmon captured using commercial fishing techniques. However, the concept of "wild" salmon as used by the Alaska Seafood Marketing Institute includes stock enhancement fish produced in hatcheries that have historically been considered ocean ranching. The percentage of the Alaska salmon harvest resulting from ocean ranching depends upon the species of salmon and location, [3] however it is all marketed as "wild Alaska salmon".
Methods of salmon aquaculture originated in late 18th century fertilization trials in Europe. In the late 19th century, salmon hatcheries were used in Europe and North America. From the late 1950s, enhancement programs based on hatcheries were established in the United States, Canada, Japan and the USSR. The contemporary technique using floating sea cages originated in Norway in the late 1960s.[4]
Salmon are usually farmed in 2 stages and in some places maybe more. First, the salmon are hatched from eggs and raised on land in freshwater tanks. When they are 12 to 18 months old, the smolt (juvenile salmon) are transferred to floating sea cages or net pens anchored in sheltered bays or fjords along a coast. This farming in a marine environment is known as mariculture. There they are fed pelleted feed for another 12 to 24 months, when they are harvested.[5]
Norway produces 33 percent of the world's farmed salmon, and Chile produces 31 percent.[1] The coastlines of these countries have suitable water temperatures and many areas well protected from storms. Chile is close to large forage fisheries which supply fish meal for salmon aquaculture. Scotland and Canada are also significant producers.[6]
Modern salmon farming systems are intensive. Their ownership is often under the control of huge agribusiness corporations, operating mechanised assembly lines on an industrial scale. In 2003, nearly half of the world’s farmed salmon was produced by just five companies.[7]
Modern commercial hatcheries for supplying salmon smolts to aquaculture net pens have been shifting to Recycle Aquaculture Systems (RAS) where the water is recycled within the hatchery. This allows location of the hatchery to be independent of a significant fresh water supply and allows economical temperature control to both speed up and slow down the growth rate to match the needs of the net pens.
Conventional hatchery systems operate flow through where spring water or other water source flow into the hatchery. The eggs are then hatched in trays and the salmon smolts produced in raceways. The waste products from the growing salmon fry and the feed are usually discharged into the local river. Conventional flow through hatcheries, such as are being used by most of the Alaska ocean ranching enhancement hatcheries use more than 100 tons of water to produce to produce a kg of smolts.
An alternative method to hatching in freshwater tanks 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. However, exposing fish to wild parasites and pathogens using uncontrolled water supplies, combined with the high cost of spawning channels, makes this technology unsuitable for salmon aquaculture businesses. This type of technology is only useful for stock enhancement programs.
Sea cages, also called sea pens or net pens, are usually made of mesh framed with steel or plastic. They can be square or circular, 10 to 32 metres across and 10 metres deep, with volumes between 1,000 and 10,000 cubic metres. A large sea cage can house up to 90,000 fish.
They are usually placed side by side to form a system called a seafarm or seasite, with a floating wharf and walkways along the net boundaries. Additional nets can also surround the seafarm to keep out predatory marine mammals. Stocking densities range from 8 to 18 kilograms per cubic metre for Atlantic salmon and 5 to 10 kilograms per cubic metre for Chinook salmon.[5][8]
In contrast to closed or recirculating systems, the open net cages of salmon farming lower production costs, but provide no effective barrier to the discharge of wastes, parasites and disease into the surrounding coastal waters.[7] Farmed salmon in open net cages can escape into wild habitats, for example, during storms.
An emerging wave in aquaculture is applying the same farming methods used for salmon to other carnivorous finfish species, such as cod, bluefin tuna, halibut and snapper. However, this is likely to have the same environmental drawbacks as salmon farming.[7][9]
A second emerging wave in aquaculture is the development of copper alloys as netting materials. Copper alloys have become important netting materials because they are antimicrobial (i.e., they destroy bacteria, viruses, fungi, algae, and other microbes) and they therefore prevent biofouling (i.e., the undesirable accumulation, adhesion, and growth of microorganisms, plants, algae, tubeworms, barnacles, mollusks, and other organisms). By inhibiting microbial growth, copper alloy aquaculture cages avoid costly net changes that are necessary with other materials. The resistance of organism growth on copper alloy nets also provides a cleaner and healthier environment for farmed fish to grow and thrive.
