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Atlantic salmon

Introduction

The Atlantic salmon, Salmo salar, is the only salmon that lives in the Atlantic Ocean. These fish are known for the good fight they put up when hooked by an angler and for their delicious pink flesh.

Atlantic salmon are native to Canadian waters. In eastern Canada, salmon occur naturally in Newfoundland, Labrador, the Maritimes and Quebec. There are also landlocked salmon populations that live permanently in lakes. In French, they are known by the Montagnais name, ouananiche, which means “the little lost one.” In Canada, the ouananiche is found throughout the regions occupied by sea-run salmon.

- Taxonomy -

Family
Salmonidae
Genus
Salmo
Species
Salmo salar Linnaeus, 1758
Common name
Atlantic salmon

- Physiology -

The Atlantic salmon has a fusiform body shape similar to that of other salmonids. The body is elongated with slightly compressed flanks. In breeding males, the lower jaw ends in a hook called a “kype.” The skin of Atlantic salmon varies in colour depending on the age of the fish and the environment where they are found:

  • At sea, the back is brown, green or blue, while the flanks and abdomen are silvery white.
  • When in the river, the adults turn darker, losing their silvery sheen.
  • During spawning, adults turn bronze and sometimes reddish spots appear on the body and head.
  • After spawning, adults that overwinter in the river are quite dark and are called “black salmon” or “kelt.”
  • Completely black salmon are very rare. When found, the fish are blind.

- Life cycle -

Taking into account natural mortality and predation, both in the river and at sea, only 4 adult salmon out of the 8,000 eggs initially laid by a female will return to their river after a stay in the ocean, for a 0.05% survival rat:

  • 8,000 eggs, 5 mm diameter (about ¼ inch)
  • 500 alevins, 2 to 3 cm long (about 1 inch) or fry, 3 to 9 cm long (1 to 3½ inches)
  • 300 parr (first year) and 120 parr (second year), 9 to 15 cm long (3½ to 6 inches)
  • 50 smolts, 15 to 20 cm long (6 to 8 inches)
  • 4 spawners that return to spawn

The salmon’s life cycle can be broken down into three phases, as shown in the figure below: reproduction, growth in the river and growth at sea.

Schéma du cycle de vie du saumon atlantique (Salmo salar).

 

 

  • Reproduction

    • Returning to the river

      After spending one to three years at sea, salmon return to their home or “natal” river. They are silvery in colour at this point. Once they reach the mouth of the river, they stop feeding and swim upstream to the same site where they hatched.

    • Selecting the spawning grounds

      Around mid-October, the salmon prepare to leave the river’s pools and move to spawning grounds upstream, seeking areas with shallow water, a swift current and a gravelly bed. When the water temperature drops to about 6 °C, the salmon begin exploring the spawning grounds.

    • Forming pairs

      Mating pairs form gradually. Males are distinguished from females by their “kype,” a pronounced upward hook of the extended lower jaw. While the females are locating favourable spawning spots, the males will attempt to establish a territory around each female. Only the most aggressive males manage to defend their territory against the tireless attempts of other males to approach the females despite continuously being chased away by the dominant males. Violent confrontations between males are sometimes observed.

    • Preparing the redd

      At the spot she chooses, the female will dig a “redd” – a depression in the gravel about 10 to 20 centimetres (4 to 8 inches) deep that serves as a nest. With a quick, strong fanning movement of her tail, she tosses the gravel up. The current washes the gravel disturbed by the females, so groups of redds in spawning grounds can be easily recognized by the pale yellow colour of this washed gravel.

    • Spawning

      When spawning, the male and female are positioned side by side. The male’s whole body shakes or quivers as he stretches and gapes. Thus stimulated, the female does the same. When the quivering subsides, the female lays her eggs and the male fertilizes them by releasing his milt (sperm and fluids). Very often, other “satellite” males and young “precocious” parr that have not yet been to sea sneak in to participate in reproduction.

    • Eggs protecting

      After depositing her eggs, the female hurries to cover them with gravel before other fish species such as brook trout can rush in to devour them, as often happens. The female will repeat the ritual of digging a redd and then depositing and covering the eggs until all her eggs are laid.

      Female salmon lay about 1,800 eggs per kilogram of body weight (800 eggs per pound), so a 4.5-kilogram female (10 pound) will deposit around 8,000 eggs in several redds.

    • Returning to the sea

      After spawning, Atlantic salmon can return downstream to the sea, and some will come back to the river once or twice more to spawn in subsequent years.

  • Growth in fresh water

    • Egg development

      The eggs laid in the fall will develop under the gravel all winter, oxygenated by the water constantly flowing through the gravel bed. However, in addition to predators, many dangers await the eggs: fungi; fine sediments filling the interstices between the gravel; freezing of the eggs; and scraping of the riverbed by ice.

    • Hatching

      The eggs hatch in April, releasing tiny transparent fry with their yolk sacs still attached, called “yolk-sac fry” or “alevins.” The alevins burrow a little deeper into the gravel, which keeps them safe from being carried away by ice scraping the bottom of the riverbed during the spring break-up. They stay buried for about five to six weeks while being nourished by their attached yolk sac. By the end of May or the beginning of June, the fry emerge from the gravel and begin to feed on small insect larvae. They frequent shallow areas of the river where the current is weak. At this stage, many fry will become victims of predation.

    • The first year of growth

      At the end of their first summer, the fry are about 5 centimetres long and are now called “parr.” Needing more food, they spend more time in rapids and whitewater habitats where insect larvae are carried by the strong currents. Parr are well adapted to this environment, with well-developed fins that allow them to stay at the bottom of the river, waiting for prey. They become progressively territorial as the season advances. Those that do not fall victim to predators like Kingfishers will spend the winter buried under large pebbles and stones.

       

       

       

       

       

       

       

       

       

       

       

       

       

       

       

       

       

       

       

    • The subsequent years of growth

      After two or three years in the river, the parr reach about 15 centimetres in length and are ready to go to sea. At this stage, they are called smolts. This is the phase during which young salmon, while still in fresh water, undergo the process of physiological adaptation to life in saltwater called “smolting.” They also memorize the scent of their river at this stage, and develop the silvery colouring that is almost identical to that of adults.

  • Growth at sea

    • Downstream migration

      After a spring freshet (high meltwater), the smolts descend the river and begin their migration to the sea. They form large schools that move through the estuaries and into the open ocean, which helps reduce predation. The grilse that return to the river after only one year at sea are almost invariably male.

    • Migration at sea

      After their downstream migration, the smolts are found in the ocean or in coastal waters. Feeding on the abundant resources in these waters allows them to grow rapidly. After only one year at sea, the salmon can reach 60 centimetres in length. The Atlantic salmon from Quebec’s rivers migrate to Newfoundland and Labrador, and even as far as the coast of Greenland.

    • Return to natal rivers

      After one to three years at sea, instinct drives the salmon to return to the river where they hatched in order to spawn. Guided by ocean currents and other mechanisms that remain obscure (stars, magnetic fields), they approach their natal river where the scent finally guides them home. Before entering the fresh water of the river, they stop feeding and subsequently survive on their reserves of fat and protein.

    • Capable of spawning several times

      In contrast to Pacific salmon, Atlantic salmon do not die, although they are weak after spawning. These survivors of the spawning season, called “kelts” or “black salmon,” often take refuge at the bottom of a pool for the winter and only move downstream with the high water the following spring.

       

      Black salmon