Salmon Life Cycle
Life Cycle of Atlantic Salmon
Irish salmon are Atlantic salmon and spend their juvenile phase in rivers before migrating to sea to grow and mature. To complete their life cycle, they must return to their river of origin to spawn. The salmon who adopt this life cycle are called anadromous. A continuous decline in abundance highlights the importance of understanding this complex life cycle.
Life begins
Salmon begin life as small, pea-sized eggs hidden away under loose gravel in cool clean rivers entering the North Atlantic Ocean. Against the odds the parents of these little eggs have succeeded in returning to freshwater to spawn, completing their life cycle before giving rise to a new generation. To do this both male and female adults ceased to feed on entering freshwater in response to gonadal development, directing all their energy instead to reproduction. The migration of adults in winter to suitable habitat can commence up to a year before spawning takes place. Spawning typically occurs in headwater, though it may happen anywhere in a river if a suitable substrate of well oxygenated loose gravel is available. At spawning time (November to January), the female digs a depression in the gravel with her tail to deposit her eggs at a depth of between 15 - 30cm. One or more males discharge milt over the falling eggs to fertilize. Quickly the female covers the eggs with gravel which forms a nest or redd on the riverbed. Buried deep inside the gravel the ova are safe from the impact of debris carried along in heavy floods and from attack by predators such as eels (Anguilla anguilla), trout (Salmo trutta) or cormorants (Phalacrocorax carbo).
Ova
The rate of egg or ova development is dependent on water temperature. Eyes inside the pea sized orange ova are visible and increasing movement can be detected as the yolk sac containing food is consumed. The number of ova deposited in the redd is determined by the size of the female with larger females over 10 kg depositing 15,000 each. This high fecundity (ova per female) is critical as survival in the wild is extremely low, especially in freshwater. For example, in the Burrishoole River on the west coast of Ireland survival rates for juveniles is less than 1%.
Alevins
The just-hatched salmon are called alevins and still have the yolk sac attached to their bodies in spring. When their yolk sac is absorbed, the alevins become increasingly active and begin their journey up through the gravel of the riverbed. When strong enough the small fish must rise to the surface of the water and gulp air. By doing this they fill their swim bladder to gain neutral buoyancy making it easier to swim and hold their position in fast flowing streams. 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, they are called fry.
Fry
Fry have eight fins, which are used to maintain their position in fast flowing streams and manoeuvre about in the water during the summer months. Fry feed on microscopic invertebrates and their abundance is regulated by temperature, predation, pollution, and competition for food with other fry and other species of fish. The ability to use both visible light and infrared vision allows salmon see colour while still navigating in low visibility floodwaters. Flexibility to change their visual sensitivity allows salmon adapt to different environments from shallow rivers to ocean depths and is a critical survival trait. Healthy skin functions to maintain osmotic balance and protect the fish from pathogens and environmental contaminants. Gills are essential for absorbing oxygen from the water and releasing carbon dioxide and other waste products from the blood. The presence of salmon in a river is synonymous with a healthy aquatic environment, and as they are extremely sensitive to changes in water quality, habitat and climate, salmon are a good indicator of freshwater and marine ecosystem status.
Parr
Over the autumn the fry develops into parr with vertical stripes and spots for camouflage to avoid predators. Nostrils give the salmon an acute sense of smell. 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 25 cm in body length, they undergo a pre-adaptation to life in seawater by smolting. Smolting refers to the physiological transformation a juvenile undergoes as it prepares for life in the sea. This is evident by changes in their appearance as they become silvery and swim with the current instead of against it. Lateral lines develop from eye to tail with specialized cells containing magnetic particles that allow salmon to detect the earth's magnetic field and low frequency sound waves in their environment. Salmon that live in freshwater need to keep salts in and get rid of extra water but at sea they do the opposite - keeping water in and getting rid of extra salt. This amazing ability is known as osmoregulation. Brain function and memory ability increase rapidly, and together these adaptations prepare the smolt for its journey to the ocean.
Smolt
In Spring, large numbers of 1-, 2- and 3-year-old smolts, leave Irish rivers to migrate along the North Atlantic Current, and into the rich feeding grounds of the Norwegian Sea and the greater expanse of the North Atlantic Ocean. Here they feed on zooplankton, especially Northern shrimp (Pandalus borealis) and Northern krill (Meganyctiphanes norvegica) which gives Atlantic salmon flesh its colour. They also feed on Amphipoda and the copepod Calanus finmarchicus. Important prey species of fish include capelin (Mallotus villosus), herring (Alosa spp.), and sand eel (Ammodytes spp.). By feeding on fish with a high calorific value they grow quickly so fewer predators can feed on them. Their rate of growth is therefore critical to the marine survival of Atlantic salmon.
Adult salmon
Salmon that reach maturity after one year at sea are called Grilse; these return to their river in summer weighing from 0.8 to 4 kg. If it takes two or more years at sea to mature the multi-sea-winter Salmon will return earlier in the year and larger at 3 to 15 kg, and because of their size they are greatly sought after by fishers. Salmon exhibits remarkable homing instinct with a 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 migrating over 3,000 km to feeding grounds north of the Arctic Circle in the Norwegian Sea and at West Greenland. There is great excitement when adult salmon return to rivers as many are seen leaping acrobatically into the air and jumping over waterfalls while moving upstream. Salmon that survive fishers, poachers and pollution may still have to scale large dams built across rivers before eventually finding refuge in lakes and deep pools. Arriving upstream to their spawning grounds among big boulders in icy headwaters the life cycle begins again, so ensuring survival of the species for another generation.
Kelts
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, kelts are susceptible to disease and predators. Pacific salmon species die after spawning but some Atlantic salmon do survive and commence their epic journey again. Scientists studying salmon initially used the rings or circuli laid down on scales, much like tree rings, to determine the age and growth of salmon in freshwater and at sea. By doing this they established that a few kelts succeeded in spawning three times! Now a new record exists of an Irish salmon that reached maturity after less than one year at sea - a zero sea winter salmon. The salmon left the Bundorragha River in Co. Mayo on the 27/04/2007 as a 1-year-old smolt of 49 g only to return from the sea on the 05/11/2007 at 810 g.
Zero Sea Winter Salmon Scale
Salmon survival has fallen across the North Atlantic, however scientists today have a greater array of techniques to study the complex life cycle of this important species. To protect salmon populations, society must adopt a dual strategy: proactively reducing known threats like pollution, habitat degradation and invasive species, while preparing for emerging risks such as climate change and novel pollutants. Scientific research needs to inform policy, identify critical habitats, and develop regulations to support long-term conservation efforts for salmon and their ecosystems.
Atlantic salmon, the climate change canary of the North Atlantic moves easily from freshwater to roam the ocean feeding constantly on migration while avoiding predators, then homes to their natal river, jumping over almost impassable falls to reach their exact place of birth. In Irish mythology, An Bradán Feasa (The Salmon Of Knowledge) is a legendary fish whose flesh contains all the world's wisdom. In the North Atlantic Salmo salar is the King of Fish.