Salmonid aquaculture involves farming and harvesting salmonid fish, such as salmon and rainbow trout, under controlled conditions for commercial and recreational use. Salmonids, along with carp and tilapia, are the three most important fish groups in aquaculture. The Atlantic salmon (Salmo salar) is the most commonly farmed salmonid for commercial purposes.

In the United States, Chinook salmon and rainbow trout are the most frequently farmed salmonids for recreational and subsistence fishing through the National Fish Hatchery System. In Europe, brown trout are the most commonly raised fish for recreational restocking. Other commonly farmed fish groups include tilapia, catfish, black sea bass, and bream. In 2007, salmonid aquaculture was valued at USD $10.7 billion globally. From 1982 to 2007, salmonid aquaculture production increased more than ten times. In 2012, the leading producers of salmonids were Norway, Chile, Scotland, and Canada.

There is significant concern about the environmental and health effects of intensive salmonid aquaculture. Particular worries include the impact on wild salmon and other marine life.

Methods

Salmon farming, also called aquaculture, is different from catching wild salmon using fishing methods. However, the term "wild" salmon, as used by the Alaska Seafood Marketing Institute, includes fish raised in hatcheries that were once called ocean ranching. The amount of Alaska's salmon harvest from ocean ranching depends on the type of salmon and where it is caught. Salmon farming methods began in the late 1700s with experiments in Europe. In the late 1800s, salmon hatcheries were built in Europe and North America. Starting in the late 1950s, hatchery-based enhancement programs were created in the United States, Canada, Japan, and the USSR. The modern method of using floating sea cages for salmon farming started in Norway in the late 1960s.

Salmon are usually farmed in two stages, and sometimes more. First, salmon eggs are hatched and raised in freshwater tanks on land. Increasing the heat units in the water during incubation can shorten the time it takes for eggs to hatch. When the salmon are 12 to 18 months old, they are moved to floating sea cages or net pens in protected coastal areas. This part of farming, called mariculture, lasts another 12 to 24 months before the salmon are harvested.

Norway produces 33% of the world's farmed salmon, and Chile produces 31%. These countries have coastlines with suitable water temperatures and areas that are safe from storms. Chile is near large fisheries that provide fish meal for salmon farming. Scotland and Canada are also major producers. In 2012, the Norwegian government controlled a large part of Canada's salmon farming industry.

Modern salmon farming is highly intensive. Large agribusiness companies often own and operate these farms using industrial methods. In 2003, about half of the world's farmed salmon came from just five companies.

Modern hatcheries that supply salmon to sea cages are moving to recirculating aquaculture systems (RAS), where water is reused within the hatchery. This allows hatcheries to be built anywhere, not just near freshwater sources, and helps control water temperature to adjust growth rates as needed.

Traditional hatcheries use flow-through systems, where water from springs or other sources flows into the hatchery. Eggs are hatched in trays, and young salmon are raised in raceways. Waste from the fish and uneaten feed is usually released into nearby rivers. For example, most of Alaska's hatcheries use more than 100 tonnes of water to produce one kilogram of salmon smolts.

An alternative to hatching in freshwater tanks is using spawning channels. These are artificial streams built next to natural streams, with concrete or gravel bases. Water from the nearby stream is directed into the channel. Spawning success in these channels is often better than in natural streams because floods that destroy natural nests are avoided. However, without floods, sediment must be cleaned from the channels. Unlike hatcheries, spawning channels do not use chemicals to prevent disease, preserving natural selection. However, the risk of wild parasites and high costs make this method unsuitable for commercial salmon farming. Spawning channels are only used for stock enhancement programs.

Sea cages, also called sea pens or net pens, are usually made of mesh held together by steel or plastic. They are square or circular, ranging from 10 to 32 meters wide and 10 meters deep. A large cage can hold up to 90,000 fish. These cages are placed side by side to form a seafarm, with floating docks and walkways. Extra nets may be added to keep out predators like seals. Salmon are raised at densities of 8 to 18 kilograms per cubic meter for Atlantic salmon and 5 to 10 kilograms per cubic meter for Chinook salmon.

Unlike closed systems, open net cages lower production costs but do not stop waste, parasites, or disease from entering the surrounding water. Farmed salmon can escape during storms.

A new trend in aquaculture is using salmon farming methods for other carnivorous fish, such as cod, tuna, halibut, and snapper. However, this may cause the same environmental problems as salmon farming.

