Know your Scottish Salmon

Scottish Salmon Farming

Scottish salmon farming has grown steadily since the 1970’s and is now a major part of Scottish aquaculture, which has a present worth of over £1.8bn annually and provides more than 8,000 jobs. Scotland is the largest producer of farmed Atlantic salmon in the EU and the third largest in in the world after Norway and Chile.

salmon life cycle
salmon farm locations
salmon farm structures
The alevin stage is very small, <2.5 cm, and has a sac containing the remainder of the yolk which they feed on. The yolk sac is slowly absorbed over time, following which alevins will “swim up” in the water column, showing readiness to first feed.Eggs are obtained from specialised suppliers (commonly based in Norway or Ireland), which remove eggs from broodstock selected from seasite production stocks.  The eggs are laid down in trays or silo systems. After 25-30 days fertilised eggs show “eyes” and unfertilised eggs are removed by a process called “shocking”. They can take 2-6 months to hatch.Once a young salmon has absorbed its yolk sac it is called a fry.  This is a rapid growing stage.Salmon fry that reach a length of 6.5 cm (2.5 inches) are called parr. The parr develop camouflaging vertical stripes. Vaccinations are given at the late part of this freshwater stage at 30-40g weight.‘Smoltification’ is a complex developmental transformation, involving physiological, biochemical, morphological and behavioural changes which allows young parr salmon to transition from living in freshwater to living in saltwater.  Light and temperature regimes can be manipulated artificially to induce early smoltification.

Smolts are initially 12-15 cm (5-6 inches) long, 50-80 g and distinguished by bright silvery colouring with scales that easily rub off.
At around 100g smolt are moved from freshwater to sea water, where they develop into the adult salmon stage.  On farms it takes 14-24 months to reach a full-size adult.  In the wild the salmon return to their natal river after 24-36 months to spawn.

There has been a trend to keep smolt size (post-smolt) before transferring to seawater, from between 100 to 1000 g.

The average size of an Atlantic salmon in the wild is 71-76 cm (28-30 inches) long and 3.6-5.4 kg (8-12 pounds) after two years at sea. Adults can grow to be 13.6 kg (30 pounds). Harvest size in farms is about 4.5-5.5 kg.

Salmon Life Cycle

Most commercially available salmon is farmed Atlantic salmon (Salmo salar), one of several species from the family Salmonidae.  Salmon are anadromous, being born in freshwater, migrating to the ocean as young adults and returning to freshwater to reproduce.  The juvenile freshwater stage of Atlantic salmon (Salmo salmar) takes 24-36 months in the wild, and 6-12 months in farms, developing from egg through the alevin, fry and parr stages to the smolt stage which can then move to saltwater.

Fish farm siting in Scotland

Atlantic salmon culture began in the 19th century in the UK in freshwater as a means of stocking waters with juvenile (“parr”) stages to enhance wild populations for anglers.  Open sea pens or cages were first used in the 1960s in Norway to raise fish to marketable size, and the success led to development of salmon farming in Scotland, and subsequently Ireland, the Faroes Islands, Canada, the North Eastern seaboard of the USA, Chile and Australia (Tasmania), with some minor production in New Zealand, France and Spain.

In Scotland fish farms are based on the west and north-west coasts, due to a presumption in Scottish Planning Policy against marine finfish farm developments on the north and east coasts to safeguard migratory fish species.

Scottish strains of salmon tend to mature early, reducing the value of the fish as they reach market size, so Norwegian strains were introduced to reduce the problem.  Generations of cross breeding have produced hybrid strains which are now used in most production areas.  Scotland hatcheries now import fertilised eggs to produce larval and juvenile fish.  Once hatched, these are grown on in tanks either using freshwater flow-through or land-based recirculating systems (RAS), or subsequently in lake cage systems.  At the final juvenile (“smolt”) stage they are transferred to on-growing systems, usually coastal open net pen farms, to reach harvest size.

Salmon Farm Structures

Atlantic salmon eggs are obtained from broodstock fish, bred on selected characteristics such as growth, disease resistance, maturation rate or colour.  In Scotland, hatcheries use imported fertilised eggs to produce larval and juvenile fish.  Once hatched, fry can be grown on in tanks either using freshwater flow-through or land-based recirculating systems (RAS), or subsequently in lake cage systems through parr stages to smolt.  Smolt are transferred to on-growing systems, usually open net pen farms, to reach harvest size.  The majority of smolt are produced by salmon farming companies for their own use, and usually released into seawater twice a year.

The three main production systems are shown below. RAS technology can be used for either the freshwater or saltwater stage of salmon aquaculture.

Flow-through hatcheries take water from nearby lochs or rivers, where it flows through grow-out tanks and raceways before being filtered and returned to its source. The majority of smolt production in Scotland takes place in land-based, single-pass flow-through farms.Recirculating aquaculture systems use enclosed tanks, and “recirculate” (recycle and re-use) water in a loop. The water can be either from a nearby water source such as a loch or river, or from the mains water supply. It is then connected to the sewer for waste disposal.  Conversion to RAS is being undertaken and considered by many producers.When smolt are ready to go to sea, they are transported from hatcheries in specially designed boats (wellboats) or trucks (tankers) to seasites. Ongrowing at sea normally takes place in open sea net pens, cages consisting of large nets suspended from various floating walkway systems anchored to the seabed. Some production has been carried out in pump-ashore seawater tank systems.  Larger sites may have cages as large as 24 m2 or 100 m in diameter, suspending nets that may reach depths of 15-18 m, enclosing water volumes of thousands of cubic metres.  Several cages may be grouped together to form a seasite.

Seasites are selected on their suitability with regard to water temperature, salinity, flow and exchange rates, proximity to other farms and/or wild fisheries and in compliance with local licensing regulations.  Atlantic salmon grow best where water temperature is in the range of 6-16⁰C. and salinities close to oceanic levels (33-34 ‰). Water flows need to be sufficient to eliminate waste and supply well oxygenated water (approximately 8ppm). Maximum stocking densities of 20 kg/m3 are usual.

On-growing in seasites takes up to 2 years, with harvesting of fish from 2 kg upwards.  Seasites normally contain a single generation of fish.  Harvesting takes place throughout the year, although most takes place in the last quarter of the year as this is the period of best growth. After a site is harvested, good practice is to fallow the location for a period of 6 weeks or more before introducing a new generation of fish.