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How are farmed Atlantic salmon bred?
In salmon farming, the lifecycle of the fish starts with laying down of eggs in the hatchery during the early winter months. Eggs and milt from selected breeding stock are mixed together to produce fertilised eggs. The fertilised eggs are then placed in purpose-built incubators at specialised hatcheries where their environment aims to mimic egg incubation in the wild, for example, by providing substrate in which eggs can nestle and clean water flow providing plenty of oxygen for the eggs to grow. As the small pea-sized eggs develop, the eyes of the salmon can be seen as a black dot on the orange egg. The incubation period is measured in ‘degree days’ and is around 450 degree days. This means that if, for example, the water temperature during incubation is 8 degrees, the incubation period is around 56 days. The hatchlings (called ‘alevins’) absorb nutrients from a yolk sac attached to their bodies and they remain in the hatching environment for another month or so at which time they are able to feed independently.
Once the hatchlings are able to feed independently they are referred to as ‘fry’ and are transferred to small freshwater tanks within the hatchery. Whilst in the hatchery, the fish are vaccinated against diseases that they may be exposed to later in life. As the fish grow, they are transferred to bigger tanks. Distinct vertical markings appear on the fish which, in the wild, serve as camouflage. At this stage, fish are referred to as ‘parr’. As they grow out in large tanks at the hatchery, the fish undergo further transformation: the vertical markings are replaced by a silvery sheen and the edges of the fins darken. At the same time, a physiological change occurs internally which allows the Atlantic salmon to survive in seawater. This process is called ‘smoltification’ and the fish, at this stage, are referred to as ‘smolts’ and they are able to be transferred to sea.
When farmed Atlantic salmon are able to be transferred to sea, they are pumped out of their hatchery tanks through water-filled pipes and transported in large water-filled tanks to the sea shore. From here, they may be transferred via pipes directly into their sea pens or to water-filled tanks in purpose-built boats (called ‘well boats’) that then take the fish to their sea pens where they will grow out for about the next year and a half. The sea pens are large, netted enclosures which not only prevent the fish escaping but also protect them from predators such as seals. Fish grow out to an average weight of around 5kg by which time they are ready to be harvested.
In salmon aquaculture systems, fish spend 10 to 16 months in fresh water (on land) plus 14 to 18 months in the sea pens before they are ready for harvest.
At harvest, farmed Atlantic salmon may be removed from their sea pens through large pipes and transferred to special harvesting boats that sit alongside the pen. Alternatively, well boats are used or the whole pen is slowly towed towards shore where fish are transferred directly to a slaughter plant. Before slaughter, fish may be fasted for a few days in order to reduce the oxygen demand required to digest their food. By reducing this oxygen demand the fish are better able to cope with the harvest process. The harvest and slaughter process keeps fish in water as long as possible before they are stunned using a percussive blow to the head. Subsequently, the unconscious fish are bled and immersed in ice slurry for transport to processing plants where they are gutted, washed and processed into fresh, frozen or smoked product.
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