Habitat and biology
Coho salmon, which are native to the coastal area of the North Pacific Basin, have been introduced into many areas of North America, Asia, Europe and Latin America, although most attempts to establish naturalized populations have been unsuccessful. Self-sustaining stocks have been reported in the Great Lakes of America and in Chile, where they are a major species in a rapidly expanding aquaculture industry.
Coho salmon are produced entirely in hatcheries.
Maturing adults are obtained from either seafarm production stocks or selected groups specifically managed as broodfish. Because coho salmon mature after one year in the sea, two separate broodstocks – odd year and even year spawners – are required to maintain continuous production. In autumn, maturing broodfish are selected from groups reared at seasites and moved into freshwater tanks where final maturation and spawning takes place.
Female salmon are ready for spawning when their eggs can be expressed by only slight pressure to the abdomen. Female fish are humanely killed, their eggs removed and fertilized with milt from one or more male fish. After water hardening and disinfection, the eggs are placed in hatchery trays for incubation. At the eyed stage of development, the eggs are 'shocked' by pouring them from one container into another. Unfertilized eggs become opaque and are removed. Egg incubation usually takes place in water less than 12 °C. Hatching takes place in the hatchery trays, where the alevins remain until the yolk sac is completely absorbed and they are ready to accept formulated feeds.
The fry are then transferred to tanks or raceways within the hatchery for start feeding. Subsequently, they can be grown to the smolt stage in tanks or raceways, or moved to lake cage systems. Fish are usually maintained in ambient water temperature and natural photoperiod regimens to produce smolts in the spring of the year of hatching, but temperature and photoperiod can be manipulated to induce early smoltification. In either case, the fish must attain a certain 'threshold' size before smoltification can occur. Coho salmon smolts generally weigh 30–80 g when they are transferred to the sea.
Coho salmon are ready for transfer to seafarms when parr marks (large, vertically oriented bars of pigment in a row along the lateral line of juvenile salmon) are no longer visible and after seawater tolerance tests verify that smoltification is complete. After grading to remove undersized fish, the smolts are placed into specialized transfer tanks and transported by road or sea-going vessel.
Ongrowing at sea takes place in cages consisting of large nets suspended from floating structures anchored to the seabed. The floating cage structure may be either square or circular in shape, and is constructed of high-density polyethylene or steel. Typically, producers use plastic circular cages 25–30 m in diameter or square steel cages (25 m x 25 m), from which nets 16–18 m in depth are suspended. Several cages are usually grouped together to form a 'seafarm' or 'seasite'. Cage systems may also be entirely surrounded by additional nets to protect them from predatory marine mammals.
Seafarm locations are selected on the basis of season water temperature profile, dissolved oxygen content, salinity, depth and current velocity; exposure to storms; presence of harmful algae and diatoms; proximity to other farms; and in compliance with local regulations. Coho salmon tolerate a wide array of oceanic conditions but grow best when temperatures are in the range of 9–15 °C, and water currents are sufficient to disperse wastes and provide a continuous supply of well-oxygenated water. Stocking densities at harvest should not exceed 8–12 kg/m³.
Coho salmon remain in seawater for 10–12 months and are harvested after reaching 2.5–3.5 kg. Best management practices dictate that a seafarm should contain only a single year class of fish. This practice reduces the risk of disease transmission to arriving smolts.
Formulated feeds for coho salmon contain various mixtures of oilseed meals (e.g. cottonseed meal, soybean meal), fish meal and fish oil, and various grains (e.g. wheat, corn), supplemented with essential vitamins and minerals. Small quantities of a naturally-occurring carotenoid called astaxanthin, which is found in the prey of wild salmon and gives them their characteristic pink colour, is also added to the feed mixture.
Feeds are pelleted using a process called extrusion, in which the feed mixture is moistened and cooked with live steam before being forced at high pressure through holes in a 'die' plate. After cooling, the feeds are bagged and distributed to the farms.
Modern feeds for coho salmon typically contain 29–30 percent lipid and 40–43 percent protein, which is efficiently converted to flesh, often at food conversion ratios below 1:1. The best results are obtained when the fish are fed using computerized, automated systems equipped with feed-back mechanisms to detect when the fish have finished feeding. This permits the fish to be fed to satiation without overfeeding and consequent feed wastage. Good results can also be obtained by trained personnel using operator-controlled blower systems.
Methods vary but the fish are generally starved for several days before harvesting. Small numbers of fish are crowded to the side of the pen using a sweep net (a net that is long and deep enough to reach across and to the bottom of the pen; when stretched across the pen and dropped to the bottom, the lead line is gradually raised by means of lines attached to the bottom corners, thus crowding a limited number of fish into the purse, from which they can be netted) and either transferred alive to a water-filled well boat and transferred to a processing plant, or slaughtered nearby. Harvesting methods are specifically designed to prevent scale loss and release of stress hormones which have detrimental effects on flesh quality.
Handling and processing
Modern processing plants follow strict sanitation and waste disposal procedures to prevent transmission of fish disease, or contamination of the product with potential human pathogens. Before processing, the fish are generally sedated by asphyxiation in carbon dioxide saturated water and/or immersion in iced water. The fish are then bled by cutting the gill arches and immediately immersing them in iced water. Following this, the fish are gutted, washed, graded, and packed on ice. At this stage, the fish can either be frozen or filleted. Generally, premium quality fish are sold either as fresh salmon or frozen. Lesser quality fish are often filleted, trimmed, de-boned or set aside for smoking.
Production costs are dependent on a number of factors, including:
- Farm production capacity.
- Feed cost and conversion ratio.
- Disease status and mortality.
- Smolt cost (in Chile, FOB, 60–80 g are USD 0.45–0.55/smolt).
- Location and associated logistical costs.
- Processing method and product type.
In Chile, production costs range from USD 1.40–1.70/kg live weight, excluding harvest, transport, and processing, assuming that mortality is distributed across the entire production cycle and does not exceed 15 percent (cumulative) and that a feed conversion ratio of 1.15:1 is achieved.