Trouts and Salmons
Culture of trouts and salmons (Salmonidae) originated much later than culture of carp, but greater scientific effort has been concentrated on these groups. A considerable part of the basic information relevant to fish culture has been derived from laboratory and field investigations, especially in the case of trout. Salmonid culture has a relatively long history in Europe and North America. The main interest of early salmonid culturists was hatchery production of young ones for introduction in new areas or to enhance existing populations in the native habitats of the species, mainly to improve or maintain sport fisheries. It is only in the last few decades that the feasibility of applying the available technical know-how to commercial production of this high-valued group of fish has been fully appreciated. The general concept of salmonids as highly expensive fish to raise in farms and as a luxury food beyond the reach of the common man has been brought into question by the rapid expansion of trout farming in Europe. Here production has increased steeply and prices have come down to a level comparable to the less expensive species without affecting the producer’s profit to any appreciable extent. Atlantic salmon aquaculture, initially confined to Norway and Scotland, has now spread over to about 20 countries and, as indicated earlier, Chile has lately taken over as the lead farmed salmon producer country. Owing mainly to constraints of space in protected inshore areas such as the Norwegian fjords, the salmon farms are now set up in the open seas with appropriate modifications in the floating cage set-up (Fredheim and Krokstad, 2002). South Africa is now ready to begin commercial farming of Atlantic salmon, where the coastal water conditions have been found to be compatible for cage farming. As estimated by FAO the global production of farmed Atlantic salmon has increased 3.5 times, from 22554 tons in 1990 to 797560 tons in 1999. As predicted, salmon has become less expensive and is now a common product in many areas (Pillay, 1990); according to FAO, the price has decreased by 47%, from US $5.22/kg in 1990 to US $2.45/kg in 1999.
There are divergent views concerning the impacts of the expanding salmon aquaculture. In some areas intensive cage culture has led to deterioration of the environment, owing to the pollution caused by release of the faecal and excretory wastes as well as uneaten feeds, and also to conflicts over the impact on the wild stocks of salmon, but it appears that the salmon farming industry has brought in suitable mitigatory measures to reduce these impacts. For example in Norway, with a farmed salmon production of about 390000 tons (1998), discharging 4225 tons of P and 20286 tons of N, the average specific loading was estimated as 10.8kg P and 52.0kg N per ton of produced fish, but the latter has been reduced to half that since 1990, mainly through improved feed conversion efficiency achieved by utilization of improved feed quality and better feeding procedures (Bergheim, 2001). Salmon escapes from cages seem to be a serious problem which unless avoided can lead to further conflicts. It has been estimated that out of 150 million salmon and rainbow trout stocked annually in Norwegian cage farms, about 20 million individuals are lost, mainly due to disease but about 2 per cent of the total losses (500000) are escaped fish (Bergheim, 2000).
Another issue that is becoming real is the genetic modification of salmon, even though the possible conflicts seem to be exaggerated. There is growing evidence to show that salmon aquaculture has also some very positive social impacts, as in the case of the Alaskan fishermen who have lost an avocation but have been employed in salmon aquaculture, and in many cases the farming enterprises in remote areas (farmed salmon production is about 1 ton per caput of the population in Faroe Islands) have led to overall economic development. All these factors, however, suggest that there is need for close observation of aquaculture developments and their interactions. There must be serious efforts to develop sustainable aquaculture systems through regulations supported by appropriate research inputs and the ensured participation of all stakeholders concerned.
A major constraint to expansion of salmonid culture is the availability of adequate quantities of water of the required quality. Water quantity requirements depend on temperature conditions and the type and intensity of culture. It has been suggested that a fresh-water rainbow trout farm using surface water in a temperate climate should have available a supply of about 5l/s for every ton of fish produced, although a lower level may be sufficient when temperatures decrease. Because of the need to have clean water of the appropriate temperature, water from springs, bore-wells or clean-flowing streams has been used for culture. Spring water is considered essential for a trout hatchery and is recommended for use in rearing up to swim-up stage. However, such water sources are limited and the water available may not be adequate. The quantity of water naturally limits the number of ponds or other culture facilities, even when methods of aeration are adopted. Recirculation of water could improve water availability but, as pointed out, the high cost involved restricts its wider use. It is to overcome this limitation that salmonid culturists have turned increasingly to cage and pen farming in fresh or sea water.
Among water quality requirements, the most important are temperature and oxygen concentration. Temperatures around 10–18°C are considered optimal for the growth of rainbow trout and are not allowed to exceed 21°C. Slightly lower temperatures are preferred for other salmonids. Water for salmonid hatcheries usually has 100 per cent oxygen saturation. A pH of 7–8 is preferred, and must be maintained when surface water is used during periods of rain. Spring water may sometimes have a high dissolved iron content and in a hatchery it can precipitate as a result of bacterial action and settle on eggs or the gills of fry. Such water should be avoided or treated to remove the iron before use. Water from bore-wells or cooling water from power-stations (often used to supplement water supplies and to raise water temperature for better growth in cold climates) may be supersaturated with nitrogen. Supersaturation of around 107 per cent can cause gas bubble disease and mortality of fish. Nitrogen absorbed into the blood at a supersaturated level begins to fall to the normal saturation level and during this process the gaseous nitrogen comes out of solution in the blood vessels, causing gas bubbles to form and eventual mortality of the fish. So, such water has to be ‘degassed’ by exposure to air by suitable means, such as allowing it to fall through a stack of perforated aluminium plates. The concentration of CO2 has to be maintained below 10mg/l.