Salmon are carnivorous and are currently being fed compound fish feeds containing fish meal and other feed ingredients, ranging from wheat byproducts to soybean meal and feather meal. Being an aquatic carnivore, salmon don't tolerate or properly metabolize many plant based carbohydrates and use fats instead of carbohydrates as a primary energy source. With the amount of world wide fish meal production being almost a constant amount for the last 30+ years and at maximum sustainable yield (MSY), much of the fish meal market has shifted from chicken and pig feed to fish and shrimp feeds as aquaculture has grown in this time period[10]. Work continues on substituting vegetable proteins and protein concentrates for fish meal in the salmon diet. Many substitutions for fish meal are known and diets containing zero fish meal are possible. However, commercial economic animal diets are determined by least cost linear programming models that are effectively competing with similar models for chicken and pig feeds for the same feed ingredients and these models show that fish meal is more useful in aquatic diets than in chicken diets, where they can make the chickens taste like fish[11]. Unfortunately though, this substitution can result in lower levels of the highly valued omega-3 content in the farmed product. However, when vegetable oil is used in the growing diet as an energy source and a different finishing diet containing high omega-3 content fatty acids from either fish oil, algae oils or some vegetable oils are used a few months before harvest, this problem is eliminated[12]. At the present time, more than 50 percent of the world fish oil production is feed to farmed salmon.[13] Farm raised salmon are also fed the carotenoids astaxanthin and canthaxanthin, so that their flesh color matches wild salmon, which also contain the same carotenoid pigments from their diet in the wild.[14] On a dry-dry basis, it takes 2–4 kg of wild caught fish to produce one kg of salmon.[15] Wild salmon require about 10kg of forage fish to produce a kg of salmon, as part of the normal trophic level energy transfer. The difference between the two numbers is related to farmed salmon feed containing other ingredients beyond fish meal and the fact that farmed fish don't spend a lot of metabolic energy catching a dinner that doesn't want to be caught.
Modern harvesting methods are shifting towards using wet well ships to transport the live salmon to the processing plant. This allows the fish to be killed, bleed, and filleted before rigor has occurred. This results in superior product quality to the customer along with more humane processing. To obtain maximum quality, it is necessary to minimize the level of stress in the live salmon until actually being electrically and percussively killed and the gills slit for bleeding.[16] These improvements in processing time and freshness to the final customer are commercially significant and forcing the commercial wild fisheries to upgrade their processing to the benefit of all seafood consumers.
An older method of harvesting is to use a sweep net, which operates a bit like a purse seine net. The sweep net is a big net with weights along the bottom edge. It is stretched across the pen with the bottom edge extending to the bottom of the pen. Lines attached to the bottom corners are raised, herding some fish into the purse, where they are netted. Before killing, the fish are usually rendered unconscious in water saturated in carbon dioxide, although this practice is being phased out in some countries due to ethical and product quality concerns. More advanced systems use a percussive-stun harvest system that kills the fish instantly and humanely with a blow to the head from a pneumatic piston. They are then bled by cutting the gill arches and immediately immersing them in iced water. Harvesting and killing methods are designed to minimise scale loss, and avoid the fish releasing stress hormones, which negatively affect flesh quality.[8]
Wild salmon are captured from wild habitats using commercial fishing techniques. Most wild salmon is caught in North American, Japanese and Russian fisheries. The following table shows the changes in production of wild salmon and farmed salmon over a period of 25 years, as reported by the FAO.[17] Russia, Japan and Alaska all operate major hatchery based stock enhancement programs that are really ocean ranching. The resulting fish hatchery fish are defined as "wild" for FAO and marketing purposes.
