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Rice

Rice is a cereal grain and in its domesticated form is the staple food of over half of the world's population, particularly in Asia and Africa. Rice is the seed of the grass species Oryza sativa (Asian rice)—or, much less commonly, Oryza glaberrima (African rice). Asian rice was domesticated in China some 13,500 to 8,200 years ago; African rice was domesticated in Africa about 3,000 years ago. Rice has become commonplace in many cultures worldwide; in 2021, 787 million tons were produced, placing it fourth after sugarcane, maize, and wheat. Only some 8% of rice is traded internationally. China, India, and Indonesia are the largest consumers of rice. A substantial amount of the rice produced in developing nations is lost after harvest through factors such as poor transport and storage. Rice yields can be reduced by pests including insects, rodents, and birds, as well as by weeds, and by diseases such as rice blast. Traditional rice polycultures such as rice-duck farming, and modern integrated pest management seek to control damage from pests in a sustainable way.

Many varieties of rice have been bred to improve crop quality and productivity. Biotechnology has created Green Revolution rice able to produce high yields when supplied with nitrogen fertiliser and managed intensively. Other products are rice able to express human proteins for medicinal use; flood-tolerant or deepwater rice; and drought-tolerant and salt-tolerant varieties. Rice is used as a model organism in biology.

Dry rice grain is milled to remove the outer layers; depending on how much is removed, products range from brown rice to rice with germ and white rice. Some is parboiled to make it easy to cook. Rice contains no gluten; it provides protein but not all the essential amino acids needed for good health. Rice of different types is eaten around the world. Long-grain rice tends to stay intact on cooking; medium-grain rice is stickier, and is used for sweet dishes, and in Italy for risotto; and sticky short-grain rice is used in Japanese sushi as it keeps its shape when cooked. White rice when cooked contains 29% carbohydrate and 2% protein, with some manganese. Golden rice is a variety produced by genetic engineering to contain vitamin A.

Production of rice is estimated to have caused over 1% of global greenhouse gas emissions in 2022. Predictions of how rice yields will be affected by climate change vary across geographies and socioeconomic contexts. In human culture, rice plays a role in certain religions and traditions, such as in weddings.

Description
The rice plant can grow to over 1 m tall; if in deep water, it can reach a length of 5 m. A single plant may have several leafy stems or tillers. The upright stem is jointed with nodes along its length; a long slender leaf arises from each node. The self-fertile flowers are produced in a panicle, a branched inflorescence which arises from the last internode on the stem. There can be up to 350 spikelets in a panicle, each containing male and female flower parts (anthers and ovule). A fertilised ovule develops into the edible grain or caryopsis.

Rice is a cereal belonging to the family Poaceae. As a tropical crop, it can be grown during the two distinct seasons (dry and wet) of the year provided that sufficient water is made available. It is normally an annual, but in the tropics it can survive as a perennial, producing a ratoon crop.

Growing
Like all crops, rice depends for its growth on both biotic and abiotic environmental factors. The principal biotic factors are crop variety, pests, and plant diseases. Abiotic factors include the soil type, whether lowland or upland, amount of rain or irrigation water, temperature, day length, and intensity of sunlight.

Rice grains can be planted directly into the field where they will grow, or seedlings can be grown in a seedbed and transplanted into the field. Direct seeding needs some 60 to 80 kg of grain per hectare, while transplanting needs less, around 40 kg per hectare, but requires far more labour. Most rice in Asia is transplanted by hand. Mechanical transplanting takes less time but requires a carefully-prepared field and seedlings raised on mats or in trays to fit the machine. Rice does not thrive if continuously submerged. Rice can be grown in different environments, depending upon water availability. The usual arrangement is for lowland fields to be surrounded by bunds and flooded to a depth of a few centimetres until around a week before harvest time; this requires a large amount of water. The "alternate wetting and drying" technique uses less water. One form of this is to flood the field to a depth of 5 cm (2 in), then to let the water level drop to 15 cm (6 in) below surface level, as measured by looking into a perforated field water tube sunk into the soil, and then repeating the cycle. Deepwater rice varieties tolerate flooding to a depth of over 50 centimetres for at least a month. Upland rice is grown without flooding, in hilly or mountainous regions; it is rainfed like wheat or maize.

Harvesting
Across Asia, unmilled rice or "paddy" (Indonesian and Malay padi), was traditionally the product of smallholder agriculture, with manual harvesting. Larger farms make use of machines such as combine harvesters to reduce the input of labour. The grain is ready to harvest when the moisture content is 20–25%. Harvesting involves reaping, stacking the cut stalks, threshing to separate the grain, and cleaning by winnowing or screening. The rice grain is dried as soon as possible to bring the moisture content down to a level that is safe from mould fungi. Traditional drying relies on the heat of the sun, with the grain spread out on mats or on pavements.

Phylogeny
The edible rice species are members of the BOP clade within the grass family, the Poaceae. The rice subfamily, Oryzoideae, is sister to the bamboos, Bambusoideae, and the cereal subfamily Pooideae. The rice genus Oryza is one of eleven in the Oryzeae; it is sister to the Phyllorachideae. The edible rice species O. sativa and O. glaberrima are among some 300 species or subspecies in the genus.

History

Oryza sativa rice was first domesticated in China 9,000 years ago, by people of Neolithic cultures in the Upper and Lower Yangtze, associated with Hmong-Mien-speakers and pre-Austronesians, respectively. The functional allele for nonshattering, the critical indicator of domestication in grains, as well as five other single-nucleotide polymorphisms, is identical in both indica and japonica. This implies a single domestication event for O. sativa. Both indica and japonica forms of Asian rice sprang from a single domestication event in China from the wild rice Oryza rufipogon. Despite this evidence, it appears that indica rice arose when japonica arrived in India about 4,500 years ago and hybridised with another rice, whether an undomesticated proto-indica or wild O. nivara.

Rice was introduced early into Sino-Tibetan cultures in northern China by around 6000 to 5600 years ago, and to the Korean peninsula and Japan by around 5500 to 3200 years ago. It was also carried into Taiwan by the Dapenkeng culture by 5500 to 4000 years ago, before spreading southwards via the Austronesian migrations to Island Southeast Asia, Madagascar, and Guam, but did not survive the voyage to the rest of the Pacific. It reached Austroasiatic and Kra-Dai-speakers in Mainland Southeast Asia and southern China by 5000 years ago.

Rice spread around the rest of the world through cultivation, migration and trade, eventually to the Americas as part of the Columbian exchange after 1492. The now less common Oryza glaberrima (African rice) was independently domesticated in Africa around 3,000 years ago, and introduced to the Americas by the Spanish.

Production
In 2021, world production of rice was 787 million tonnes, led by China and India with a combined 52% of the total. This placed rice fourth in the list of crops by production, after sugarcane, maize, and wheat. Other major producers were Bangladesh, Indonesia and Vietnam. 90% of world production is from Asia.

Yield records
The average world yield for rice was 4.7 MT/ha, in 2022. Yuan Longping of China's National Hybrid Rice Research and Development Center set a world record for rice yield in 1999 at 17.1 MT/ha on a demonstration plot. This employed specially developed hybrid rice and the System of Rice Intensification (SRI), an innovation in rice farming.

Food security
Rice is a major food staple in Asia, Latin America, and some parts of Africa, feeding over half the world's population. However, a substantial part of the crop can be lost post-harvest through inefficient transportation, storage, and milling. A quarter of the crop in Nigeria is lost after harvest. Storage losses include damage by mould fungi if the rice is not dried sufficiently. In China, losses in modern metal silos were just 0.2%, compared to 7–13% when rice was stored by rural households.

Processing
The dry grain is milled to remove the outer layers, namely the husk and bran. These can be removed in a single step, in two steps, or as in commercial milling in a multi-step process of cleaning, dehusking, separation, polishing, grading, and weighing. Brown rice only has the inedible husk removed. Further milling removes bran and the germ to create successively whiter products. Parboiled rice is subjected to a steaming process before it is milled. This makes the grain harder, and moves some of the grain's vitamins and minerals into the white part of the rice so these are retained after milling. Rice does not contain gluten, so is suitable for people on a gluten-free diet. Rice is a good source of protein and a staple food in many parts of the world, but it is not a complete protein as it does not contain all of the essential amino acids in sufficient amounts for good health.

Trade
World trade figures are much smaller than those for production, as less than 8% of rice produced is traded internationally. China, an exporter of rice in the early 2000s, had become the world's largest importer of rice by 2013. Developing countries are the main players in the world rice trade; by 2012, India was the largest exporter of rice, with Thailand and Vietnam the other largest exporters.

Worldwide consumption
As of 2016, the countries that consumed the most rice were China (29% of total), India, and Indonesia. By 2020, Bangladesh had taken third place from Indonesia. On an annual average from 2020-23, China consumed 154 million tonnes of rice, India consumed 109 million tonnes, and Bangladesh and Indonesia consumed about 36 million tonnes each. Across the world, rice consumption per capita fell in the 21st century as people in Asia and elsewhere ate less grain and more meat. An exception is Sub-Saharan Africa, where both per capita consumption of rice and population are increasing.

