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salmon

Any of the various bony fishes of the family Salmonidae. More specifically the name is applied to several species of game fishes of the genera Salmo and Oncorhynchus of North America and Eurasia that mature in the ocean but, to spawn, return to the freshwater streams where they were born. Their normal colour is silvery with a few dark spots, but the colour changes at the spawning season.

Life cycle

The spawning season is between September and January, although they occasionally spawn at other times. Once the salmon pair, the female hollows out her redd in the riverbed and in it deposits her orange eggs, about 6 mm/0.25 in in diameter, to be fertilized by the male. Once fertilized, the eggs are then covered with gravel by the female. The incubation period is from five weeks to five months. The young hatched fish are known as alevins, and when they begin feeding they are called parr. At about two years old, their coat becomes silvery, and they are called smolts. Depending on the species, they may spend up to four years at sea before returning to their home streams to spawn (at this stage called grilse), often overcoming great obstacles to get there and die.

As the salmon migrate up stream, their colouring changes to include greens, pinks, and blues and they increase in size, some growing to as much 1 m/ 3 ft. The jaws of the males undergo restructuring to become hook shaped to enable them to drive rival males from spawning areas. Internally, the reproductive organs increase in size in both males and females and the gut decreases to such an extent that salmon fast for their migration.

Salmon farming

Salmon are increasingly farmed in cages, and ‘ranched’ (selectively bred, hatched, and fed before release to the sea). Stocking rivers indiscriminately with hatchery fish may destroy the precision of their homing instinct owing to interbreeding between fish originating in different rivers.

Genetically engineered salmon The first genetically engineered salmon were hatched in January 1996 at a high-security hatchery at Loch Fyne, Scotland. The eggs were injected with a gene sequence from another fish (ocean pout) plus the salmon growth hormone gene. Normally, salmon produce the growth hormone only in certain tissues and at certain times of the year, but the transgenic salmon have no such restrictions; at one year they should be on average five times as big as other salmon of the same age.

Conservation The Environment Agency has started work on narrowing some rivers and streams in southern England, including the Test, the Avon, and the Itchen, in order to increase the flow of water and thus save wild salmon from extinction. The gravel in which the salmon lay their eggs has been silting up, and recent droughts have reduced the flow of water, especially where the rivers are fed by underground chalk springs. Measures to be taken include fencing the banks to prevent cattle from breaking down the river banks, and the planting of reed beds. The scheme has already been successful in Winchester, where Brandy Stream was narrowed, and the number of young salmon produced from eggs greatly increased.

angling grilse

parr Salmon (river)

Salmon Arm Salmon, Robert

smolt