| Salmon production in tonnes by species | ||||
|---|---|---|---|---|
| 1982 | 2007 | |||
| Species | Wild | Farmed | Wild | Farmed |
| Atlantic salmon | 10,326 | 13,265 | 2,989 | 1,433,708 |
| Salmon trout | 171,946 | 604,695 | ||
| Coho salmon | 42,281 | 2,921 | 17,200 | 115,376 |
| Chinook salmon | 25,147 | 8,906 | 11,542 | |
| Pink salmon | 170,373 | 495,986 | ||
| Chum salmon | 182,561 | 303,205 | ||
| Sockeye salmon | 128,176 | 164,222 | ||
| Total salmon production | ||||
|---|---|---|---|---|
| 1982 | 2007 | |||
| tonnes | percent | tonnes | percent | |
| Wild | 558,864 | 75 | 992,508 | 31 |
| Farmed | 188,132 | 25 | 2,165,321 | 69 |
| Overall | 746,996 | 3,157,831 | ||
There is currently much controversy about the ecological and health impacts of intensive salmon aquaculture. There are particular concerns about the impacts on wild salmon and other marine life and on the incomes of commercial salmon fishermen. The interest of the commercial fishermen are trying to prevent competition.[2]
In 1972, Gyrodactylus, a monogenean parasite, spread from Norwegian hatcheries to wild salmon, and devastated some wild salmon populations.[18]
In 1984, infectious salmon anemia (ISAv) was discovered in Norway in an Atlantic salmon hatchery. Eighty percent of the fish in the outbreak died. ISAv, a viral disease, is now a major threat to the viability of Atlantic salmon farming. It is now the first of the diseases classified on List One of the European Commission’s fish health regime. Amongst other measures, this requires the total eradication of the entire fish stock should an outbreak of the disease be confirmed on any farm. ISAv seriously affects salmon farms in Chile, Norway, Scotland and Canada, causing major economic losses to infected farms.[19] As the name implies, it causes severe anemia of infected fish. Unlike mammals, the red blood cells of fish have DNA, and can become infected with viruses. The fish develop pale gills, and may swim close to the water surface, gulping for air. However, the disease can also develop without the fish showing any external signs of illness, the fish maintain a normal appetite, and then they suddenly die. The disease can progress slowly throughout an infected farm and, in the worst cases, death rates may approach 100 percent. It is also a threat to the dwindling stocks of wild salmon. Management strategies include developing a vaccine and improving genetic resistance to the disease.[20]
In the wild, diseases and parasites are normally at low levels, and kept in check by natural predation on weakened individuals. In crowded net pens they can become epidemics. Diseases and parasites also transfer from farmed to wild salmon populations. A recent study in British Columbia links the spread of parasitic sea lice from river salmon farms to wild pink salmon in the same river.[7] The European Commission (2002) concluded “The reduction of wild salmonid abundance is also linked to other factors but there is more and more scientific evidence establishing a direct link between the number of lice-infested wild fish and the presence of cages in the same estuary.”[21] It is reported that wild salmon on the west coast of Canada are being driven to extinction by sea lice from nearby salmon farms.[22] These predictions have been disputed by other scientists[23] and recent harvests have indicated that the predictions were in error. Antibiotics and pesticides are often used to control the diseases and parasites.
Salmon farms are typically sited in marine ecosystems with good water quality, high water exchange rates, current speeds fast enough to prevent pollution of the bottom but slow enough to prevent pen damage, protection from major storms, reasonable water depth and a reasonable distance from major infrastructure such as ports, processing plants and logistical facilities like airports. Logistical considerations are significant and feed and maintenance labor must be transported to the facility and the product returned. Siting decisions are complicated by complex politically driven permitting problems in many countries that prevents optimal locations for the farms.
