Atlantic Salmon

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Lore & Background

The Atlantic salmon's life cycle begins in a gravel bed in the upper reaches of a stream or river. After 2 to 6 months the eggs hatch into tiny larvae called sac fry or alevin, which stay hidden in the gravel while feeding on their yolk sac. When the yolk is gone, they leave the gravel and start feeding on plankton, becoming fry. By the end of summer, they develop into juvenile fish called parr, which feed on small invertebrates and are camouflaged with spots and vertical bars. They remain in this stage for up to three years. As they prepare to migrate to the sea, the parr lose their camouflage bars and undergo physiological changes to survive the shift from freshwater to saltwater, becoming smolt. Smolt spend time in brackish waters while their body chemistry adjusts to higher salt levels, and they grow silvery scales. When mature in late spring, about 15 to 20 centimetres long, they swim out to sea. There they spend their first year as post-smolt, forming schools and finding deep-sea feeding grounds, then spend up to four more years as adult ocean salmon developing full swimming and reproductive capacity. Then they return from the saltwater ocean back to a freshwater river to spawn.

In Their Own Story

After several years wandering huge distances in the ocean, most Atlantic salmon return to the natal rivers where they were spawned. They swim up the rivers until they reach the very spawning ground that was their original birthplace. They use geomagnetic and chemical cues to guide them. They may be sensitive to the Earth's magnetic field, allowing them to navigate back to the estuary of their natal stream. Salmon have a strong sense of smell. In 1951, Hasler hypothesised that once near the estuary, salmon use chemical cues unique to their natal stream to home onto the entrance. In 1978, Hasler and his students found that salmon locate their home rivers because they can recognise their characteristic smell, which becomes imprinted when they transform into smolts just before migrating out to sea. Homecoming salmon can also recognise characteristic smells in tributary streams as they move up the main river. They may also be sensitive to pheromones given off by juvenile conspecifics. There is evidence they can discriminate between two populations of their own species. Tagging studies show a small number of fish stray to other nearby streams, which is important for colonizing new habitats. In 1984, Quinn hypothesized a dynamic equilibrium, controlled by genes, between homing and straying.

Reader's Guide

**Habitat & Range:** Atlantic salmon are found in the Atlantic Ocean and run up rivers on both sides of the ocean. They do not thrive in warm water. Their spawning grounds are gravel beds in the upper reaches of streams or rivers, which also serve as nurseries. **Life Cycle Stages:** Eggs hatch into alevin after 2 to 6 months. Alevin become fry, then parr (up to three years), then smolt (about 15 to 20 centimetres long in late spring), then post-smolt, then adult ocean salmon (up to four more years). **Migration & Navigation:** Salmon return to their natal rivers using geomagnetic cues and their sense of smell. The smell of their river becomes imprinted when they transform into smolts. They can recognise characteristic smells in tributary streams and may use pheromones from juvenile conspecifics. **Conservation Status:** The source does not provide conservation status information for Atlantic salmon.

Did You Know?

• Atlantic salmon is the only species of salmon found in the Atlantic Ocean.
• Salmon eggs hatch into larvae called alevin after 2 to 6 months.
• Parr, the juvenile stage, can remain in freshwater for up to three years before becoming smolt.
• Smolt are about 15 to 20 centimetres long when they migrate to the sea in late spring.
• In 1978, Hasler and his students demonstrated that salmon locate their home rivers by recognising their characteristic smell, imprinted when they become smolts.