Harvesting and processing
In the type of high-density culture adopted for trout and salmon, harvesting does not pose any special problems. In land-based large pond farms and in enclosures, seining is adopted. In most cases ponds are drained and the fish gathered in a sump. Methods of harvesting from cages and raceways have been briefly described. As the marketability of the high-valued salmonids depends very much on quality, it is necessary to ensure that the fish are handled with the utmost care and speed during harvest, sorting and transport. The best way for long-distance transport of trout is in live tanks.
Portionsize fish are usually allowed to die by suffocation, but larger fish have to be slaughtered as quickly as possible. If allowed to die slowly, lactic acids are released into the tissues and these will accelerate the autolytic process and deterioration of the flesh. Where possible the fish are kept in a suitably-sized tank (for example 2 x 1.5 x 1.5m) with water and killed instantly by passing electricity (about 600 volts) into the water. The practice of bleeding carcasses in Norway has been described.
Portionsize trout and salmon are preferred in many markets in the live or fresh condition. Both small and large fish are also marketed in the frozen, smoked (hot- or cold-smoked), filleted and packaged forms. The product form naturally depends on the consumer preferences, but the demand for ready-to-cook products is increasing in most urban areas. Much of the success of Atlantic salmon farming appears to have been due to the high quality of smoked salmon exported to lucrative markets. Hot-smoking is performed mainly for smaller fish, which are partially cooked in the process. Larger fish of around 2.5kg are split and, after removal of the backbone, are smoked slightly above the ambient temperature. For hot-smoking, fish are brined by immersion in a brine solution for two to five hours depending on the size of the fish and then smoked at different temperatures, starting with 30°C for half an hour, followed by 50°C for another half hour and then 80°C for three-quarters to one hour. The smoked fish are cooled down to 4–10°C before being packed. If quick-frozen, they can be stored for about a year; otherwise they will stay fresh for only a week at low temperatures of 0–4°C. For cold-smoking, a fish with a fat content of at least 15 per cent of the body weight is preferred. Large fish are gutted and cleaned and the sides cut back from the neck to the tail or the vent. The sides which are carved across are salted by placing them on a layer of salt. After 12–14 hours they are washed to remove the surface salt. Another method is to immerse the fish in 8–10 per cent brine solution. Dry-salted or brined fish are dried in cool air and smoked at ambient temperature, not exceeding 28°C, in natural or forced-draught kilns.