- Name: Atlantic salmon
- Latin: Salmo salar
- Classification: Ray-finned fish
- Origin: North Atlantic ocean
- Lifespan: 4-6 years
- AKA: Black salmon
Taxonomy
- Kingdom: Animalia (Animals)
- Phylum: Chordata (Vertebrates)
- Class: Actinopterygii (Ray-finned fish)
- Order: Salmoniformes (Salmonids)
- Family: Salmonidae (Salmonids)
- Genus: Salmo (Salmon)
- Species: Salmo salar (Atlantic salmon)
Appearance
- Length: 28-30in (70-75cm)
- Weight: 8-12lbs (3.5-5.5kg)
The Atlantic salmon is a large fish with a small head and eyes and a blunt nose. Its mouth reaches back below the eye and is filled with short, conical teeth. Its caudal fins are slightly forked and has an adipose fin on its back, present in all species of trout. It is distinguished from its Pacific cousins by its anal fin, which has a maximum of 12 rays. It also has a narrow ‘wrist’ on the tail, which differentiates it from sea trout.
The Atlantic salmon has large scales and during feeding periods, the flanks are silver and back dark green-blue and black spots are flecked above the lateral line. During spawning, adults become darker in colour, with their flanks becoming red or bronze with reddish spots. Females can appear to be almost black. During spawning, males develop a hooked jaw (called a ‘kype’) used for fighting other males for fertilization rights.
Relatives
- Chinook salmon (Oncorhynchus tshawytscha) -NOT EVALUATED-
- Sockeye salmon (Oncorhynchus nerka) -LEAST CONCERN-
- Pink salmon (Oncorhynchus gorbuscha) -LEAST CONCERN-
- Coho salmon (Oncorhynchus kisutch) -NOT EVALUATED-
- Chum salmon (Oncorhynchus keta) -NOT EVALUATED-
Habitat & Distribution
Atlantic salmon alternate between freshwater and ocean habitats. They are born in coastal rivers in northeastern North America, Iceland and Europe where they spend the first 2-3 years of their life. Here, they prefer to live near gravel and boulders where the water is clean and well oxygenated or near natural grassy banks and overhanging deciduous trees. Insects (which young salmon feed on) will occasionally fall into the rivers and estuaries and the trees provide shade and some cover from predators. While in these rivers, salmon will undergo a physiological change called ‘smoltification’ which prepares them for life in the ocean. Their bodies become more streamlined and develop a silver colouring. It is also thought that they imprint on the rivers they will return to later in life to spawn.
Atlantic salmon will then migrate through the North Atlantic Ocean and share feeding grounds off the coast of Greenland to feed, as the ocean has a much better supply of nutritious food than their river birthing grounds. They will stay in the ocean for a further 2-3 years before return to their rivers to spawn and reproduce.
Diet
Soon after the yolk sac is absorbed, young salmon begin to hunt and feed off very small organisms and occasionally, salmon eggs. The most common include caddisflies, blackflies, mayflies, stoneflies and plankton and will hunt in both the substrate and the current.
After the smoltification process, the salmon grow larger, move downstream into the ocean and feed on much larger prey, including Arctic squid, sand eels, amphipods, Arctic shrimp and sometimes herring.
Reproduction
After spending 2-3 years in the ocean, Atlantic salmon will migrate as much as thousands of miles back to their natal coastal rivers to spawn during the summer and autumn. They will swim against river currents and have to leap over several waterfalls, only the fittest will reach the spawning grounds and reproduce. They stop feeding as they enter freshwater and rely on their fat reserves for energy. Their stomach disintegrates internally to make room for eggs and sperm.
Males will arrive first and stake out territories. During this time, the mouths of the males change and become more hooked and pincer-like, called a ‘kype’. This displays the male’s dominant characteristics to females and is used in fighting other males for females. Territories with shallow, gravel beds are preferred with the right amount of river flow. Enough to oxygenate the eggs but not so strong that they wash eggs away.
When females arrive, they initiate spawning by digging a shallow nest in the gravel, known as a ‘redd’ and is created by the female turning into one side and fanning her tail. The female then signals to a male that she is ready to lay by touching her anal fin to the gravel. The male joins the female, and side by side, they release their gametes (eggs and sperm) above the redd.
The female released 2,000-10,000 small, orange eggs about the size of a small pea. The male releases a while milt, which contains millions of sperm over the eggs, and fertilization occurs externally. The pair then moves upstream and repeat the process until they are out of their energy or gametes. The males may spawn with several females, however, females will typically only spawn with a single male. The entire reproduction process takes a day or two once the pair reach the spawning grounds.
Fertilized eggs hatch within 3-4 months and emerge in the form on an alevin, where they still have the yolk sac attached to their bodies near the throat, which lasts about a month before the young begin to hunt for themselves. After 5-10 weeks they are known as a ‘fry’ and after several months, when they have developed their ‘tiger-like’ markings, they are called a ‘parr’.