Another new development is using copper alloys for netting. These materials are antimicrobial, killing bacteria, viruses, and algae, which prevents biofouling (the buildup of unwanted organisms). Copper nets reduce the need for frequent net replacements and create a cleaner environment for fish.

Fish meal production has remained nearly the same for over 30 years. As aquaculture has grown, the fish meal market has shifted from use in chicken and pig feed to fish and shrimp feed.

Research is ongoing to create salmon diets using plant-based protein. By 2014, a process using enzymes could reduce the carbohydrate content in barley, making it a high-protein feed for salmon. Other fish meal substitutes exist, and some farms now use ragworms, algae, and amino acids instead of fish meal. Omega-3 fatty acids like EPA and DHA can be replaced with algae oil from land-based sources, reducing the need to catch wild fish for fish meal.

Commercial animal diets are determined by cost-effective models that prioritize ingredients for maximum use. These models show fish meal is more valuable in fish diets than in chicken feed, where it can make the meat taste fishy. However, using vegetable oil in the growing phase and a high-omega-3 finishing diet before harvest can restore omega-3 levels in farmed fish.

To produce 1 kilogram of farmed salmon, 2 to 4 kilograms of wild-caught fish are needed. This ratio can decrease if non-fish ingredients are used. Wild salmon require about 10 kilograms of forage fish to produce 1 kilogram of salmon, as part of the natural food chain.

Wild versus farmed

Wild salmon and related fish are caught in their natural environments using fishing methods used by businesses. Most of these fish are caught in fisheries located in North America, Japan, and Russia. The table below shows how much wild salmon and farmed salmon were produced over a 25-year period, as reported by the FAO. Russia, Japan, and Alaska all run large programs to raise fish in hatcheries and release them into the wild. The fish raised in hatcheries are considered "wild" by the FAO and for selling purposes.

Issues

The United States dietary guidelines from 2010 recommend eating 8 ounces of seafood each week, with 12 ounces for lactating mothers. There are no limits on how much seafood can be eaten, and no restrictions on eating farmed or wild salmon. In 2018, Canadian guidelines suggested eating at least two servings of fish weekly, such as char, herring, mackerel, salmon, sardines, and trout.

There is much disagreement about the effects of raising salmon in large numbers on the environment and human health. Concerns include harm to wild salmon and other sea life, as well as impacts on the income of people who fish for wild salmon. However, the increased production of young salmon from hatcheries—such as those that contribute up to 50% of Alaska’s yearly wild salmon catch—also causes debate. Alaska’s salmon harvest depends on hatchery operations managed by local groups. The sustainability of salmon caught from hatcheries is often discussed by scientists and policymakers. This debate led to a pause in re-certifying Alaska’s salmon fisheries by the Marine Stewardship Council (MSC) in 2012. Later, Alaska’s fisheries regained certification, but a valuable area near Prince William Sound remained under review for several years.

In 1972, a type of parasite called Gyrodactylus was brought to Norway from Sweden through infected fish. Hatcheries then released infected fish into rivers to help wild salmon populations, but this caused harm to some wild salmon.

In 1984, a virus called infectious salmon anemia (ISAv) was found in Norway’s salmon hatcheries. Eighty percent of the fish in the outbreak died. ISAv is now a major threat to salmon farming. It is classified as a top priority by the European Union, requiring the complete removal of fish if an outbreak occurs. ISAv affects salmon farms in Norway, Chile, Scotland, and Canada, causing large financial losses. The virus causes severe anemia in fish. Unlike mammals, fish red blood cells can be infected by viruses, leading to pale gills and fish swimming near the surface to breathe. In some cases, infected fish show no signs of illness but die suddenly. In extreme cases, nearly all fish in a farm may die. ISAv also threatens wild salmon populations. Efforts to manage the disease include creating vaccines and breeding salmon with better resistance.

In nature, diseases and parasites are usually rare because predators control sick animals. In crowded fish pens, diseases can spread quickly. Parasites and diseases from farmed fish can also harm wild salmon. A study in British Columbia linked sea lice from salmon farms to wild pink salmon in the same river. A 2002 European Union report said that the presence of fish farms in an area is connected to fewer wild salmon. Some scientists claim that sea lice from farms are causing wild salmon to disappear in western Canada, but others disagree, and recent fishing results suggest these claims may be incorrect. In 2011, Scottish salmon farms began using a type of fish called ballan wrasse to clean sea lice from farmed salmon.

