Life for a salmon begins between November and February in the gravel bed of a freshwater stream. The male fends off intruders while the female excavates several small hollows up to 30 cm deep. Together they lay and fertilize several thousand eggs in each hollow. The female then protects the eggs by covering them with gravel.
In March or April, a peculiar-looking fish emerges from the egg. Called an alevin, it is only about 3 cm long and has an ungainly yolk sac attached underneath. Initially the fish stays hidden under the gravel, feeding off its portable food supply. After four or five weeks, when its yolk sac has been absorbed, the fry, as it is now called, wriggles out from beneath the stones into the main stream. It is about 5 cm long and now looks like a proper fish. There are only two things on its mind. First, finding a new food source-small insects and plankton-and second, finding a safe place to live. At this stage more than 90 percent of the salmon fry perish because of lack of food or space or because they are eaten by predators, such as trout, kingfishers, herons, and otters.
When it`s about a year old,the salmon gets to be about 8 cm or 10 cm long. It is now called a parr and has a distinctive marking of dark patches along each side of its body. When its length reaches about 15 cm, the dark markings give way to a uniform brilliant silver. Now some remarkable and complicated changes occur that set the salmon apart from most other fish.
Between May and June, the fish, now called a smolt, is prompted by some internal signal and joins thousands of others in an exodus downstream to the estuaries. But surely a freshwater creature cannot survive in the sea, can it? Normally, no, but complex changes occur around its gills, which enable it to filter out the salts found in seawater. When the changes are complete, the smolt, which is small enough to fit into the palm of your hand, sets off on an epic journey.
Life at Sea
Why does such a small fish leave its familiar river? Where does it go? The young salmon needs to get to its feeding grounds in order to become fully mature. If it avoids predators, such as cormorants, seals, dolphins, and even killer whales, it will arrive there and feed on certain large zooplankton and sand eels, as well as herring, capelin, and other fish. After a year its weight will have increased 15-fold-from a couple of hundred grams to nearly 3 kg. If it stays in the ocean for five years, its weight could reach 18 kg or more. A few have been known to exceed 45 kg!
The exact location of the feeding grounds was unknown until the 1950’s, when commercial fishermen began catching large numbers of salmon off the coast of Greenland. Another major feeding ground was later discovered around the Faeroe Islands, north of Scotland. More feeding grounds have since been discovered. There are even reports of salmon feeding under the Arctic ice! With the discovery of these feeding grounds, the troubles really began for the Atlantic salmon. Huge fisheries were built in Greenland and the Faeroe Islands. Thousands of tons of fish were caught by commercial fishermen, and suddenly the numbers returning to breed in the freshwater rivers plummeted. Realizing the seriousness of this problem, governments set various restrictions and quotas for fishermen. This has helped to protect the salmon while at sea.
The Return From the Sea
Eventually the mature salmon returns to the river where it was hatched, finds a mate, and the cycle begins again. Amazingly, the remarkable salmon unerringly navigates thousands of miles of ocean that it has never seen before! How it does this continues to baffle scientists. Some say that salmon navigate using the earth’s magnetism, ocean currents, or even the stars. It is thought that once it is back in the estuary, the salmon recognizes its home river by its ‘smell,’ or its chemical composition.
They adapt to freshwater life once more, and enter the river. This homing instinct is so strong that even if waterfalls or rapids are in the way, these salmon, now much bigger and stronger, will stubbornly struggle to get over each hurdle.
It`s a miracle of nature that the process of spawning to adulthood continues, as the odds seem to be stacking against it as the years go by.
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