Atlantic salmon (Salmon salar)
Also known as: bay salmon, black salmon, caplin-scull salmon, fiddler, sebago salmon, silver salmon, outside salmon and winnish.
Morphology & Biology
Atlantic salmon are a species of ray-finned fish. They are the largest species within their genus, salmo, and are sometimes called the King of Fish. Young Atlantic salmon have blue and red spots, while the adults have a silvery-blue sheen. They have a fusiform (torpedo-shaped) body, with black-bordered fins. After two years at sea, salmon reach an average of 71–76 cm, and 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, where the young fish spend the first 1-4 years of their lives. Once the fish reach a certain size, they undergo smoltification: a series of changes which transform the fish so that it is adapted to live in the sea, rather than the river. Before smolting, the fish has a relatively drab coloration with large bands and spots, which provides camouflage in the river environment. During smolting, these marks fade, and the the skin undergoes extensive silvering so that it is much more reflective. The body shape lengthens and becomes more streamlined. They also experience hormonal changes, which allow them to tolerate the higher salinity of seawater. After smoltification, the fish change their behaviour, swimming with the current, rather than against it, and so swim down the river to reach the sea.
When the Atlantic salmon reaches sexual maturity 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. Unlike many of the Pacific salmon species, the Atlantic salmon often returns to the ocean after spawning to regain its strength and is able to spawn several times. The Atlantic salmon are therefore able to grow to a much larger size than other salmon species.
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.
Unfortunately, wild salmon disappeared from many rivers during the twentieth century due to fishing, dams and pollution. Numbers dropped to critically low levels by the year 2000. Within the Baltic region, Atlantic salmon populations have declined rapidly, with annual production of fish larvae decreasing by 95% in the past century. Despite this, the IUCN classifies Atlantic salmon as being of ‘least concern’.
The primary threat to wild Atlantic salmon is from the fishing and aquaculture industry, in part due to the direct removal of large numbers of wild fish, but also through pollutants and parasites discharged into the ocean through the aquaculture of 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 wiped out soon. Construction of hydroelectric dams is a serious problem, restricting the salmon’s migration and ultimately blocking their access to historical spawning grounds. Finally, disease is another considerable factor limiting wild salmon populations, often environmentally related.
Fisheries & Aquaculture
The offshore fisheries in the Baltic region rely on the use of drift nets and longlines. 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 2,319 tons, while 2.3 million tons were produced in aquacultures. After extensive habitat damage and overfishing of 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, such as pathogen and parasite transfers to the wild populations that live near commercial facilities, and interbreeding between wild and farmed individuals. In the last few years an increasing number of commercial salmon farms have 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.
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 that killed thousands of marine animals.
As males enter the spawning season, many salmonoids develop a ‘kype’. This is a hooked structure that grows from the jaw. The kype influences the hierarchy of spawning groups, determining who is most dominant: the bigger the kype, the more dominant the salmon. The size of the kype is believed to determine male spawning frequency.