Global salmon production dropped about 9% in 2015, partly because of sea lice outbreaks in Scotland and Norway. Lasers are used to reduce lice on farmed fish.

Between the 1980s and 1990s, a disease called bacterial kidney disease (BKD), caused by a bacteria called Renibacterium salmoninarum, harmed Chinook salmon hatcheries in Idaho. The disease causes swelling and abscesses in the fish’s organs.

Salmon farms are usually built in areas with clean water, strong currents to prevent pollution, and protection from storms. They need to be far enough from ports and other infrastructure. Transporting feed and workers to the farms and returning the fish to markets is important. Farm locations are also limited by complex rules and permits in many countries.

In areas with weak currents, heavy metals like copper and zinc can build up on the seafloor near salmon farms.

Both farmed and wild salmon can contain harmful substances like PCBs, dioxins, and other chemicals. In 2002, Health Canada found that farmed salmon had higher levels of these chemicals than wild salmon. However, a 2012 European study said farmed salmon had much lower levels of dioxins and PCBs than wild salmon. A 2004 study in Science found that farmed salmon had more harmful chemicals than wild salmon, with European (especially Scottish) salmon having the highest levels and Chilean salmon the lowest. The FDA and Health Canada set a limit of 2000 parts per billion for PCBs in fish. A follow-up study found that farmed salmon had up to ten times more harmful chemicals than wild Pacific salmon. However, farmed salmon also had higher levels of healthy fatty acids, which are important for the body. A risk analysis showed that eating farmed salmon can meet recommended levels of healthy fats with acceptable cancer risks. PCBs are stored in fat, so farmed salmon, which has more fat, has higher levels of PCBs. Eating farmed salmon can help meet healthy fat needs, but not enough to fully replace wild salmon for certain nutrients.

In 2005, Russia banned importing chilled fish from Norway after testing found harmful chemicals in Norwegian farmed fish.

Land-raised salmon

Recirculating aquaculture systems allow salmon to be farmed completely on land. This is an ongoing project in the industry as of 2019. However, large salmon farming companies like Mowi and Cermaq were not investing in these systems beyond the early stages of raising fish. In the United States, a major supporter of this effort was Atlantic Sapphire, which planned to sell salmon raised in Florida by 2021. Other companies involved in this effort include Nordic Acquafarms and Whole Oceans.

Nephrocalcinosis is a new problem in salmon farming that has become more common with the use of recirculating aquaculture systems. A major cause of this issue is when the water used to raise salmon contains very high levels of carbon dioxide.

Species

Atlantic salmon are highly valued fish for recreation, especially by fly anglers who catch them during their yearly migration to spawn. In the past, wild Atlantic salmon were important for commercial fishing and as a food source. However, today, wild Atlantic salmon are no longer commercially viable due to habitat loss and overfishing. Only 0.5% of Atlantic salmon sold in global markets are wild, while the rest are farmed, mainly in countries like Chile, Canada, Norway, Russia, the United Kingdom, and Tasmania.

Atlantic salmon is the most commonly farmed fish species. It is easy to handle, grows well in sea cages, has high market value, and adapts well to being raised away from its natural habitat.

To breed Atlantic salmon, adult males and females are put under anesthesia. Eggs and sperm are removed after the fish are cleaned and dried. The sperm and eggs are mixed, washed, and placed in fresh water. The adults are then placed in flowing, clean water to recover. Some scientists study freezing eggs for storage.

After hatching, young salmon (fry) are raised in large freshwater tanks for 12 to 20 months. When they reach the smolt stage, they are moved to the ocean, where they grow for up to two years in large cages near the coasts of Canada, the United States, or parts of Europe. Cages are made of two nets: inner nets hold the salmon, while outer nets, supported by floats, protect them from predators.

Some farmed salmon escape from cages. These escaped fish can reduce the genetic diversity of wild salmon populations, lowering survival and catch rates. On the West Coast of North America, non-native Atlantic salmon could become invasive, competing with native species for resources. Efforts are being made to prevent escapes and spread. Despite past introductions of Atlantic salmon to the Pacific Coast by governments in the 1900s, no wild populations have been found there.

In 2007, 1,433,708 tonnes of Atlantic salmon were harvested worldwide, valued at $7.58 billion. By 2017, over 2 million tonnes of farmed Atlantic salmon were harvested.

