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Atlantic salmon Salmo salar; Linnaeus, 1758. Family: Salmonidae

Salmon Biology and Life Cycle

The Atlantic salmon (Salmo salar) is an anadromous fish that moves between fresh water and the sea during the course of its life. Although salmon spawn in fresh water and the young fish spend the first part of their life there, they grow the most in the sea. Young fish, which are first known as fry and later as parr, live and grow in streams and rivers. In general, they live discretely among the stones and boulders of the stream-bed, emerging at times to feed nearby. As they grow, the young fish move from shallow areas to deeper ones nearby but after this stage parr tend to live in the same places for prolonged periods.

After one or more years - commonly two or three years in Scottish rivers - the young fish forsake this somewhat sedentary life-style when they change their appearance and habits as a prelude to migration. At this stage the fish become known as smolts. Most smolts range from 100 to 150 mm in length. Smolts are distinguished from dark-coloured parr by their slim, silvery appearance and their adoption of a free-swimming, migratory life-style. Smolts move down rivers in April and May to reach the sea and head northwards and westwards in prolonged migrations of up to many thousands of kilometres that last for one or more years. Some Scottish fish travel as far as the Davis Strait between the western coast of Greenland and Baffin Island in the Canadian Arctic.

After one, two or sometimes three years of feeding in the northern oceans, the adult fish return to the same coasts that they left a year or more before. Soon afterwards they enter the rivers where they previously lived. In late autumn or early winter they spawn near to the places where they lived when they were parr.

Female salmon place their eggs in depressions that they prepare in suitable places in the stones and gravel of the stream-bed. The females turn on their side and use a swimming-like motion to create turbulence that lifts and clears stones from the chosen site. Often, the same places are used for spawning year after year. Male fish fertilise the female's eggs are as they are shed into the nests. Finally, the female fish covers the eggs by moving gravel over the completed nest in the same way as before.

Although most fish migrate to sea before they spawn, some male parr become sexually mature while they remain in fresh water. These parr take part in spawning and, in spite of their small size, are responsible for fertilising large numbers of eggs. The eggs remain within the nest until they hatch in late spring - five or six months later. The newly-hatched fish, which are known as alevins, remain within the gravel near to the nests for several more weeks. In May or June, the young fish - the new generation of fry - leave the shelter of the streambed to start life in the stream itself.

Salmon (Salmo salar L.) have always been a highly sought-after commodity. The earliest fisheries used box traps placed in gaps in purpose-built weirs in rivers, known as 'cruives'. Later on, the fisheries expanded as new fishing methods were devised. Even until quite recently, major net fisheries existed in the sea, on the coasts and in estuaries in Scotland. In the last few years, most net fisheries have declined, often as a result of buy-outs by those with interests in angling. In former years the net catch greatly exceeded the rod catch but now the angling catch is the greater.

In the past, a large seasonal market for salmon existed. To a large extent, this commerce has been supplanted by the production and distribution of farmed salmon in Scotland and elsewhere. The advent of salmon farming has reduced the requirement for wild salmon by the food industry. Because numbers of wild salmon are declining, many of the fish caught by anglers are now returned to rivers unharmed so that they may go on to spawn. This practice is known as catch-and-release.

Scotland's salmon stocks are of unique value. Not only do Scotland's coasts and rivers produce large catches but the diversity of the resource has few parallels elsewhere. In most other countries the fisheries are limited to the summer and early autumn months when returning fish pass along the coasts and run the rivers. However, salmon enter Scotland's rivers throughout the year and, with the exception of December, when fisheries are closed to protect spawning fish, it is possible to fish for salmon most months in the year.

From an economic viewpoint, the diversity of Scotland's salmon runs is of inestimable value in spreading the fisheries over so much of the year. Sporting opportunities exist in Scotland at times of year which do not occur elsewhere.

The diversity of the salmon runs is based on genetic traits that affect the timing of river entry and it therefore has an additional, intrinsic value as part of the biodiversity resource.

Indeed, the six or seven major rivers of the Scottish east coast are the repository for the greater part of the remaining world resource of early-running or so-called spring salmon.