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Morphology & Biology

Atlantic salmon, also known as the King of Fish, are a species of ray-finned fish, the largest species within their genus. Young Atlantic salmon have blue and red spots, while the adults have a more silver-blue sheen. They have a fusiform body and all their fins are bordered with black. After two years at sea, salmon can reach an average of 71 - 76 cm and between 3.6 - 5.4 kg in weight. Large Atlantic salmon can grow up to 150 cm in length and reach a maximum weight of 30 kg.

Atlantic salmon begin their life in rivers, with the young fish spending 1-4 years in their natal river. When large enough, they undergo smoltification, changing their colouring from large, grey spots to shiny sides. During this transition, they also experience some endrocrinological changes to adapt to the osmotic differences between freshwater and seawater. After this smoltification, the young fish swim with the current rather than against it. Once they have reached the sea, the smolt follow ocean surface currents, feeding on plankton or fry from other fish species, such as herring.

LEFT Juvenile salmon with blue and red dots (Roger Tabor, USFWS) / RIGHT Adult Salmon with black dots (Maritime Aquarium Norwalk)

When the Atlantic Salmon reaches sexual it returns from the ocean to the river where it was born, and even to its specific natal site. The female salmon selects the spawning site very carefully, because the gravel of the river bed has to have a specific size and the water depth needs to be just right. She will then dig a small hole, where her eggs will sink into and be covered by a layer of gravel. A female salmon releases between 8,000 - 25,000 eggs during a spawning season. Most males die after spawning, while 10 - 40% of the females survive and return to the sea.

Distribution

Water temperature is the main driver of the salmon’s distribution. There are three groups of Atlantic salmon: North American, European, and Baltic. They spawn in the coastal rivers of northeastern North America, Iceland, Europe, and northwestern Russia. European and North American populations of Atlantic salmon intermix while living in the ocean, where they share feeding grounds off Greenland during summer.

Threats

Unfortunately, wild salmon have disappeared from many rivers during the twentieth century due to overfishing, dams and pollution. In fact, numbers dropped to critically low levels by the year 2000. Within the Baltic region, Atlantic salmon populations have declined rapidly, with annual production of smolt decreasing by 95% in the past century. 

The primary threat to wild Atlantic salmon is the overexploitation by offshore fisheries, in part due to the direct overfishing, but also through harvesting farmed salmon. Since their distribution is dependent on water temperature, climate change is also a threat to Atlantic salmon. Some of the southern populations in warm countries like Spain are growing smaller and are expected to be extirpated soon. Construction of hydroelectric dams is also a severe problem, restricting the salmon’s migration. Finally, disease is another considerable factor limiting wild salmon populations, often environmentally related.

Chief Joseph Dam on the Columbia River in Washington is a hydroelectric pose plant (U.S. Army Corps of Engineers)

Fisheries & Aquaculture

The offshore fisheries in the Baltic region rely on the use of drift nets and longlines. But in coastal areas, the most commonly used fishing gear is trap nets. In rivers, seine nets and sport fishing are the major methods used.

In 2014 the FAO reported a global capture of Atlantic salmon of only 2319 tons, while 2.3 million tons were produced in aquacultures. After the extensive habitat damage and overfishing of the wild stocks, wild fish make up only 0.5% of the Atlantic salmon available in world fish markets. The rest are farmed, mainly from aquacultures in Chile, Canada, Norway, Russia and the UK.

The commercial aquaculture of Atlantic salmon is well-known for environmental scandals, like interbreeding between wild and farmed individuals as well as pathogen and parasite transfers to the wild populations that live near commercial facilities. In the last years more and more commercial salmon farms reported massive outbreaks of sea lice, a small parasite, which lives by attaching itself to a fish and feeding on its blood and skin. As of 2017, nearly half of Scotland’s salmon farms are infested with salmon lice.

LEFT Salmon farming in Norway (Gerd Meissner) / RIGHT Sea lice on farmed Atlantic salmon, Canada (Wikipedia)

Chile is the world’s second-largest producer of salmon, more than 80% of which is for export. In 2016 Greenpeace uncovered an environmental disaster in Chiloé, where industrial salmon farmers dumped more than 9,000 tonnes of rotting salmon into the ocean, causing a red tide killing thousands of marine animals.


More information on Atlantic Salmon

World Register of Marine Species

IUCN

Fishbase

NOAA Fisheries

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