Eating qualities
Rice is a commonly-eaten food around the world. The varieties of rice are typically classified as short-, medium-, and long-grained. Oryza sativa indica varieties are usually long-grained; Oryza sativa japonica varieties are usually short- or medium-grained. Short-grain rice, with the exception of Spanish Bomba, is usually sticky when cooked, and is suitable for puddings. Thai Jasmine rice is aromatic, and unusually for a long-grain rice has some stickiness, with a soft texture. Indian Basmati rice is very long-grained and aromatic. Italian Arborio rice, used for risotto, is of medium length, oval, and quite sticky. Japanese sushi rice is a sticky short-grain variety.

Nutrition
Cooked white rice is 69% water, 29% carbohydrates, 2% protein, and contains negligible fat (table). In a reference serving of 100 g, cooked white rice provides 130 calories of food energy, and contains moderate levels of manganese (18% DV), with no other micronutrients in significant content (all less than 10% of the Daily Value). In 2018, the World Health Organization strongly recommended fortifying rice with iron, and conditionally recommended fortifying it with vitamin A and with folic acid.

Golden rice
Golden rice is a variety produced through genetic engineering to synthesize beta-carotene, a precursor of vitamin A, in the endosperm of the rice grain. It is intended to be grown and eaten in parts of the world where Vitamin A deficiency is prevalent. Golden rice has been opposed by activists, such as in the Philippines. In 2016 more than 100 Nobel laureates encouraged the use of genetically modified organisms, such as golden rice, for the benefits these could bring.

Greenhouse gases from rice production

In 2022, greenhouse gas emissions from rice cultivation were estimated at 5.7 billion tonnes CO2eq, representing 1.2% of total emissions. Within the agriculture sector, rice produces almost half the greenhouse gas emissions from croplands, some 30% of agricultural methane emissions, and 11% of agricultural nitrous oxide emissions. Methane is released from rice fields subject to long-term flooding, as this inhibits the soil from absorbing atmospheric oxygen, resulting in anaerobic fermentation of organic matter in the soil. Emissions can be limited by planting new varieties, not flooding continuously, and removing straw.

It is possible to cut methane emissions in rice cultivation by improved water management, combining dry seeding and one drawdown, or executing a sequence of wetting and drying. This results in emission reductions of up to 90% compared to full flooding and even increased yields.

Effects of climate change on rice production
Predictions of climate change's effects on rice cultivation vary. Global rice yield has been projected to decrease by around 3.2% with each 1°C increase in global average temperature while another study predicts global rice cultivation will increase initially, plateauing at about 3°C warming (2091–2100 relative to 1850–1900).

The impacts of climate change on rice cultivation will vary across geographic location and socioeconomic contexts. For example, rising temperatures and decreasing solar radiation during the later years of the 20th century decreased rice yield by between 10% and 20% across 200 farms in seven Asian countries. This may have been caused by increased night-time respiration. IRRI has predicted that Asian rice yields will fall by some 20% per 1°C rise in global mean temperature. Further, rice is unable to yield grain if the flowers experience a temperature of 35 °C or more for over one hour, so the crop would be lost under these conditions.

In the Po Valley in Italy, the arborio and carnaroli risotto rice varieties have suffered poor harvests through drought in the 21st century. The Ente Nazionale Risi is developing drought-resistant varieties; its nuovo prometeo variety has deep roots that enable it to tolerate drought, but is not suitable for risotto.

Pests and weeds

Rice yield can be reduced by weed growth, and a wide variety of pests including insects, nematodes, rodents such as rats, snails, and birds. Major rice insect pests include armyworms, rice bugs, black bugs, cutworms, field crickets, grasshoppers, leafhoppers, mealybugs, and planthoppers. High rates of nitrogen fertiliser application may worsen aphid outbreaks. Weather conditions can contribute to pest outbreaks: rice gall midge outbreaks are worsened by high rainfall in the wet season, while thrips outbreaks are associated with drought.

Diseases

Rice blast, caused by the fungus Magnaporthe grisea, is the most serious disease of growing rice. It and bacterial leaf streak (caused by Xanthomonas oryzae pv. oryzae) are perennially the two worst rice diseases worldwide; they are both among the ten most important diseases of all crop plants. Other major rice diseases include sheath blight (caused by Rhizoctonia solani), false smut (Ustilaginoidea virens), and bacterial panicle blight (Burkholderia glumae). Viral diseases include rice bunchy stunt, rice dwarf, rice tungro, and rice yellow mottle.

Pest management
Crop protection scientists are developing sustainable techniques for managing rice pests. Sustainable pest management is based on four principles: biodiversity, host plant resistance, landscape ecology, and hierarchies in a landscape—from biological to social. Farmers' pesticide applications are often unnecessary. Pesticides may actually induce resurgence of populations of rice pests such as the brown planthopper, both by destroying beneficial insects and by enhancing the pest's reproduction. The International Rice Research Institute (IRRI) demonstrated in 1993 that an 87.5% reduction in pesticide use can lead to an overall drop in pest numbers.

Farmers in China, Indonesia and the Philippines have traditionally managed weeds and pests by the polycultural practice of raising ducks and sometimes fish in their rice paddies. These produce valuable additional crops, eat small pest animals, manure the rice, and in the case of ducks also control weeds.

Rice plants produce their own chemical defences to protect themselves from pest attacks. Some synthetic chemicals, such as the herbicide 2,4-D, cause the plant to increase the production of certain defensive chemicals and thereby increase the plant's resistance to some types of pests. Conversely, other chemicals, such as the insecticide imidacloprid, appear to induce changes in the gene expression of the rice that make the plant more susceptible to certain pests.

Plant breeders have created rice cultivars incorporating resistance to various insect pests. Conventional plant breeding of resistant varieties has been limited by challenges such as rearing insect pests for testing, and the great diversity and continuous evolution of pests. Resistance genes are being sought from wild species of rice, and genetic engineering techniques are being applied.

Ecotypes and cultivars

The International Rice Research Institute maintains the International Rice Genebank, which holds over 100,000 rice varieties. Much of southeast Asia grows sticky or glutinous rice varieties. High-yield cultivars of rice suitable for cultivation in Africa, called the New Rice for Africa (NERICA), have been developed to improve food security and alleviate poverty in Sub-Saharan Africa.

The complete genome of rice was sequenced in 2005, making it the first crop plant to reach this status. Since then, the genomes of hundreds of types of rice, both wild and cultivated, and including both Asian and African rice species, have been sequenced.

High-yielding varieties
The high-yielding varieties are a group of crops created during the Green Revolution to increase global food production radically. The first Green Revolution rice variety, IR8, was produced in 1966 at the International Rice Research Institute through a cross between an Indonesian variety named "Peta" and a Chinese variety named "Dee Geo Woo Gen". Green Revolution varieties were bred to have short strong stems so that the rice would not lodge or fall over. This enabled them to stay upright and productive even with heavy applications of fertiliser.

Expression of human proteins
Ventria Bioscience has genetically modified rice to express lactoferrin and lysozyme which are proteins usually found in breast milk, and human serum albumin. These proteins have antiviral, antibacterial, and antifungal effects. Rice containing these added proteins can be used as a component in oral rehydration solutions to treat diarrheal diseases, thereby shortening their duration and reducing recurrence. Such supplements may also help reverse anemia.

Flood-tolerance

In areas subject to flooding, farmers have long planted flood tolerant varieties known as deepwater rice. In South and South East Asia, flooding affects some 20 e6ha each year. Flooding has historically led to massive losses in yields, such as in the Philippines, where in 2006, rice crops worth $65 million were lost to flooding.

Standard rice varieties cannot withstand stagnant flooding for more than about a week, since it disallows the plant access to necessary requirements such as sunlight and gas exchange. The Swarna Sub1 cultivar can tolerate week-long submergence, consuming carbohydrates efficiently and continuing to grow. So-called "scuba rice" with the Sub1A transgene is robustly tolerant of submergence for as long as two weeks, offering much improved flood survival for farmers' crops. IRRI has created Sub1A varieties and distributed them to Bangladesh, India, Indonesia, Nepal, and the Philippines.

Drought-tolerance
Drought represents a significant environmental stress for rice production, with 19-23 e6ha of rainfed rice production in South and South East Asia often at risk. Under drought conditions, without sufficient water to afford them the ability to obtain the required levels of nutrients from the soil, conventional commercial rice varieties can be severely affected—as happened for example in India early in the 21st century.

The International Rice Research Institute conducts research into developing drought-tolerant rice varieties, including the varieties Sahbhagi Dhan, Sahod Ulan, and Sookha dhan, currently being employed by farmers in India, the Philippines, and Nepal respectively. In addition, in 2013 the Japanese National Institute for Agrobiological Sciences led a team which successfully inserted the DEEPER ROOTING 1 (DRO1) gene, from the Philippine upland rice variety Kinandang Patong, into the popular commercial rice variety IR64, giving rise to a far deeper root system in the resulting plants. This facilitates an improved ability for the rice plant to derive its required nutrients in times of drought via accessing deeper layers of soil, a feature demonstrated by trials which saw the IR64 + DRO1 rice yields drop by 10% under moderate drought conditions, compared to 60% for the unmodified IR64 variety.

Salt-tolerance
Soil salinity poses a major threat to rice crop productivity, particularly along low-lying coastal areas during the dry season. For example, roughly 1 e6ha of the coastal areas of Bangladesh are affected by saline soils. These high concentrations of salt can severely affect rice plants' physiology, especially during early stages of growth, and as such farmers are often forced to abandon these areas.

