Atlantic salmon (Salmo salar)

KingdomAnimalia
PhylumChordata
ClassActinopterygii
OrderSalmoniformes
FamilySalmonidae
GenusSalmo
SizeLength: males 150 cm, females 120 cm
Weightup to 39 kg

The Atlantic salmon is classified as Least Concern (LC) on the IUCN Red List (1), and is listed on Appendix III of the Berne Convention. Freshwater populations are listed on Annex II of the EC Habitats Directive and Schedule 3 of the Conservation Regulations (1994).

The Atlantic salmon (Salmo salar), 'the leaper', has been called the king of fish, due primarily to its spectacular ability to clear seemingly insurmountable obstacles. The large body is long and hydrodynamic, measuring up to 150 centimetres in length and weighing up to 39 kilograms. Adult Atlantic salmon are usually silvery-grey with some black spotting, but become reddish with purple spots in the breeding season, when males also develop a hooked lower jaw for fighting.

Historically the Atlantic salmon could be found throughout the North Atlantic, in a range spreading from Quebec to Connecticut in the west and from the Arctic Circle to Portugal in the east. In the last 30 years however, the Atlantic salmon population has suffered a devastating decline with catches falling by more than 80 percent. Today many populations are teetering on the brink of extinction or have already been lost, and it is believed that Atlantic salmon numbers are only reasonably healthy in four countries: Norway, Ireland, Iceland and Scotland. Across the UK, populations in nearly 50 percent of salmon rivers are at risk and over 30 percent are endangered.

Outside the breeding season adult Atlantic salmon are found in the open ocean at depths between two and ten metres. They return to freshwater to spawn in the streams in which they themselves were born, often only returning to pristine river systems. This makes the Atlantic salmon a useful indicator species of a river's quality.

Until the early 19th century the life cycle of the Atlantic salmon was not understood and juvenile stages were thought to be different species. Females dig a depression in the gravel into which eggs and sperm are released simultaneously. The first juvenile stage (alevins) hatch and remain in the gravel, feeding on yolk sacs that are attached to the body. The next stage (fry) feed on microscopic particles in the stream. When vertical markings appear on the young fish's body the juveniles are known as parr. This stage remains in the river for two to six years before they transform into 'smolt'. A silvery colouration develops and complex internal changes occur to allow survival in salt water.

Adult Atlantic salmon spend most of their lives at sea where they roam vast distances in small groups in search of food. At sea their diet consists of squid, shrimp and small fish such as herring or cod. After one or more years the salmon return to their birthplace in order to spawn, and do not eat during this phase of their life cycle. It appears that an olfactory sense (sense of smell) enables the salmon to identify their exact natal location and they are able to leap vertical distances of up to an amazing 12 feet in their endeavour to return there.

The Atlantic salmon has shown a steady decline over the last two centuries, seemingly related to increased industrial development throughout their traditional home range. The situation has become drastically worse since the 1970s and catches of wild salmon have fallen by 80 percent. River pollution caused by industrialisation can severely damage local populations as can the increased number of man-made obstacles, such as dams, weirs or the alteration of watercourses, which makes migration impossible.

Salmon has become an extremely popular dish in the western world and commercial farming can affect wild populations in a number of ways; escaped salmon may erode the gene pool through interbreeding, or farms may act as foci for the spread of parasites and diseases to wild stocks.

A further identified threat to the Atlantic salmon is global climate change. As the species’ developmental rate is directly related to water temperature, it is possible that increasing temperatures could result in more rapidly developing juveniles entering the ocean before their planktonic food source has reached sufficiently high levels.

Increasing freshwater temperatures could also create a thermal barrier to migrating salmon, as hotter waters require additional energy to navigate around. Such barriers can also delay or even prevent spawning. In addition, as air temperatures warm, much of the snow that feeds the river systems is expected to melt earlier. This will lead to a reduction in the flow of many rivers in the spring and summer, which will increase water temperatures further and is likely to reduce the overall habitat available to the Atlantic salmon.

In the Atlantic salmon’s marine habitat, climate change may disrupt many food webs. For example, the timing of the planktonic blooms required by the young salmon is governed by climatic factors. Changes in the timing of these blooms could cause a scarcity of food at a critical stage of the salmon’s life cycle.

The North Atlantic Salmon Conservation Organisation was established in 1983 under the Convention for the Conservation of Salmon in the North Atlantic. It is an international organisation that aims to conserve and promote the rational management of salmon stocks in the wild. The organisation includes all countries in which the Atlantic salmon is historically found and many different measures have been taken to reduce exploitation and protect the salmon. However, numbers of salmon are not recovering and further research is being carried out into why this is the case.

For further information on the Atlantic salmon: 

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
arkive@wildscreen.org.uk

  1.  IUCN Red List (February, 2011)
    http://www.iucnredlist.org/