In sites without adequate currents there can be an accumulation of heavy metals on the benthos (seafloor) near the salmon farms, particularly copper and zinc.[8]
Toxins are also found in the flesh of farmed salmon.[24] A 2004 study, reported in Science, analysed farmed and wild salmon for organochlorine contaminants. They found the contaminants were higher in farmed salmon. Within the farmed salmon, European (particularly Scottish) salmon had the highest levels, and Chilean salmon the lowest.[25] A follow up study confirmed this, and found levels of dioxins, chlorinated pesticides, PCBs and other toxins up to ten times greater in farmed salmon than wild Pacific salmon.[26] On a positive note, further research using the same fish samples used in the previous study, showed that farmed salmon contained levels of beneficial fatty acids that were two to three times higher than wild salmon.[27] A follow up benefit-risk analysis on salmon consumption balanced the cancer risks with the (n-3) fatty acid advantages of salmon consumption. They found that recommended levels of (n-3) fatty acid consumption can be achieved eating farmed salmon with acceptable carcinogenic risks, but recommended levels of EPA+DHA intake cannot be achieved solely from farmed (or wild) salmon without unacceptable carcinogenic risks.[28] The conclusions were that
"...consumers should not eat farmed fish from Scotland, Norway and eastern Canada more than three times a year; farmed fish from Maine, western Canada and Washington state no more than three to six times a year; and farmed fish from Chile no more than about six times a year. Wild chum salmon can be consumed safely as often as once a week, pink salmon, Sockeye and Coho about twice a month and Chinook just under once a month."[24]
Farmed salmon can, and often do, escape from sea cages. If the farmed salmon is not native, it can compete with native wild species for food and habitat.[29][30][31] If the farmed salmon is native, it can interbreed with the wild native salmon. Such interbreeding can reduce genetic diversity, disease resistance and adaptability.[32] In 2004, about 500,000 salmon and trout escaped from ocean net pens off Norway. Around Scotland, 600,000 salmon were released during storms.[7] Commercial fishermen targeting wild salmon not infrequently catch escaped farm salmon. At one stage, in the Faroe Islands, 20 to 40 percent of all fish caught were escaped farm salmon.[33]
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.[34][35] Sea lice are ectoparasites which feed on mucous, blood, and skin, and migrate and latch onto the skin of wild salmon during free-swimming, planktonic nauplii and copepodid larval stages, which can persist for several days.[36][37][38] 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.[39][40] 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 some regions is commonly over 80%.[41]
A 2008 meta-analysis of available data shows that salmon farming reduces the survival of associated wild salmon populations. This relationship has been shown to hold for Atlantic, steelhead, pink, chum, and coho salmon. The decrease in survival or abundance often exceeds 50 percent.[42] However, these studies are all correlation analysis and correlation doesn't equal causation, especially when similar salmon declines were occurring in Oregon and California, which have no salmon aquaculture or marine net pens. Independent of the predictions of the failure of salmon runs in Canada indicated by these studies, the wild salmon run in 2010 was a record harvest.[43]
Wild salmon are anadromous. They spawn inland in fresh water and when young migrate to the ocean where they grow up. Most salmon return to the river where they were born, although some stray to other rivers. There is concern about of the role of genetic diversity within salmon runs. The resilience of the population depends on some fish being able to survive environmental shocks, such as unusual temperature extremes. It is also unclear what the effect of hatchery production has been on the genetic diversity of salmon.[4]
Salmon have been genetically modified in laboratories so they can grow faster. There is opposition to the commercial use of these fish, and, so far, no approval has been given.[44] A Canadian company, Aqua Bounty Farms, has developed a modified Atlantic salmon which grows several times faster, and is more disease resistant and cold tolerant. This was achieved using a chinook salmon gene sequence affecting growth hormones, and a promoter sequence from the ocean pout affecting antifreeze production.[45] Normally, salmon produce growth hormones only in the presence of light. The modified salmon doesn't switch growth hormone production off. The company first submitted the salmon for FDA approval in 1996.[46][47] A concern with transgenic salmon is what might happen if they escape into the wild. One study, in a laboratory setting, found that modified salmon mixed with their wild cohorts were aggressive in competing, but ultimately failed.[48]
The use of forage fish for fish meal production has been almost a constant for the last thirty years and at the maximum sustainable yield, while the market for fish meal has shifted from chicken, pig and pet food to aquaculture diets.[49] The fact that this market shift at constant production is an economic decision having no impact on the forage fish harvest rates for fish meal implies that the development of salmon aquaculture had no impact on forage fish harvest rates.
