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Atlantic Salmon

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Atlantic Salmon

Adult Salmon

Salmon are native to the world's two biggest oceans and the rivers draining into them.  The Atlantic Ocean has only one species, the Atlantic salmon (Salmo salar), while in the Pacific Ocean there are several species: pink salmon (Oncorhynchus gorbuscha), chum (O. keta), sockeye (O. nerka), coho (O. kisutch), chinook (O. tschawytscha) and amago (O. rhodurus).

Life Cycle of Atlantic Salmon

Irish salmon are Atlantic salmon, and spend their juvenile phase in rivers before migrating to sea to grow. To complete their life cycle they must return to their river of origin to spawn. Fish with this life cycle are called anadromous.


All salmon spawn naturally in freshwater. Spawning typically occurs in the headwater and tributary streams of rivers, though it can happen anywhere in a river if the substrate is suitable. The migration to suitable habitat may commence up to a year before spawning takes place in autumn-winter, salmon ceasing to feed, directing all their energy instead to reproduction. Usually the female salmon will excavate a depression in the gravel with her tail, and deposit her eggs into this. One or more males discharge sperm over the falling eggs to fertilize. The eggs are then covered with gravel to a depth of several centimetres by the female. The parents then leave the eggs in the nest or "redd", and there is no further parental care.



The eggs (ova) begin developing right after fertilization, and will hatch after about 180 days at normal water temperatures. The fertilized orange pea-sized eggs will not become "eyed" (i.e., the eyes of the embryo can be seen as two black dots) until January-February, before hatching in March-April.



The just-hatched fish are called alevins, and still have a yolk sac attached to their bodies, with the remains of food supplied from the egg. When most of the their yolk sac has been consumed, the alevins become active and begin their journey up through the gravel. They soon grow all eight fins, which will be used to maintain their position in fast flowing streams and manoeuvre about in the water.



The small fish must rise to the surface of the water to take a gulp of air with which they fill their swim bladder, giving them neutral buoyancy, which makes it easier to swim and hold their position in the water column. This critical period is therefore referred to as "swim-up" and exposes the young to dangerous predators for the first time. Once they begin to swim freely (three to six weeks after hatching), they are called fry. Their survival is temperature dependant and heavily influenced by predation and competition for food.



Fry quickly develop into parr with vertical stripes and spots for camouflage. They feed on aquatic insects and grow for one to three years in their natal stream. Once the parr have grown to 10–24 cm in body length, they undergo a physiological pre-adaptation to life in seawater while still in freshwater, by smolting. In addition to the internal changes in the salt-regulating mechanisms of the body, the appearance and behaviour of the fish also change. The smolts become silvery and change from swimming against the current to moving with it. This adaptation prepares the smolt for its journey to the oceans.



In spring, large numbers of smolts leave Irish rivers to migrate north along the slope current into the rich feeding grounds of the Norwegian Sea and the greater expanse of the North Atlantic Ocean. Here they feed primarily on fish (piscivorous), such as capelin (Mallotus villosus), herring (Alosa spp.), and sand eel (Ammodytes spp.). As they grow fewer predators are able to feed on them. Their rate of growth is therefore critical to survival.

Adult Salmon


Some Irish salmon, called grilse will reach maturity after one year at sea and return to their river in summertime weighing from 1 to 4kg. If it takes two or more years to mature, the salmon will return considerably earlier in the year and larger at 3 to 15kg - becoming a highly prized fish but also a very rare one. Salmon exhibit a remarkable "homing instinct", by which a very high proportion are able to locate their river of origin using the earth's magnetic field, the chemical smell of their river and pheromones (chemical substances released by other salmon in the river). A journey of up to 5000km makes salmon "the king of fish".



Having spawned, the salmon are referred to as "kelts". Weakened by not having eaten any food since their arrival in freshwater and losing energy in a bid to reproduce successfully they are susceptible to disease and predators. Mortality after spawning can be significant, especially for males but some do survive and commence their epic jouney again. In exceptional cases, some Irish salmon are known to have spawned up to three times!



Fish that shed their spawn late or not at all. Such fish are occasionally caught in springtime on the early rivers and indeed some salmon may spawn as late as March. Baggots can be distinguished by their soft flesh, distended bellies and sometimes open vents. Baggots are not dissimilar in appearance to coloured hens but their bellies are flabby. They must not be killed because it is illegal to do so.