Progress has been made in developing rice varieties capable of tolerating such conditions; the hybrid created from the cross between the commercial rice variety IR56 and the wild rice species Oryza coarctata is one example. O. coarctata can grow in soils with double the limit of salinity of normal varieties, but does not produce edible rice. Developed by the International Rice Research Institute, the hybrid variety utilises specialised leaf glands that remove salt into the atmosphere. It was produced from one successful embryo out of 34,000 crosses between the two species; this was then backcrossed to IR56 with the aim of preserving the genes responsible for salt tolerance that were inherited from O. coarctata.

Cold tolerance
Rice is sensitive to temperatures below 12C. Sowing takes place once the daily average temperature is reliably above this limit. Average temperatures below that reduce growth; if sustained for over four days, germination and seedling growth are harmed and seedlings may die. In larger plants subjected to cold, rice blast is encouraged, seriously reducing yield. As of 2022, researchers continue to study the mechanisms of chilling tolerance in rice and its genetic basis.

Reducing methane emissions
Producing rice in paddies is harmful for the environment due to the release of methane by methanogenic bacteria. These bacteria live in the anaerobic waterlogged soil, consuming nutrients released by rice roots. Putting the barley gene SUSIBA2 into rice creates a shift in biomass production from root to shoot, decreasing the methanogen population, and resulting in a reduction of methane emissions of up to 97%. Further, the modification increases the amount of rice grains.

Model organism
Rice is used as a model organism for investigating the mechanisms of meiosis and DNA repair in higher plants. For example, study using rice has shown that the gene OsRAD51C is necessary for the accurate repair of DNA double-strand breaks during meiosis.

In human culture

Rice plays an important role in certain religions and popular beliefs. In Hindu wedding ceremonies, rice, denoting fertility, prosperity, and purity, is thrown into the sacred fire, a custom modified in Western weddings, where people throw rice. In Malay weddings, rice features in multiple special wedding foods such as sweet glutinous rice. In Japan and the Philippines, rice wine is used for weddings and other celebrations. Dewi Sri is a goddess of the Indo-Malaysian archipelago, who in myth is transformed into rice or other crops. The start of the rice planting season is marked in Asian countries including Nepal and Cambodia with a Royal Ploughing Ceremony.

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Clay

Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, Al2Si2O5(OH)4). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impurities, such as a reddish or brownish colour from small amounts of iron oxide.

Clays develop plasticity when wet but can be hardened through firing. Clay is the longest-known ceramic material. Prehistoric humans discovered the useful properties of clay and used it for making pottery. Some of the earliest pottery shards have been dated to around 14,000 BCE, and clay tablets were the first known writing medium. Clay is used in many modern industrial processes, such as paper making, cement production, and chemical filtering. Between one-half and two-thirds of the world's population live or work in buildings made with clay, often baked into brick, as an essential part of its load-bearing structure.

Clay is a very common substance. Shale, formed largely from clay, is the most common sedimentary rock. Although many naturally occurring deposits include both silts and clay, clays are distinguished from other fine-grained soils by differences in size and mineralogy. Silts, which are fine-grained soils that do not include clay minerals, tend to have larger particle sizes than clays. Mixtures of sand, silt and less than 40% clay are called loam. Soils high in swelling clays (expansive clay), which are clay minerals that readily expand in volume when they absorb water, are a major challenge in civil engineering.

Properties
The defining mechanical property of clay is its plasticity when wet and its ability to harden when dried or fired. Clays show a broad range of water content within which they are highly plastic, from a minimum water content (called the plastic limit) where the clay is just moist enough to mould, to a maximum water content (called the liquid limit) where the moulded clay is just dry enough to hold its shape. The plastic limit of kaolinite clay ranges from about 36% to 40% and its liquid limit ranges from about 58% to 72%. High-quality clay is also tough, as measured by the amount of mechanical work required to roll a sample of clay flat. Its toughness reflects a high degree of internal cohesion.

Clay has a high content of clay minerals that give it its plasticity. Clay minerals are hydrous aluminium phyllosilicate minerals, composed of aluminium and silicon ions bonded into tiny, thin plates by interconnecting oxygen and hydroxide ions. These plates are tough but flexible, and in moist clay, they adhere to each other. The resulting aggregates give clay the cohesion that makes it plastic. In kaolinite clay, the bonding between plates is provided by a film of water molecules that hydrogen bond the plates together. The bonds are weak enough to allow the plates to slip past each other when the clay is being moulded, but strong enough to hold the plates in place and allow the moulded clay to retain its shape after it is moulded. When the clay is dried, most of the water molecules are removed, and the plates hydrogen bond directly to each other, so that the dried clay is rigid but still fragile. If the clay is moistened again, it will once more become plastic. When the clay is fired to the earthenware stage, a dehydration reaction removes additional water from the clay, causing clay plates to irreversibly adhere to each other via stronger covalent bonding, which strengthens the material. The clay mineral kaolinite is transformed into a non-clay material, metakaolin, which remains rigid and hard if moistened again. Further firing through the stoneware and porcelain stages further recrystallizes the metakaolin into yet stronger minerals such as mullite.

The tiny size and plate form of clay particles gives clay minerals a high surface area. In some clay minerals, the plates carry a negative electrical charge that is balanced by a surrounding layer of positive ions (cations), such as sodium, potassium, or calcium. If the clay is mixed with a solution containing other cations, these can swap places with the cations in the layer around the clay particles, which gives clays a high capacity for ion exchange. The chemistry of clay minerals, including their capacity to retain nutrient cations such as potassium and ammonium, is important to soil fertility.

Clay is a common component of sedimentary rock. Shale is formed largely from clay and is the most common of sedimentary rocks. However, most clay deposits are impure. Many naturally occurring deposits include both silts and clay. Clays are distinguished from other fine-grained soils by differences in size and mineralogy. Silts, which are fine-grained soils that do not include clay minerals, tend to have larger particle sizes than clays. There is, however, some overlap in particle size and other physical properties. The distinction between silt and clay varies by discipline. Geologists and soil scientists usually consider the separation to occur at a particle size of 2 μm (clays being finer than silts), sedimentologists often use 4–5 μm, and colloid chemists use 1 μm. Clay-size particles and clay minerals are not the same, despite a degree of overlap in their respective definitions. Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils' Atterberg limits. ISO 14688 grades clay particles as being smaller than 2 μm and silt particles as being larger. Mixtures of sand, silt and less than 40% clay are called loam.

Some clay minerals (such as smectite) are described as swelling clay minerals, because they have a great capacity to take up water, and they increase greatly in volume when they do so. When dried, they shrink back to their original volume. This produces distinctive textures, such as mudcracks or "popcorn" texture, in clay deposits. Soils containing swelling clay minerals (such as bentonite) pose a considerable challenge for civil engineering, because swelling clay can break foundations of buildings and ruin road beds.

Formation

Clay minerals most commonly form by prolonged chemical weathering of silicate-bearing rocks. They can also form locally from hydrothermal activity. Chemical weathering takes place largely by acid hydrolysis due to low concentrations of carbonic acid, dissolved in rainwater or released by plant roots. The acid breaks bonds between aluminium and oxygen, releasing other metal ions and silica (as a gel of orthosilicic acid).)

The clay minerals formed depend on the composition of the source rock and the climate. Acid weathering of feldspar-rich rock, such as granite, in warm climates tends to produce kaolin. Weathering of the same kind of rock under alkaline conditions produces illite. Smectite forms by weathering of igneous rock under alkaline conditions, while gibbsite forms by intense weathering of other clay minerals.

There are two types of clay deposits: primary and secondary. Primary clays form as residual deposits in soil and remain at the site of formation. Secondary clays are clays that have been transported from their original location by water erosion and deposited in a new sedimentary deposit. Secondary clay deposits are typically associated with very low energy depositional environments such as large lakes and marine basins.

Varieties
The main groups of clays include kaolinite, montmorillonite-smectite, and illite. Chlorite, vermiculite, talc, and pyrophyllite are sometimes also classified as clay minerals. There are approximately 30 different types of "pure" clays in these categories, but most "natural" clay deposits are mixtures of these different types, along with other weathered minerals. Clay minerals in clays are most easily identified using X-ray diffraction rather than chemical or physical tests.

Varve (or varved clay) is clay with visible annual layers that are formed by seasonal deposition of those layers and are marked by differences in erosion and organic content. This type of deposit is common in former glacial lakes. When fine sediments are delivered into the calm waters of these glacial lake basins away from the shoreline, they settle to the lake bed. The resulting seasonal layering is preserved in an even distribution of clay sediment banding.

Quick clay is a unique type of marine clay indigenous to the glaciated terrains of Norway, North America, Northern Ireland, and Sweden. It is a highly sensitive clay, prone to liquefaction, and has been involved in several deadly landslides.

Uses
Modelling clay is used in art and handicraft for sculpting. Clays are used for making pottery, both utilitarian and decorative, and construction products, such as bricks, walls, and floor tiles. Different types of clay, when used with different minerals and firing conditions, are used to produce earthenware, stoneware, and porcelain. Prehistoric humans discovered the useful properties of clay. Some of the earliest pottery shards recovered are from central Honshu, Japan. They are associated with the Jōmon culture, and recovered deposits have been dated to around 14,000 BCE. Cooking pots, art objects, dishware, smoking pipes, and even musical instruments such as the ocarina can all be shaped from clay before being fired.