Steelhead trout were reclassified in 1989 as a type of Pacific trout, Oncorhynchus mykiss, replacing older names. Steelhead are a type of rainbow trout that migrate between lakes, rivers, and the ocean. They are also called steelhead salmon or ocean trout.

Steelhead are farmed in many countries. Production has grown since the 1950s, especially in Europe and Chile. In 2007, 604,695 tonnes of farmed steelhead were harvested, valued at $2.59 billion. Chile is the largest producer. Inland farms in countries like Italy, France, and Spain raise rainbow trout for domestic markets. Steelhead and rainbow trout eat aquatic insects, fish eggs, and small fish. As they grow, they consume more fish. In the ocean, adult steelhead eat other fish, squid, and small crustaceans. Farmed steelhead are fed a diet similar to their natural food, including fish meal, vitamins, and pigments.

Steelhead are vulnerable to a disease called enteric redmouth disease, which affects fish but not humans.

Coho salmon are the state animal of Chiba, Japan. They mature in one year in the ocean, requiring two groups of breeding fish that alternate each year. Broodfish are moved from the ocean to freshwater tanks for spawning.

In 2007, 115,376 tonnes of farmed Coho salmon were harvested, valued at $456 million. Chile produces about 90% of the world’s Coho salmon, with Japan and Canada producing the rest.

Chinook salmon are the state fish of Oregon and are called "king salmon" because of their size and flavor. Salmon from Alaska’s Copper River are known for their color, taste, and high omega-3 content. Alaska banned finfish aquaculture in 1989.

In 2007, 11,542 tonnes of farmed Chinook salmon were harvested, valued at $83 million. New Zealand is the largest producer, farming most Chinook salmon in sea cages. Smolt are raised in freshwater hatcheries and then moved to sea cages, where they are fed high-protein fishmeal. Chinook are also farmed in freshwater rivers and hydroelectric canals, such as in New Zealand’s Tekapo, which is the world’s highest salmon farm at 677 meters above sea level.

Before being killed, farmed salmon are sometimes anesthetized with herbal extracts. They are then stunned by a brain spike, and their gills are sliced to bleed them. This method keeps the flesh firm and long-lasting. New Zealand farmers avoid using antibiotics because farmed salmon are healthy and raised in low-density cages.

Timeline

• 1527: Hector Boece of the University of Aberdeen, Scotland, wrote about the life of the Atlantic salmon.
• 1763: Scientists in Germany tried experiments to help Atlantic salmon reproduce. Later, scientists in Scotland and France improved these methods.
• 1854: The Dohulla Fishery in Ballyconneely, Ireland, built areas for salmon to lay eggs and places to raise young salmon along a river.
• 1864: People released young Atlantic salmon from a hatchery into the River Plenty in Tasmania. This effort failed to create a population in Australia.
• 1892: People released young Atlantic salmon from a hatchery into the Umkomass River in South Africa. This effort failed to create a population in Africa.
• Late 19th century: Salmon hatcheries were used in Europe, North America, and Japan to help increase wild salmon numbers.
• 1961: People released young Atlantic salmon from a hatchery into the rivers of the Falkland Islands. This effort failed to create a population in the South Atlantic.
• Late 1960s: The first salmon farms were built in Norway and Scotland.
• 1970: People released young Atlantic salmon from a hatchery into the rivers of the Kerguelen Islands. This effort failed to create a population in the Indian Ocean.
• Early 1970s: Salmon farms were built in North America.
• 1973: Rainbow trout were first bred in Thailand on Doi Inthanon, the highest mountain in Thailand, as part of the Royal Project started by King Bhumibol Adulyadej (Rama IX).
• 1975: A small type of parasite called Gyrodactylus spread from Norwegian hatcheries to wild salmon, likely through fishing equipment, and caused harm to some wild salmon populations.
• Late 1970s: Salmon farms were built in Chile and New Zealand.
• 1984: A serious disease called infectious salmon anemia was discovered in a Norwegian salmon hatchery. About 80% of the fish there died.
• 1985: Salmon farms were built in Australia.

In popular culture

In Chapter 14 of Paul Torday's 2007 novel Salmon Fishing in the Yemen, the book describes a visit to the "McSalmon Aqua Farms," where salmon are raised in cages inside a sea loch in Scotland.