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Taxonomy and Species Description

Atlantic salmon are classified as a bony fish in the order Salmoniformes, family Salmonidae, genus Salmo, and species salar.  There are many members of the Salmonidae family, commonly called salmonids, in North America (coho, chinook, steelhead, pink, chum, cutthroat), but there are only two members of the genus Salmo in North America:  the native Atlantic salmon (Salmo salar) and the brown trout (Salmo trutta), which was introduced from Europe.  The Atlantic salmon is anadromous, spending its first 2 to 3 years in fresh water, then migrating to the ocean, where it spends typically 2 years, and returning to its natal river to spawn.  Atlantic salmon are iteroparous, meaning they can spawn multiple times, unlike Pacific salmon that spawn once and die (semelparous).  A nonanadromous (resident) variety of Atlantic salmon is found in some lakes and rivers, but for purposes of this plan, the term “Atlantic salmon” refers to the anadromous form, while “landlocked salmon” refers to members of the non-anadromous populations.

Atlantic salmon have a fusiform body shape (i.e., like a spindle, broadest in the middle and tapering at each end).  The shape is somewhat flattened towards the sides, the head is relatively small, and the ventral fins are paired.  Juvenile salmon, called parr, have 8 to 11 vertical dark bars (“parr marks”) on silvery sides.  As parr reach the age when they migrate to the ocean, most undergo a physiological process called smoltification, typically turning silvery; they are then called smolts.  In some instances, male parr can reach maturity while in freshwater and can participate in spawning activities.  These parr are referred to as precocious parr.  When adults return to freshwater to spawn they darken to a bronze color; after spawning, adults darken further and are often called “black salmon” or kelts.  Upon returning to the ocean, adults return to the silvery color.

Out-migrating smolts in Maine average 14 to 18 centimeters (cm) in length.  The size of returning adults depends upon the amount of time spent at sea.  Grilse are young salmon returning to fresh water after one winter or 1 year at sea (called a one sea-winter or 1SW fish); they average 50 to 60 cm and 1 to 2 kilograms (kg).  Adult salmon returning after 2 years at sea (2SW fish) range from 70 to 80 cm and 3.5 to 4.5 kg.  Adult salmon returning after 3 years at sea (3SW fish) are 80 to 90 cm long and often weigh more than 7 kg (Baum 1997).


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