Ancient peoples in Mesopotamia adopted clay tablets as the first known writing medium. Clay was chosen due to the local material being easy to work with and widely available. Scribes wrote on the tablets by inscribing them with a script known as cuneiform, using a blunt reed called a stylus, which effectively produced the wedge shaped markings of their writing. After being written on, clay tablets could be reworked into fresh tablets and reused if needed, or fired to make them permanent records. Purpose-made clay balls were used as sling ammunition. Clay is used in many industrial processes, such as paper making, cement production, and chemical filtering. Bentonite clay is widely used as a mold binder in the manufacture of sand castings.

Materials
Clay is a common filler used in polymer nanocomposites. It can reduce the cost of the composite, as well as impart modified behavior: increased stiffness, decreased permeability, decreased electrical conductivity, etc.

Medicine
Traditional uses of clay as medicine goes back to prehistoric times. An example is Armenian bole, which is used to soothe an upset stomach. Some animals such as parrots and pigs ingest clay for similar reasons. Kaolin clay and attapulgite have been used as anti-diarrheal medicines.

Construction
Clay as the defining ingredient of loam is one of the oldest building materials on Earth, among other ancient, naturally-occurring geologic materials such as stone and organic materials like wood. Also a primary ingredient in many natural building techniques, clay is used to create adobe, cob, cordwood, and structures and building elements such as wattle and daub, clay plaster, clay render case, clay floors and clay paints and ceramic building material. Clay was used as a mortar in brick chimneys and stone walls where protected from water.

Clay, relatively impermeable to water, is also used where natural seals are needed, such as in pond linings, the cores of dams, or as a barrier in landfills against toxic seepage (lining the landfill, preferably in combination with geotextiles). Studies in the early 21st century have investigated clay's absorption capacities in various applications, such as the removal of heavy metals from waste water and air purification.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Clay


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Birmingham (disambiguation)

Birmingham is the second-most populous city in England and the United Kingdom.

Birmingham may also refer to:

Canada

 * Birmingham, Saskatchewan, an unincorporated community

United States

 * Birmingham, Alabama, a large city in Alabama
 * Birmingham-Hoover-Cullman Combined Statistical Area
 * Birmingham, Connecticut, now Derby, a city
 * Birmingham, Indiana, an unincorporated community
 * Birmingham, Tippecanoe County, Indiana, an unincorporated community
 * Birmingham, Iowa, a city
 * Birmingham, Kansas, an unincorporated community
 * Birmingham, Kentucky, a sunken town
 * Birmingham, Michigan, a city
 * Birmingham, Missouri, a village
 * Birmingham, New Jersey, an unincorporated community
 * Birmingham, Coshocton County, Ohio, a ghost town
 * Birmingham, Erie County, Ohio, an unincorporated community
 * Birmingham, Guernsey County, Ohio, an unincorporated community
 * Birmingham, Chester County, Pennsylvania, a township
 * Birmingham, Huntingdon County, Pennsylvania, a borough
 * Birmingham, Pittsburgh, Pennsylvania, the former name of Pittsburgh's South Side neighborhood
 * Birmingham Township (disambiguation), several places

United Kingdom

 * Aston University, Birmingham, England (founded 1895)
 * University of Birmingham, Birmingham, England (founded 1900)
 * Birmingham City University, Birmingham, England (founded 1971)
 * Newman University Birmingham, Birmingham, England (founded 1968)
 * University College Birmingham, Birmingham, England (founded 1957)

United States

 * Birmingham–Southern College, Birmingham, Alabama
 * Birmingham High School, Los Angeles, California
 * University of Alabama at Birmingham, Birmingham, Alabama

People

 * Birmingham (surname), including a list of people with the name
 * De Birmingham family, which held the lordship of Birmingham in England

Royal Navy

 * HMS Birmingham (1913), a Town-class light cruiser launched in 1913 and sold in 1931
 * HMS Birmingham (C19), a cruiser launched in 1936 and broken up in 1960
 * HMS Birmingham (D86), a Type 42 destroyer in service from 1976 to 1999

United States Navy

 * USS Birmingham (CL-2), a light cruiser in service from 1908 to 1923
 * USS Birmingham (CL-62), a light cruiser, in service from 1943 to 1946 and involved in heavy fighting in the Pacific Ocean during World War II
 * USS Birmingham (SSN-695), a Los Angeles class nuclear attack submarine in service from 1978 to 1997

Songs

 * "Birmingham" (1974), a song by Randy Newman from the album Good Old Boys
 * Birmingham (Amanda Marshall song), 1996
 * "Birmingham" (2001), a song by Drive-By Truckers from the album Southern Rock Opera
 * "Birmingham" (2012), a song by Shovels & Rope
 * "Birmingham (We Are Safe)", a song by the David Crowder*Band from the album Church Music
 * "Birmingham Blues" (song), by Electric Light Orchestra, 1977
 * “Birmingham” (2021), a song by the American Punk rock band NOFX from the album Single Album

Sport

 * Birmingham Barons, a minor league baseball team
 * Birmingham Bulls (WHA), a defunct professional ice hockey team
 * Birmingham Bullets, a defunct basketball team
 * Birmingham City F.C., an English football club that currently play in the Football League Championship
 * Birmingham City L.F.C., an English ladies' football club
 * Birmingham Football Club, a defunct English football club that formerly played at the Aston Lower Grounds
 * Birmingham (horse), a British Thoroughbred racehorse
 * Birmingham Stallions, a defunct American football team that played in the USFL

Air travel

 * Birmingham Airport, Birmingham, England
 * Birmingham–Shuttlesworth International Airport, Birmingham, Alabama
 * G-BDXJ, the former British Airways Boeing 747-200 City of Birmingham

Railroads

 * Birmingham railway (disambiguation)
 * Birmingham station (disambiguation)
 * LMS Coronation Class (4)6235 City of Birmingham, an LMS Coronation Class steam railway locomotive

Other transportation

 * Birmingham Motors, U.S. automobile brand of the 1920s

Other uses

 * Birmingham, a star in the constellation Cygnus
 * Birmingham, a style of home described in the song Paint Me a Birmingham
 * Birmingham (crater), a lunar crater
 * Birmingham (HM Prison), a prison in Birmingham, England
 * Birmingham (UK Parliament constituency), constituency 1832–1885
 * Birmingham City Council, Birmingham, England
 * Birmingham gauge of wires and tubes

title: Birmingham (disambiguation)


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Backronym

A backronym is an acronym formed from an already existing word by expanding its letters into the words of a phrase. Backronyms may be invented with either serious or humorous intent, or they may be a type of false etymology or folk etymology. The word is a portmanteau of back and acronym.

A normal acronym is a word derived from the initial letters of the words of a phrase, such as radar from "radio detection and ranging". By contrast, a backronym is "an acronym deliberately formed from a phrase whose initial letters spell out a particular word or words, either to create a memorable name or as a fanciful explanation of a word's origin". Many fictional espionage organizations are backronyms, such as SPECTRE (special executive for counterintelligence, terrorism, revenge and extortion) from the James Bond franchise.

For example, the Amber Alert missing-child program was named after Amber Hagerman, a nine-year-old girl who was abducted and murdered in 1996. Officials later publicized the backronym "America's Missing: Broadcast Emergency Response".

Examples
An example of a backronym as a mnemonic is the Apgar score, used to assess the health of newborn babies. The rating system was devised by and named after Virginia Apgar. Ten years after the initial publication, the backronym APGAR was coined in the US as a mnemonic learning aid: appearance, pulse, grimace, activity, and respiration. Another example is the American Contract Bridge League's tools to address cheating in online bridge games. EDGAR was originally named for Edgar Kaplan, whose many contributions to the game included groundbreaking efforts to reduce illegal partnership communication. The new EDGAR tools expected to debut in early 2024 have been launched with the backronym "everyone deserves a game above reproach".

Many United States Congress bills have backronyms as their names; examples include the American CARES Act (Coronavirus Aid, Relief, and Economic Security Act) of 2020, the USA PATRIOT Act (Uniting and Strengthening America by Providing Appropriate Tools Required to Intercept and Obstruct Terrorism Act) of 2001, and the DREAM Act (Development, Relief, and Education for Alien Minors Act). In the 113th Congress (2013) there were over 240 bills with such names.

As false etymologies
Sometimes a backronym is reputed to have been used in the formation of the original word, and amounts to a false etymology or an urban legend. Acronyms were rare in the English language before the 1930s, and most etymologies of common words or phrases that suggest origin from an acronym are false.

Examples include posh, an adjective describing stylish items or members of the upper class. A popular story derives the word as an acronym from "port out, starboard home", referring to 19th-century first-class cabins on ocean liners, which were shaded from the sun on outbound voyages east (e.g. from Britain to India) and homeward voyages west. The word's actual etymology is unknown, but more likely related to Romani påš xåra ('half-penny') or to Urdu (borrowed from Persian) safed-pōśh ('white robes'), a term for wealthy people.

Another example is the word chav, which is a derogatory term for a working-class youth. This word is probably of Romani origin but commonly believed to be a backronym of "council-housed and violent".