Fish do not actually produce omega-3 fatty acids, but instead accumulate them from either consuming microalgae that produce these fatty acids, as is the case with forage fish like herring and sardines, or, as is the case with fatty predatory fish, like salmon, by eating prey fish that have accumulated omega-3 fatty acids from microalgae. To satisfy this requirement, more than 50 percent of the world fish oil production is fed to farmed salmon.[13]
In addition, salmon require nutritional intakes of protein, protein which is often supplied to them in the form of fish meal as the lowest cost alternative protein. 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 a higher percentage of the 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.[7]
In 2004 the World Wide Fund for Nature (WWF) initiated the Salmon Aquaculture Dialogue.[6] The aim of the dialogue is to produce an environmental standard for farmed salmon by 2010. The WWF have identified what they call "seven key environmental and social impacts", which they characterise as follows
“ 1. Benthic impacts and siting: Chemicals and excess nutrients from food and feces associated with salmon farms can disturb the flora and fauna on the ocean bottom (benthos).[50] 2. Chemical inputs: Excessive use of chemicals - such as antibiotics, anti-foulants and pesticides - or the use of banned chemicals can have unintended consequences for marine organisms and human health.[51]
3. Disease/parasites: Viruses and parasites can transfer between farmed and wild fish, as well as among farms.[52][53]
4. Escapes: Escaped farmed salmon can compete with wild fish and interbreed with local wild stocks of the same population, altering the overall pool of genetic diversity.[54]
5. Feed: A growing salmon farming business must control and reduce its dependency upon fishmeal and fishoil - a primary ingredient in salmon feed - so as not to put additional pressure on the world's fisheries. Fish caught to make fishmeal and oil currently represent one-third of the global fish harvest.[55]
6. Nutrient loading and carrying capacity: Excess food and fish waste in the water have the potential to increase the levels of nutrients in the water. This can cause the growth of algae, which consumes oxygen that is meant for other plant and animal life.[56]
7. Social issues: Salmon farming often employs a large number of workers on farms and in processing plants, potentially placing labor practices and worker rights under public scrutiny. Additionally, conflicts can arise among users of the shared coastal environment.”
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 disruption. 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.
In their natal streams, Atlantic salmon are considered a prized recreational fish, pursued by avid fly anglers during its annual runs. At one time, the species supported an important commercial fishery and a supplemental food fishery. However, the wild Atlantic salmon fishery is commercially dead; after extensive habitat damage and overfishing, wild fish make up only 0.5% of the Atlantic salmon available in world fish markets. The rest are farmed, predominantly from aquaculture in Chile, Canada, Norway, Russia, the UK and Tasmania in Australia.[57]
Atlantic salmon is, by far, the species most often chosen for farming. It is easy to handle, it grows well in sea cages, commands a high market value and it adapts well to being farmed away from its native habitats.[4]
Adult male and female fish are anaesthetised. Eggs and sperm are "stripped", after the fish are cleaned and cloth dried. Sperm and eggs are mixed, washed, and placed into fresh water. Adults recover in flowing, clean, well aerated water.[58] Some researchers have even studied cryopreservation of their eggs.[59]
Fry are generally reared in large freshwater tanks for 12 to 20 months. Once the fish have reached the smolt phase, they are taken out to sea where they are held for up to two years. During this time the fish grow and mature in large cages off the coasts of Canada, the USA, or parts of Europe.[57]
Generally, cages are made of two nets. Inner nets, which wrap around the cages, hold the salmon. Outer nets, which are held by floats, keep predators out.[58]
Many Atlantic salmon escape from cages at sea. Those salmon who further breed tend to lessen the genetic diversity of the species leading to lower survival rates, and lower catch rates. On the West Coast of Northern America, the non-native salmon can be an invasive threat, especially in Alaska and parts of Canada. This causes them to compete with native salmon for resources. Extensive efforts are underway to prevent escapes and the spread of Atlantic salmon in the Pacific and elsewhere.[60]
Atlantic salmon which have escaped from the aquaculture industry have also been found breeding in rivers tributary to the Pacific Ocean in British Columbia on Canada's west coast.