Adaptations
- The lateral line of a fish, runs along the length of its body and acts as a ‘sixth sense’. It allows the fish to detect which way is up in water. Atlantic salmon also use this line to detect the flow of the current around them, aiding them in their migration to their natal rivers.
- Atlantic salmon have four sets of gills with specialized cells that allow them to migrate between salt and fresh water, and are efficient in the dissolving of oxygen. When in saltwater, salt is taken from the water flowing over the gills and expelled via a gland. This process is called ‘osmoregulation’.
- The sense of smell of the Atlantic salmon is approximately 1,000 times greater than that of a dog. It is thought this superior sense of smell is used to aid the fish with navigation during their migration. In an experiment carried out in Canada. A salmon was able to detect a single drop of a substance (prostaglandin-A) in the equivalent of eight Olympic sized swimming pools of water.
Threats
Atlantic salmon survival can be affected immensely by river pollution. They require clean and well oxygenated water in order to spawn and for smolts to grow. Pollution can be caused by chemicals, oil and rubbish as well as acid rain and agricultural run-off. Large numbers of cattle walking through the river can also disturb salmon by kicking up mud, causing eggs to become destroyed or by making it near impossible for females to create their redds. Erosion can also cause river banks to collapse into the water which can be caused by overgrazing and deforestation. Run-off can also cause an increase in nutrients which results in ‘algal blooms’. The increased vegetation eventually die off and cause an increase in bacteria, which uses up the valuable oxygen the salmon need. Dams and other water obstructions can also prevent salmon from migrating back to their spawning grounds, resulting in a decrease of young.
Atlantic salmon are also affected by alien species, most notably the American signal crayfish (Pacifastacus leniusculus) which were introduced to Scotland in the mid-1970’s for aquaculture. This crayfish burrows into riverbanks, which weakens them and makes them more likely to collapse. It also feeds on insects, fish eggs and fry (including salmon).
Salmon can also be affected by disease such as the ulcerative dermal necrosis (UDN) which was caught by most salmon in Scotland in the 70’s and nearly wiped out the entire population. Parasites such as Gyrodactylus salaris and Henneguya salminicola can attach to salmon using their hooked feet and can affect the gills, fins and skin and occurs in over 20 Norwegian rivers. Climate change can also affect salmon survival by increasing water temperature. Smolts struggle to survive in higher temperatures and can also affect growth rates and prey availability.
Atlantic salmon are commonly farmed in many rivers throughout Norway and the west coasts of Ireland and Scotland. The high concentrations of fish can cause sea lice to multiply drastically which can effect wild fish as well as those in fish farms. Escapees from these farms can also interbreed with wild fish which reduces the fitness of future generations. These farms can also increase water pollution due to a build-up of uneaten food, feces and medication. This can also lead to a rapid spread of infection among all fish.
Conservation
IUCN Status: Least Concern
There are several non-profit organizations which work to actively protect or provide funding for the protection and conservation of the Atlantic salmon. One of which is the North Atlantic Salmon Conservation Organization (NASCO), which soon after it was established, created a fishing free sanctuary by prohibiting fishing in most parts of the North Atlantic 12 nautical miles from the coast. They have also developed Precautionary Approach agreements in relation to:
- “Management of fisheries,
- Habitat protection and restoration,
- Impacts of aquaculture, introductions and transfers of transgenics,
- Stock rebuilding programs,
- Use of socio economic factors in management decisions.”
The Atlantic Salmon Conservation Foundation also works to “promote enhances community partnerships of salmon and its habitat in Atlantic Canada and Quebec”. It operates as a source of funding for the conservation of the Atlantic salmon and its habitat and to improve public awareness, education and research of the animal and its conservation efforts.
Atlantic salmon can be actively supported by way of ‘supportive breeding’, which includes capturing wild individuals, breeding them in captivity and then release. Fish ladders can also be installed in order to aid the migration of the salmon around man-made obstacles. The salmon leap over each step in order move over the obstacle. Adding limestone to rivers can also neutralized acidity levels of the water. This process is known as ‘liming’. River banks can also be stabilized against erosion by reforesting the area (without conifers, as the needles are acidic and can cause the acidity level of the water to rise by dropping into the river).
Fun Facts
- Male: Cock
- Female: Hen
- Young: Alevin / fry / parr
- Group: run / shoal / bind / draught
- When Atlantic salmon return to their natal rivers, they lose all their teeth and grow new ones.
- Atlantic salmon will swim up to 2,500 miles to reach their feeding grounds.
- Atlantic salmon are one of only 87 species of fish to migrate between salt and freshwater.
- Mature females can carry up to 22% of their body weight in eggs.
- A female can lay up to 1,600 eggs per kilogram of her body weight.
Cherry Maureen Iris Franklin, Bsc (Hons)
1 May 2016
References
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Nature Table 