Similarly, the distress signal SOS is often believed to be an abbreviation for "save our ship" or "save our souls" but was chosen because it has a simple and unmistakable Morse code representation – three dots, three dashes, and three dots, sent without any pauses between characters.

More recent examples include the brand name Adidas, named after company founder Adolf "Adi" Dassler but falsely believed to be an acronym for "all day I dream about sport"; The word Wiki, said to stand for "what I know is", but in fact derived from the Hawaiian phrase wiki-wiki meaning 'fast'; or Yahoo!, sometimes claimed to mean "yet another hierarchical officious oracle", but in fact chosen because Yahoo's founders liked the word's meaning of "rude, unsophisticated, uncouth" (taken from Jonathan Swift's book Gulliver's Travels). The distress call "pan-pan" is commonly stated to mean "possible assistance needed", whereas it is in fact derived from the French word panne, meaning 'breakdown'.

title: Backronym


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Recursive acronym

A recursive acronym is an acronym that refers to itself, and appears most frequently in computer programming. The term was first used in print in 1979 in Douglas Hofstadter's book Gödel, Escher, Bach: An Eternal Golden Braid, in which Hofstadter invents the acronym GOD, meaning "GOD Over Djinn", to help explain infinite series, and describes it as a recursive acronym. Other references followed, however the concept was used as early as 1968 in John Brunner's science fiction novel Stand on Zanzibar. In the story, the acronym EPT (Education for Particular Task) later morphed into "Eptification for Particular Task".

Recursive acronyms typically form backwardly: either an existing ordinary acronym is given a new explanation of what the letters stand for, or a name is turned into an acronym by giving the letters an explanation of what they stand for, in each case with the first letter standing recursively for the whole acronym.

Use in computing
In computing, an early tradition in the hacker community, especially at MIT, was to choose acronyms and abbreviations that referred humorously to themselves or to other abbreviations. Perhaps the earliest example in this context is the backronym "Mash Until No Good", which was created in 1960 to describe Mung, and revised to "Mung Until No Good". It lived on as a recursive command in the editing language TECO.[3] In 1977 programmer Ted Anderson coined TINT ("TINT Is Not TECO"), an editor for MagicSix. This inspired the two MIT Lisp Machine editors called EINE ("EINE Is Not Emacs", German for one) and ZWEI ("ZWEI Was EINE Initially", German for two), in turn inspiring Anderson's retort SINE ("SINE is not EINE"). Richard Stallman followed with GNU (GNU's Not Unix).

Recursive acronym examples often include negatives, such as denials that the thing defined is or resembles something else (which the thing defined does in fact resemble or is even derived from), to indicate that, despite the similarities, it was distinct from the program on which it was based.

An earlier example appears in a 1976 textbook on data structures, in which the pseudo-language SPARKS is used to define the algorithms discussed in the text. "SPARKS" is claimed to be a non-acronymic name, but "several cute ideas have been suggested" as expansions of the name. One of the suggestions is the tail recursive "Smart Programmers Are Required to Know SPARKS".

Other examples are the YAML language, which stands for "YAML ain't markup language" and PHP language meaning "PHP: Hypertext Preprocessor".

Examples

 * Allegro: Allegro Low LEvel Game ROutines (early versions for Atari ST were called "Atari Low Level Game Routines")
 * AROS: AROS Research Operating System (originally Amiga Research Operating System)
 * ATI: ATI Technologies Inc.
 * BIRD: BIRD Internet Routing Daemon
 * CAVE: CAVE Automatic Virtual Environment
 * cURL: Curl URL Request Library
 * Darcs: Darcs Advanced Revision Control System
 * EINE: EINE Is Not Emacs
 * FIJI: FIJI Is Just ImageJ
 * GiNaC: GiNaC is Not a CAS (Computer Algebra System)
 * GNU: GNU's Not Unix
 * GNE (encyclopedia): GNE's Not Encyclopedia
 * GPE: GPE Palmtop Environment
 * gRPC: grpc Remote Procedure Calls
 * JACK: JACK Audio Connection Kit
 * KGS: KGS Go Server
 * LAME: LAME Ain't MP3 Encoder
 * LiVES: LiVES is Video Editing System
 * MINT: MINT Is Not TRAC
 * MINDY: MINDY Is Not Dylan Yet
 * MiNT: MiNT is Not TOS (later changed to "MiNT is Now TOS")
 * Mung: Mung Until No Good
 * Nagios: Nagios Ain't Gonna Insist On Sainthood (a reference to the previous name of Nagios, "Netsaint"; agios [αγιος] is the Greek word for "saint")
 * NiL: NiL Isn't Liero
 * Ninja-ide: Ninja-IDE Is Not Just Another IDE
 * PHP: PHP: Hypertext Preprocessor (from "Personal Home Page Tools", more frequently referenced as "PHP Tools." )
 * PINE: PINE Is Nearly Elm, originally; PINE now officially stands for "Pine Internet News E-mail"
 * PIP: PIP Installs Packages
 * P.I.P.S.: P.I.P.S. Is POSIX Symbian
 * PNG: officially "Portable Network Graphics", but unofficially "PNG's not GIF".
 * RPM: RPM Package Manager
 * SPARQL: SPARQL Protocol And RDF Query Language
 * TikZ: TikZ ist kein Zeichenprogramm (German; TikZ is not a drawing program)
 * TiLP: TiLP is Linking Program
 * TIP: TIP isn't Pico
 * TRESOR: TRESOR Runs Encryption Securely Outside RAM
 * UIRA: UIRA Isn't Recursive Acronym
 * WINE: WINE Is Not Emulator (Originally, Windows Emulator )
 * XAMPP: XAMPP Apache MariaDB PHP Perl
 * XBMC: XBMC Media Center (originally Xbox Media Center)
 * XINU: XINU Is Not Unix
 * XNA: XNA's Not Acronym'd
 * YAML: YAML Ain't Markup Language (initially "Yet Another Markup Language")
 * YARA: Yara: Another Recursive Acronym
 * Zinf: Zinf Is Not FreeAmp
 * ZWEI: ZWEI Was EINE Initially ("eins" and "zwei" are German for "one" and "two" respectively)

Companies and organizations

 * BWIA: BWIA West Indies Airways (formerly British West Indian Airways)
 * CYGNUS Solutions: "Cygnus, Your GNU Solutions"
 * HIJOS: Hijas e Hijos por la Identidad y la Justicia contra el Olvido y el Silencio (literally, "Daughters and Sons for Identity and Justice against Forgetfulness and Silence")
 * HIM: HIM International Music, Taiwanese independent record label
 * MIATA: MIATA is Always the Answer
 * MEGA: MEGA Encrypted Global Access
 * MOM: MOM's Organic Market
 * SAAB: Saab Automobile AB
 * TAP: TAP Air Portugal
 * VISA: Visa International Service Association
 * ZINC: ZINC Is Not Commercial
 * OIL: Oil India Limited. However it can be debatable as "Oil" is a noun and the company is in the oil industry.

In media

 * The initials for the Commodore CDTV stand for Commodore Commodore Dynamic Total Vision.
 * TTP: a technology project in the Dilbert comic strip. The initials stand for "The TTP Project".
 * GRUNGE: defined by Homer Simpson in The Simpsons episode That '90s Show as "Guitar Rock Utilizing Nihilist Grunge Energy", another uncommon example of a recursive acronym whose recursive letter is neither the first nor the last letter.
 * BOB: the primary antagonist from the series Twin Peaks. His name itself is an acronym standing for "Beware of BOB".
 * KOS-MOS: a character from the Xenosaga series of video games. "KOS-MOS" is a recursive acronym meaning "Kosmos Obey Strategical Multiple Operation System".
 * Hiroshi Yoshimura's "A・I・R" stands for "AIR IN RESORT".

Special

 * The GNU Hurd project is named with a mutually recursive acronym: "Hurd" stands for "Hird Unix-Replacing Daemons", and "Hird" stands for "Hurd Interfaces Representing Depth."
 * RPM, PHP, XBMC and YAML were originally conventional acronyms which were later redefined recursively. They are examples of, or may be referred to as, backronymization, where the official meaning of an acronym is changed.
 * Jini claims the distinction of being the first recursive anti-acronym: 'Jini Is Not Initials'. It might, however, be more properly termed an anti-backronym because the term "Jini" never stood for anything in the first place. The more recent "XNA", on the other hand, was deliberately designed that way.
 * Most recursive acronyms are recursive on the first letter, which is therefore an arbitrary choice, often selected for reasons of humour, ease of pronunciation, or consistency with an earlier acronym that used the same letters for different words, such as PHP, which now stands for "PHP: Hypertext Preprocessor", but was originally "Personal Home Page". However YOPY, "Your own personal YOPY" is recursive on the last letter.
 * A joke implying that the middle initial "B." in the name of Benoit B. Mandelbrot stands for "Benoit B. Mandelbrot" plays on the idea that fractals, which Mandelbrot studied, repeat themselves at smaller and smaller scales when examined closely.

