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 including Pink (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. Salmon, both male and female, cease to feed on entering freshwater in response to gonadal development, directing all their energy instead to reproduction.The migration of adults to suitable habitat may commence up to a year before spawning takes place in winter. Spawning typically occurs in the tributary streams of rivers, though it can happen anywhere in a river if the substrate (gravel) is suitable. At spawning time, the female 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 female covers the eggs with gravel to a depth of several centimetres. The parents then leave the eggs in the nest or "redd" with no further parental care. Buried deep inside the gravel the eggs are safe from shock during critical early days of development such as impact from debris travelling down river during heavy floods and attack from predators such as eels (Anguilla anguilla), trout (Salmo trutta) and cormorants (Phalacrocorax carbo).
The eggs (ova) begin developing right after fertilization, and depending on water temperature, they will hatch in the spring. The number of eggs deposited in the redd is determined by the size of the female; some larger salmon can deposit up to 16,000 eggs each. This high fecundity (eggs per female) is critical as survival in the wild is extremely low; in Irish rivers, less than 1% survive to leave rivers.
The just-hatched fish are called alevins and still have the yolk sac containing food attached to their bodies. When most of their yolk sac has been consumed the alevins become active and begin their journey up through the gravel of the riverbed. The small fish must rise to the surface of the water and gulp air to fill their swim bladder for neutral buoyancy, which makes it easier to swim and hold their position in the river. 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.
The fry have eight fins, which are used to maintain their position in fast flowing streams and manoeuvre about in the water. Their survival is temperature dependant and heavily influenced by predation, pollution and competition for food. The presence of salmon in a river is thus indicative of a healthy aquatic environment.
Over the autumn, the fry develop into parr with vertical stripes and spots for camouflage. They feed on aquatic insects and continue to grow for one to three years while maintaining their territory in the stream. Once the parr have grown to between 10 and 25cm in body length, they undergo a physiological pre-adaptation to life in seawater by smolting. This is evident by changes in the appearance and behaviour of the fish which become silvery and alter from swimming against the current to moving with it. There are also internal changes in the salt-regulating mechanisms of the fish. This adaptation prepares the smolt for its journey to the ocean.
In spring, large numbers of smolts leave Irish rivers to migrate along the North Atlantic Drift into the rich feeding grounds of the Norwegian Sea and the greater expanse of the North Atlantic Ocean. Here they feed primarily on fish 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.
Grilse are salmon that reach maturity after one year at sea; these return to their river in summer weighing from 1 to 4kg. If it takes two or more years at sea to mature, the salmon will return considerably earlier in the year and larger at 3 to 15kg, becoming a highly prized fish to anglers but also a very rare one. Salmon exhibit remarkable "homing instinct" with a very high proportion 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). Perfect homing precision is expected even after migrations over 3,000km to feeding grounds north of the Arctic Circle in the Norwegian Sea and West Greenland.
Having spawned, the salmon are referred to as "kelts". Weak from not eating since arriving 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 journey again. In fact there are records of Irish salmon spawning three times! Another record exists of an Irish salmon that reached maturity after less than a year at sea, an extremely rare event. The salmon left the Bundorragha River in Co. Mayo on the 27/04/2007 as a smolt of 49g only to return from the sea on the 05/11/2007 at 810g. Research continues to explore the complex life cycle of this important species.