Smolted fish at 40-120 g are transferred to seasites following the determination that fish have smolted and are adapted for seawater survival, using combinations of experience and seawater tolerance testing techniques. Transfers are usually carried out in specialized transport tanks by any combination of road, helicopters, and by sea in specialized "wellboats" (boats with large wells circulating seawater). Ongrowing at sea normally takes place in cages consisting of large nets suspended from various floating "walkway" systems anchored to the seabed, although some production has been carried out in pump-ashore seawater tank systems. Cages may be square or circular in design, and come in various sizes and systems. The 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 in sites where water temperature extremes are in the range 6-16 °C, and salinities are close to oceanic levels (33-34‰). Water flows need to be sufficient to eliminate waste and to supply well oxygenated water (approximately 8 ppm). Maximum stocking densities of up to 20 kg/m³ are usual. Atlantic salmon are ongrown in seasites for up to 2 years with harvesting of fish from 2 kg upwards. Seasites normally contain a single generation of fish. Good practice is to fallow seasites for a period of 6 weeks or more prior to the introduction of a new generation of fish.
The bulk of salmon feeds are produced by three or four large companies. Fish meal and fish oil, derived largely from the huge industrial fisheries in South America, still form the basis of salmon diets, although increasing pressure on these sources have led to increased research into the substitution of fish products with vegetable protein and oil sources in recent times. Salmon grower diets contain high levels of fish oil, which is efficiently converted by the salmon, often at food conversion ratios of close to 1:1. Feeding methods and technology have also advanced in recent years. Many seafarms use computerized systems to drive automated feeding systems, with feedback mechanisms to detect when fish have finished feeding. This allows fish to be fed to satiation without overfeeding and consequent feed wastage.
In order to produce fish with the flesh colour demanded by the market, carotenoid pigments are added to the diet (at high cost to the farmer) during the seawater growing phase of the production cycle.
Methods of harvesting vary but fish are generally starved for up to 3 days beforehand. The whole process is carried out with the aim to keep stress to a minimum, thus maximising flesh quality. The fish are crowded in pens using sweep nets and are either pumped from the holding pen alive and transported to the slaughter plant, generally by well boat, or slaughtered on the side of the pens.
Handling and processing
In Scotland, for example, most fish are initially stunned using an automated stunner or a blow to the head. Bleeding is then carried out by cutting the gill arches rapidly and the fish are immersed in iced water. Waste disposal of blood is strictly controlled in order to prevent disease transmission. The fish are then gutted, washed and chilled. Once the flesh temperature reaches approximately 3 °C, the fish are graded and packed on ice. At this stage, whole fish can be frozen for sale as whole frozen salmon or as fresh gutted salmon. However, most fish are filleted and either sold as fresh salmon fillets or set aside for smoking.
The smoking of salmon accounts for over 60 percent of total salmon use. Many different methods of smoking exist, but to summarize the process, the fillets are salted, and smoked over smouldering wood chips. The product is then trimmed, de-boned and either sliced by hand or machines.
Costs of production vary considerably depending on:
- Farm unit size.
- Prevailing health status of stocks.
- Geographic/political location.
- Availability and cost of raw materials (feed, eggs/fish etc).
- Method of calculation.
- The largest single cost for an ongrower is normally feed.
A typical cost of production in Scotland would be around £ 0.60 (USD 1.00) per smolt for a smolt producer, and around £ 1.50 per kg (USD 2.50/kg) for an ongrower.