Other

 * According to Hayyim Vital, a 16th–17th century kabbalist, the Hebrew word adam (אדם, meaning "man") is an acronym for adam, dibbur, maaseh (man, speech, deed).
 * According to Isaac Luria, a 16th-century kabbalist, the Hebrew word tzitzit (ציצת in its Biblical spelling, meaning "ritual fringes") is an acronym for tzaddik yafrid tzitziyotav tamid ("a righteous person should separate [the strings of] his tzitzit constantly").

title: Recursive acronym


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Salmon

Salmon (: salmon) is the common name for several commercially important species of euryhaline ray-finned fish from the genera Salmo and Oncorhynchus of the family Salmonidae, native to tributaries of the North Atlantic (Salmo) and North Pacific (Oncorhynchus) basins. Other closely related fish in the same family include trout, char, grayling, whitefish, lenok and taimen, all coldwater fish of the subarctic and cooler temperate regions with some sporadic endorheic populations in Central Asia.

Salmon are typically anadromous: they hatch in the shallow gravel beds of freshwater headstreams and spend their juvenile years in rivers, lakes and freshwater wetlands, migrate to the ocean as adults and live like sea fish, then return to their freshwater birthplace to reproduce. However, populations of several species are restricted to fresh waters (i.e. landlocked) throughout their lives. Folklore has it that the fish return to the exact stream where they themselves hatched to spawn, and tracking studies have shown this to be mostly true. A portion of a returning salmon run may stray and spawn in different freshwater systems; the percent of straying depends on the species of salmon. Homing behavior has been shown to depend on olfactory memory.

Salmon are important food fish and are intensively farmed in many parts of the world, with Norway being the world's largest producer of farmed salmon, followed by Chile. They are also highly prized game fish for recreational fishing, by both freshwater and saltwater anglers. Many species of salmon have since been introduced and naturalized into non-native environments such as the Great Lakes of North America, Patagonia in South America and South Island of New Zealand.

Name and etymology
The Modern English term salmon is derived from samoun, samon and saumon, which in turn are from Anglo-Norman: saumon, from saumon, and from (which in turn might have originated from salire, meaning "to leap". ). The unpronounced "l" absent from Middle English was later added as a Latinisation to make the word closer to its Latin root. The term salmon has mostly displaced its now dialectal synonym lax, in turn from lax, from leax, from lahsaz from Proto-Indo-European: *lakso-.

Species
The seven commercially important species of salmon occur in two genera of the subfamily Salmoninae. The genus Salmo contains the Atlantic salmon, found in both sides of the North Atlantic, as well as more than 40 other species commonly named as trout. The genus Oncorhynchus contains 12 recognised species which occur naturally only in the North Pacific, six of which are known as Pacific salmon while the remainder are considered trout. Outside their native habitats, Chinook salmon have been successfully introduced in New Zealand and Patagonia, while coho, sockeye and Atlantic salmon have been established in Patagonia, as well.

† Both the Salmo and Oncorhynchus genera also contain a number of trout species informally referred to as salmon. Within Salmo, the Adriatic salmon (Salmo obtusirostris) and Black Sea salmon (Salmo labrax) have both been named as salmon in English, although they fall outside the generally recognized seven salmon species. The masu salmon (Oncorhynchus masou) is actually considered a trout ("cherry trout") in Japan, with masu actually being the Japanese word for trout. On the other hand, the steelhead and sea trout, the anadromous forms of rainbow trout and brown trout respectively, are from the same genera as salmon and live identical migratory lives, but neither is termed "salmon".

The extinct Eosalmo driftwoodensis, the oldest known Salmoninae fish in the fossil record, helps scientists figure how the different species of salmon diverged from a common ancestor. The Eocene salmon's fossil from British Columbia provides evidence that the divergence between Pacific and Atlantic salmon had not yet occurred 40 million years ago. Both the fossil record and analysis of mitochondrial DNA suggest the divergence occurred 10 to 20 million years ago during the Miocene. This independent evidence from DNA analysis and the fossil record indicate that salmon divergence occurred long before the Quaternary glaciation began the cycle of glacial advance and retreat.

Non-salmon species of "salmon"
There are several other species of fish which are colloquially called "salmon" but are not true salmon. Of those listed below, the Danube salmon or huchen is a large freshwater salmonid closely related (from the same subfamily) to the seven species of salmon above, but others are fishes of unrelated orders, given the common name "salmon" simply due to similar shapes, behaviors and niches occupied:

Distribution

 * Atlantic salmon (Salmo salar) reproduce in northern rivers on both coasts of the Atlantic Ocean.
 * Landlocked Atlantic salmon (Salmo salar m. sebago) is a potamodromous (migratory only between fresh waters) subspecies/morph that live in a number of lakes in eastern North America and in Northern Europe, for instance in lakes Sebago, Onega, Ladoga, Saimaa, Vänern and Winnipesaukee. They are not a different species from the sea-run Atlantic salmon but have independently evolved a freshwater-only life cycle, which they maintain even when they could access the ocean.
 * Chinook salmon (Oncorhynchus tshawytscha) are also known in the United States as king salmon or "blackmouth salmon", and as "spring salmon" in British Columbia, Canada. Chinook salmon is the largest of all Pacific salmon, frequently exceeding 6 ft and 30 lb. The name tyee is also used in British Columbia to refer to Chinook salmon over 30 pounds and in the Columbia River watershed, especially large Chinooks were once referred to as June hogs. Chinook salmon are known to range as far north as the Mackenzie River and Kugluktuk in the central Canadian arctic, and as far south as the Central Californian Coast.
 * Chum salmon (Oncorhynchus keta) is known as dog salmon or calico salmon in some parts of the US, and as keta in the Russian Far East. This species has the widest geographic range of the Pacific species: in the eastern Pacific from north of the Mackenzie River in Canada to south of the Sacramento River in California and in the western Pacific from Lena River in Siberia to the island of Kyūshū in the Sea of Japan.
 * Coho salmon (Oncorhynchus kisutch) are also known in the US as silver salmon. This species is found throughout the coastal waters of Alaska and British Columbia and as far south as Central California (Monterey Bay). It is also now known to occur, albeit infrequently, in the Mackenzie River.
 * Masu salmon (Oncorhynchus masou), also known as "cherry trout" (桜鱒 サクラマス) in Japan, are found only in the western Pacific Ocean in Japan, Korea, and Russian Far East. A landlocked subspecies known as the Taiwanese salmon or Formosan salmon (Oncorhynchus masou formosanus) is found in central Taiwan's Chi Chia Wan Stream.
 * Pink salmon (Oncorhynchus gorbuscha), known as humpback salmon or "humpies" in southeast and southwest Alaska, are found in the western Pacific from Lena River in Siberia to Korea, found throughout northern Pacific, and in the eastern Pacific from the Mackenzie River in Canada to northern California, usually in shorter coastal streams. It is the smallest of the Pacific species, with an average weight of 3.5 to 4.0 lb.
 * Sockeye salmon (Oncorhynchus nerka) is also known as red salmon in the US (especially Alaska). This lake-rearing species is found in the eastern Pacific from Bathurst Inlet in the Canadian Arctic to Klamath River in California, and in the western Pacific from the Anadyr River in Siberia to northern Hokkaidō island in Japan. Although most adult Pacific salmon feed on small fish, shrimp, and squid, sockeye feed on plankton they filter through gill rakers. Kokanee salmon are the landlocked form of sockeye salmon.
 * Danube salmon, or huchen (Hucho hucho), are the largest permanent freshwater salmonid species.

Life cycle

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 six 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. Only 10% of all salmon eggs are estimated to survive to this stage.

The smolt body chemistry changes, allowing them to live in salt water. While a few species of salmon remain in fresh water throughout their life cycle, the majority are anadromous and migrate to the ocean for maturation: in these species, smolts spend a portion of their out-migration time in brackish water, where their body chemistry becomes accustomed to osmoregulation in the ocean. This body chemistry change is hormone-driven, causing physiological adjustments in the function of osmoregulatory organs such as the gills, which leads to large increases in their ability to secrete salt. Hormones involved in increasing salinity tolerance include insulin-like growth factor I, cortisol, and thyroid hormones, which permits the fish to endure the transition from a freshwater environment to the ocean.

The salmon spend about one to five years (depending on the species) in the open ocean, where they gradually become sexually mature. The adult salmon then return primarily to their natal streams to spawn. 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 Canada, Britain, and Ireland. Grilse may be present at spawning, and go unnoticed by large males, releasing their own sperm on the eggs.

Prior to spawning, depending on the species, salmon undergo changes. They may grow a hump, develop canine-like teeth, or develop a kype (a pronounced curvature of the jaws in male salmon). All change from the silvery blue of a fresh-run fish from the sea to a darker colour. 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 mi and climb nearly 7000 ft 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), postspawning 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 sqft. The eggs usually range from orange to red. One or more males approach the female in her redd, depositing 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 may make as many as seven redds before her supply of eggs is exhausted.

Each year, the fish experiences a period of rapid growth, often in summer, and one of slower growth, normally in winter. This results in ring formation around an earbone called the otolith (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 associated 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.

Salmon not killed by other means show greatly accelerated deterioration (phenoptosis, or "programmed aging") at the end of their lives. Their bodies rapidly deteriorate right after they spawn as a result of the release of massive amounts of corticosteroids.

Diet
Salmon are mid-level carnivores whose diet change according to their life stage. Salmon fry predominantly feed upon zooplanktons until they reach fingerling sizes, when they start to consume more aquatic invertebrates such as insect larvae, microcrustaceans and worms. As juveniles (parrs), they become more predatory and actively prey upon aquatic insects, small crustaceans, tadpoles and small bait fishes. They are also known to breach the water to attack terrestrial insects such as grasshoppers and dragonflies, as well as consuming fish eggs (even those of other salmon).