Worldwide, in 2007, 1,433,708 tonnes of Atlantic salmon were harvested with a value of 7.578 billion US dollars.[61]
In 1992 the salmon trout, Salmo gairdneri, was reclassified as a true salmon, Oncorhynchus mykiss. Salmon trout take an anadromous form and a non-anadromous form. The anadromous form, who migrate between lakes and rivers and the ocean, are known as steelhead salmon or ocean trout, while the non-anadromous form, who spend their entire life in freshwater, are known as rainbow trout.
Salmon trout are raised in many countries throughout the world. Since the 1950s production has grown exponentially, particularly in Europe and recently in Chile. Worldwide, in 2007, 604,695 tonnes of farmed salmon trout were harvested with a value of 2.589 billion US dollars.[62] The largest producer is Chile. In Chile and Norway, the ocean cage production of steelhead has expanded to supply export markets. Inland production of rainbow trout to supply domestic markets has increased strongly in countries such as Italy, France, Germany, Denmark and Spain. Other significant producing countries include the USA, Iran, Germany and the UK.[62]
Salmon trout have tender flesh and a mild, somewhat nutty flavour. Steelhead meat is pink like that of other salmon, and is more flavourful than the light-coloured meat of rainbow trout.[63] Both are highly desired food. However, farmed trout and those taken from certain lakes have a pronounced earthy flavour which many people find unappealing; many shoppers therefore make it a point to ascertain the source of the fish before buying. Salmon trout that are wild have a diet of scuds (freshwater shrimp), insects such as flies, and crayfish are the most appealing. Dark red/orange meat indicates that it is either an anadromous steelhead or a farmed rainbow trout given a supplemental diet with a high iodine content. The resulting pink flesh is marketed under monikers like Ruby Red or Carolina Red.
The salmon trout is especially susceptible to enteric redmouth disease. There has been considerable research conducted on redmouth disease, as its implications for salmon trout farmers are significant. The disease does not affect humans.[64]
The Coho salmon[8] is the state animal of Chiba, Japan.
Coho salmon mature after only one year in the sea, so two separate broodstocks (spawners) are needed, alternating each year. Broodfish are selected from the salmon in the seasites and transferred to freshwater tanks for maturation and spawning."[8]
Worldwide, in 2007, 115,376 tonnes of farmed Coho salmon were harvested with a value of 456 million US dollars.[65] Chile, with about 90 percent of word production, is the primary producer with Japan and Canada producing the rest.[8]
In Alaska, Chinook salmon are the state fish, and are known as "king salmon" because of their large size and flavoursome flesh. Those from the Copper River in Alaska are particularly known for their colour, rich flavour, firm texture, and high Omega-3 oil content.[66]
Worldwide, in 2007, 11,542 tonnes of farmed Chinook salmon were harvested with a value of 83 million US dollars.[67] New Zealand is the largest producer of farmed king salmon, accounting for over half of world production (7,400 tonnes in 2005).[68] Most of the salmon are farmed in the sea (mariculture) using a method sometimes called sea-cage ranching. Sea-cage ranching takes place in large floating net cages, about 25 metres across and 15 metres deep, moored to the sea floor in clean, fast-flowing coastal waters. Smolt (young fish) from freshwater hatcheries are transferred to cages containing several thousand salmon, and remain there for the rest of their life. They are fed fishmeal pellets high in protein and oil.[68]
King salmon are also farmed in net cages placed in freshwater rivers or raceways, using techniques similar to those used for sea-farmed salmon. A unique form of freshwater salmon farming occurs in some hydroelectric canals. A site in Tekapo, fed by fast cold waters from the Southern Alps, is the highest salmon farm in the world, 677 metres above sea level.[69]
Before they are killed, cage salmon are anaesthetised with a herbal extract. They are then spiked in the brain. The heart beats for a time as the animal is bled from its sliced gills. Relaxing the salmon like this when it is killed produces firm, long-keeping flesh.[68] Lack of disease in wild populations and low stocking densities used in the cages means that New Zealand salmon farmers do not use antibiotics and chemicals that are often needed elsewhere.[70]
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