As adults, salmon behave like other mid-sized pelagic fish, eating a variety of sea creatures including smaller forage fish such as lanternfish, herrings, sand lances, mackerels and barracudina. They also eat krill, squid and polychaete worms.

Ecology

In the Pacific Northwest and Alaska, salmon are keystone species. The migration of salmon represent a massive retrograde nutrient transfer, rich in nitrogen, sulfur, carbon and phosphorus, from the ocean to the inland freshwater ecosystems. Predation by piscivorous land animals (such as ospreys, bears and otters) along the journey serve to transfer the nutrients from the water to land, and decomposition of salmon carcass benefits the forest ecosystem.

In the case of Pacific salmon, most (if not all) of the salmon that survive to reach the headwater spawning grounds will die after laying eggs and their dead bodies sink to cover the gravel beds, with the nutrients released from the biodegradation of their corpses providing a significant boost to these otherwise biomass-poor shallow streams.

Bears
Grizzly bears function as ecosystem engineers, capturing salmon and carrying them into adjacent dry land to eat the fish. There they deposit nutrient-rich urine and feces and partially eaten carcasses. Bears preparing for hibernation tend to preferentially consume the more nutrient- and energy-rich salmon roes and brain over the actual flesh, and are estimated to discard up to half the salmon they've harvested uneaten on the forest floor, in densities that can reach 4000 kg per hectare, providing as much as 24% of the total nitrogen available to the riparian woodlands. The foliage of spruce trees up to 500 m from a stream where grizzlies fish salmon have been found to contain nitrogen originating from the fished salmon.

Beavers

Beavers also function as ecosystem engineers; in the process of tree-cutting and damming, beavers alter the local ecosystems extensively. Beaver ponds can provide critical habitat for juvenile salmon.

An example of this was seen in the years following 1818 in the Columbia River Basin. In 1818, the British government made an agreement with the U.S. government to allow U.S. citizens access to the Columbia catchment (see Treaty of 1818). At the time, the Hudson's Bay Company sent word to trappers to extirpate all furbearers from the area in an effort to make the area less attractive to U.S. fur traders. In response to the elimination of beavers from large parts of the river system, salmon runs plummeted, even in the absence of many of the factors usually associated with the demise of salmon runs. Salmon recruitment can be affected by beavers' dams because dams can:
 * Slow the rate at which nutrients are flushed from the water system; nutrients provided by adult salmon dying throughout the fall and winter remain available in the spring to newly hatched juveniles
 * Provide deeper salmon pools where young salmon can avoid avian predators
 * Increase productivity through algal photosynthesis and by enhancing the conversion efficiency of the cellulose-powered detritus cycle
 * Create slow-water environments where juvenile salmon put the food they ingest into growth rather than into fighting currents
 * Increase structural complexity with many physical niches where salmon can avoid predators

Beaver dams are able to nurture salmon juveniles in estuarine tidal marshes where the salinity is less than 10 ppm. Beavers build small dams of generally less than 2 ft high in channels in the myrtle zone. These dams can be overtopped at high tide and hold water at low tide. This provides refuges for juvenile salmon so they do not have to swim into large channels where they are subject to predation by larger fish.

Lampreys
It has been discovered that rivers which have seen a decline or disappearance of anadromous lampreys, loss of the lampreys also affects the salmon in a negative way. Like salmon, anadromous lampreys stop feeding and die after spawning, and their decomposing bodies release nutrients into the stream. Also, along with species like rainbow trout and Sacramento sucker, lampreys clean the gravel in the rivers during spawning. Their larvae, called ammocoetes, are filter feeders which contribute to the health of the waters. They are also a food source for the young salmon, and being fattier and oilier, it is assumed predators prefer them over salmon offspring, taking off some of the predation pressure on smolts. Adult lampreys are also the preferred prey of seals and sea lions, which can eat 30 lampreys to every salmon, allowing more adult salmon to enter the rivers to spawn without being eaten by the marine mammals.

Parasites
According to Canadian biologist Dorothy Kieser, the myxozoan parasite Henneguya salminicola is commonly found in the flesh of salmonids. It has been recorded in the field samples of salmon returning to the Haida Gwaii 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 complex life cycles, 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 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 life cycle. 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, "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, 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".

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 C. clemensi and C. rogercresseyi, can cause deadly infestations of both farm-grown and wild salmon. Sea lice are ectoparasites which feed on mucus, 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.

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 some regions is commonly over 80%.

Effect of pile driving
The risk of injury caused by underwater pile driving has been studied by Dr. Halvorsen and her co-workers. The study concluded that the fish are at risk of injury if the cumulative sound exposure level exceeds 210 dB relative to 1 μPa2 s.

Commercial
As can be seen from the production chart at the left, the global capture reported by different countries to the FAO of commercial wild salmon has remained fairly steady since 1990 at about one million tonnes per year. This is in contrast to farmed salmon (below) which has increased in the same period from about 0.6 million tonnes to well over two million tonnes.

Nearly all captured wild salmon are Pacific salmon. The capture of wild Atlantic salmon has always been relatively small, and has declined steadily since 1990. In 2011 only 2,500 tonnes were reported. In contrast, about half of all farmed salmon are Atlantic salmon.

Recreational
Recreational salmon fishing can be a technically demanding kind of sport fishing, not necessarily intuitive for beginning fishermen. A conflict exists between commercial fishermen and recreational fishermen for the right to salmon stock resources. Commercial fishing in estuaries and coastal areas is often restricted so enough salmon can return to their natal rivers where they can spawn and be available for sport fishing. On parts of the North American West Coast salmon sport fishing has completely replaced inshore commercial salmon fishing. In most cases, the commercial value of a salmon sold as seafood can be several times less than the value attributed to the same fish caught by a sport fisherman. This is "a powerful economic argument for allocating stock resources preferentially to sport fishing".

Farms

Salmon aquaculture is a major contributor to the world production of farmed finfish, representing about US$10 billion annually. Other commonly cultured fish species include tilapia, catfish, sea bass, carp and bream. Salmon farming is significant in Chile, Norway, Scotland, Canada and the Faroe Islands; it is the source for most salmon consumed in the United States and Europe. Atlantic salmon are also, in very small volumes, farmed in Russia and Tasmania, Australia.

Salmon are carnivorous, and need to be fed meals produced from catching other wild forage fish and other marine organisms. Salmon farming leads to a high demand for wild forage fish. As a predator, salmon require large nutritional intakes of protein, and farmed salmon consume more fish than they generate as a final product. On a dry weight basis, 2–4 kg of wild-caught fish are needed to produce one kilogram of salmon. As the salmon farming industry expands, it requires more forage fish for feed, at a time when 75% of the world's monitored fisheries are already near to or have exceeded their maximum sustainable yield. The industrial-scale extraction of wild forage fish for salmon farming affects the survivability of other wild predatory fish which rely on them for food. Research is ongoing into sustainable and plant-based salmon feeds.

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

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 released into rivers in an attempt to increase the salmon population. This system is referred to as ranching. It was very common in countries such as 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, a company is limited in benefiting financially from their investment.

Because of this, the ranching method has mainly been used by various public authorities and non-profit groups, such as the Cook Inlet Aquaculture Association, as a way to increase salmon populations in situations where they have declined due to overharvesting, construction of dams and habitat destruction or fragmentation. Negative consequences to this sort of population manipulation include genetic "dilution" of the wild stocks. 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 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 that destroy natural redds also clean the regular streams. Spawning channels preserve the natural selection of natural streams, as there is no benefit, as in hatcheries, to use prophylactic chemicals to control diseases.

Farm-raised salmon are fed the carotenoids astaxanthin and canthaxanthin to match their flesh colour to wild salmon to improve their marketability. Wild salmon get these carotenoids, primarily astaxanthin, from eating shellfish and krill.

One proposed alternative to the use of wild-caught fish as feed for the salmon, is the use of soy-based products. This should be better for the local environment of the fish farm, but producing soy beans has a high environmental cost for the producing region. The fish omega-3 fatty acid content would be reduced compared to fish-fed salmon.

Another possible alternative is a yeast-based coproduct of bioethanol production, proteinaceous fermentation biomass. Substituting such products for engineered feed can result in equal (sometimes enhanced) growth in fish. With its increasing availability, this would address the problems of rising costs for buying hatchery fish feed.

Yet another attractive alternative is the increased use of seaweed. Seaweed provides essential minerals and vitamins for growing organisms. It offers the advantage of providing natural amounts of dietary fiber and having a lower glycemic load than grain-based fish meal. In the best-case scenario, widespread use of seaweed could yield a future in aquaculture that eliminates the need for land, freshwater, or fertilizer to raise fish.

Management

Salmon population levels are of concern in the Atlantic and in some parts of the Pacific. 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 wild salmon in the Snake and Columbia River systems in northwestern United States.

Alaska fishery stocks are still abundant, and catches have been on the rise in recent decades, after the state initiated limitations in 1972. Some of the most important Alaskan salmon sustainable wild fisheries are located near the Kenai River, Copper River, and in Bristol Bay. 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.

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. Of 435 wild stocks of salmon and steelhead, only 187 of them were classified as healthy; 113 had an unknown status, one was extinct, 12 were in critical condition and 122 were experiencing depressed populations.

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.

Salmon populations have been established in all the Great Lakes. Coho stocks were planted by the state of Michigan in the late 1960s to control the growing population of non-native alewife. Now Chinook (king), Atlantic, and coho (silver) salmon are annually stocked in all Great Lakes by most bordering states and provinces. These populations are not self-sustaining and do not provide much in the way of a commercial fishery, but have led to the development of a thriving sport fishery.

Wild, self sustaining Pacific salmon populations have been established in New Zealand, Chile, and Argentina. They are highly prized by sport fishers, but others worry about displacing native fish species. Also, and especially in Chile (Aquaculture in Chile), both Atlantic and Pacific salmon are used in net pen farming.

In 2020 researchers reported widespread declines in the sizes of four species of wild Pacific salmon: Chinook, chum, coho, and sockeye. These declines have been occurring for 30 years, and are thought to be associated with climate change and competition with growing numbers of pink and hatchery salmon.

As food
Salmon is a popular food fish. Classified as an oily fish, salmon is considered to be healthy due to the fish's high protein, high omega-3 fatty acids, and high vitamin D content. Salmon is also a source of cholesterol, with a range of 23–214 mg/100 g depending on the species. According to reports in the journal Science, farmed salmon may contain high levels of dioxins. PCB (polychlorinated biphenyl) levels may be up to eight times higher in farmed salmon than in wild salmon, but still well below levels considered dangerous. Nonetheless, according to a 2006 study published in the Journal of the American Medical Association, the benefits of eating even farmed salmon still outweigh any risks imposed by contaminants. Farmed salmon has a high omega-3 fatty acid content comparable to wild salmon. The type of omega-3 present may not be a factor for other important health functions.

Salmon flesh is generally orange to red, although white-fleshed wild salmon with white-black skin colour occurs. The natural colour of salmon results from carotenoid pigments, largely astaxanthin, but also canthaxanthin, in the flesh. Wild salmon get these carotenoids from eating krill and other tiny shellfish.

The vast majority of Atlantic salmon available in market around the world are farmed (almost 99%), whereas the majority of Pacific salmon are wild-caught (greater than 80%). Canned salmon in the U.S. is usually wild Pacific catch, though some farmed salmon is available in canned form. Smoked salmon is another popular preparation method, and can either be hot or cold smoked. Lox can refer to either cold-smoked salmon or salmon cured in a brine solution (also called gravlax). Traditional canned salmon includes some skin (which is harmless) and bone (which adds calcium). Skinless and boneless canned salmon is also available.

Raw salmon flesh may contain Anisakis nematodes, marine parasites that cause anisakiasis. Before the availability of refrigeration, the Japanese did not consume raw salmon. Salmon and salmon roe have only recently come into use in making sashimi (raw fish) and sushi.

To the Indigenous peoples of the Pacific Northwest Coast, salmon is considered a vital part of the diet. Specifically, the indigenous peoples of Haida Gwaii, located near former Queen Charlotte Island in British Columbia, rely on salmon as one of their main sources of food, although many other bands have fished Pacific waters for centuries. Salmon are not only ancient and unique, but it is important because it is expressed in culture, art forms, and ceremonial feasts. Annually, salmon spawn in Haida, feeding on everything on the way upstream and down. Within the Haida nation, salmon is referred to as "tsiin", and is prepared in several ways including smoking, baking, frying, and making soup.

Historically, there has always been enough salmon, as traditional subsistence fishing methods did not result in overfishing, and people only took what they needed. In 2003, a report on First Nations participation in commercial fisheries, including salmon, commissioned by BC's Ministry of Agriculture and Food found that there were 595 First Nation-owned and operated commercial vessels in the province. Of those vessels, First Nations' members owned 564. However, employment within the industry has decreased overall by 50% in the last decade, with 8,142 registered commercial fishermen in 2003. This has affected employment for many fisherman, who rely on salmon as a source of income.

Black bears also rely on salmon as food. The leftovers the bears leave behind are considered important nutrients for the Canadian forest, such as the soil, trees and plants. In this sense, the salmon feed the forest and in return receive clean water and gravel in which to hatch and grow, sheltered from extremes of temperature and water flow in times of high and low rainfall. However, the condition of the salmon in Haida has been affected in recent decades. Due to logging and development, much of the salmon's habitat (i.e., Ain River) has been destroyed, resulting in the fish being close to endangered. For residents, this has resulted in limits on catches, in turn, has affected families diets, and cultural events such as feasts. Some of the salmon systems in danger include: the Davidon, Naden, Mamim, and Mathers. It is clear that further protection is needed for salmon, such as their habitats, where logging commonly occurs.

History
The salmon has long been at the heart of the culture and livelihood of coastal dwellers, which can be traced as far back as 5,000 years when archeologists discovered Nisqually tribe remnants. The original distribution of the genus Oncorhynchus covered the Pacific Rim coastline. History shows salmon used tributaries, rivers and estuaries without regard to jurisdiction for 18–22 million years. Baseline data is near impossible to recreate based on the inconsistent historical data, but there has been massive depletion since the 1900s. The Pacific Northwest once sprawled with native inhabitants who ensured little degradation was caused by their actions to salmon habitats. As animists, the indigenous people relied not only for salmon for food, but spiritual guidance. The role of the salmon spirit guided the people to respect ecological systems such as the rivers and tributaries the salmon used for spawning. Natives often used the entire fish and left little waste by turning the bladder into glue, and using bones for toys and skin for clothing and shoes. The original salmon ceremony, introduced by indigenous tribes on the Pacific coast, consisted of three major parts. First was the welcoming of the first catch, and then the cooking of it. Finally the bones were returned to the sea to induce hospitality so other salmon would give their lives to the people of that village.

Many tribes, such as the Yurok, had a taboo against harvesting the first fish that swam upriver in summer, but once they confirmed that the salmon run had returned in abundance they would begin to catch them in plentiful. The indigenous practices were guided by deep ecological wisdom, which was eradicated when Euro-American settlements began to be developed. Salmon have a much grander history than what is presently shown today. The salmon that once dominated the Pacific Ocean are now just a fraction in population and size. The Pacific salmon population is now less than 1–3% of what it was when Lewis and Clark arrived at the region. In his 1908 State of the Union address, U.S. President Theodore Roosevelt observed that the fisheries were in significant decline:

"The salmon fisheries of the Columbia River are now but a fraction of what they were twenty-five years ago, and what they would be now if the United States Government had taken complete charge of them by intervening between Oregon and Washington. During these twenty-five years the fishermen of each State have naturally tried to take all they could get, and the two legislatures have never been able to agree on joint action of any kind adequate in degree for the protection of the fisheries. At the moment the fishing on the Oregon side is practically closed, while there is no limit on the Washington side of any kind, and no one can tell what the courts will decide as to the very statutes under which this action and non-action result. Meanwhile very few salmon reach the spawning grounds, and probably four years hence the fisheries will amount to nothing; and this comes from a struggle between the associated, or gill-net, fishermen on the one hand, and the owners of the fishing wheels up the river."

On the Columbia River, the Chief Joseph Dam completed in 1955 completely blocks salmon migration to the upper Columbia River system.

The Fraser River salmon population was affected by the 1914 slide caused by the Canadian Pacific Railway at Hells Gate. The 1917 catch was one quarter of the 1913 catch.

The origin of the word for "salmon" was one of the arguments about the location of the origin of the Indo-European languages.

Mythology
The salmon is an important creature in several strands of Celtic mythology and poetry, which often associated them with wisdom and venerability. In Irish folklore, fishermen associated salmon with fairies and thought it was unlucky to refer to them by name. In Irish mythology, a creature called the Salmon of Knowledge plays key role in the tale The Boyhood Deeds of Fionn. In the tale, the Salmon will grant powers of knowledge to whoever eats it, and is sought by poet Finn Eces for seven years. Finally Finn Eces catches the fish and gives it to his young pupil, Fionn mac Cumhaill, to prepare it for him. However, Fionn burns his thumb on the salmon's juices, and he instinctively puts it in his mouth. In so doing, he inadvertently gains the Salmon's wisdom. Elsewhere in Irish mythology, the salmon is also one of the incarnations of both Tuan mac Cairill and Fintan mac Bóchra.

Salmon also feature in Welsh mythology. In the prose tale Culhwch and Olwen, the Salmon of Llyn Llyw is the oldest animal in Britain, and the only creature who knows the location of Mabon ap Modron. After speaking to a string of other ancient animals who do not know his whereabouts, King Arthur's men Cai and Bedwyr are led to the Salmon of Llyn Llyw, who lets them ride its back to the walls of Mabon's prison in Gloucester.

In Norse mythology, after Loki tricked the blind god Höðr into killing his brother Baldr, Loki jumped into a river and transformed himself into a salmon to escape punishment from the other gods. When they held out a net to trap him he attempted to leap over it but was caught by Thor who grabbed him by the tail with his hand, and this is why the salmon's tail is tapered.

Salmon are central spiritually and culturally to Native American mythology on the Pacific coast, from the Haida and Coast Salish peoples, to the Nuu-chah-nulth peoples in British Columbia.

title: Salmon