The trout species of the greatest importance in aquaculture is undoubtedly the rainbow trout (Oncorhynchus mykiss) (fig. 17.1). Native to the Pacific Coast drainages of North America, the rainbow trout has since 1874 been introduced to waters on all continents except Antarctica. Its range extends into low latitudes, at higher elevations. Trout waters are maintained in the upland areas of many tropical and subtropical countries of Asia, East Africa and South America, and commercial trout farming has developed in Central and South America and to a limited extent in some Asian and African countries like India and Kenya. As can be expected in a widely distributed and adaptable species like the rainbow trout, several local forms have developed, some of them described as distinct separate species or subspecies (see MacCrimmon (1971) for a list of some of them). Several strains have also been developed through mass selection and cross-breeding for improved cultural qualities.
The brown trout (S. trutta) is the indigenous trout of Central and Western Europe, and was the first trout to be artificially propagated. It has also been introduced into several countries around the world for developing sport fisheries, by stocking natural water bodies. Because of its slower growth rate, poor utilization of artificial feeds and stringent water quality requirements, commercial cultivation of this species has not developed to any appreciable extent, as compared to the rainbow trout.
Another trout which has received some attention is the brook trout (Salvelinus fontinalis), native to north-eastern North America.The species was introduced into Europe towards the end of the 18th century, and to a number of other areas where the water temperature is between 12 and 14°C (not higher than 18°C). Because of the rather demanding environmental requirements and susceptibility to infectious diseases and water pollution, its culture is not very common now in spite of its fast growth rate under favourable conditions and the high level of acceptability by consumers.
As indicated earlier, it is the rainbow trout that has now become the mainstay of large-scale salmonid aquaculture on a worldwide basis. The two main varieties of importance are the sea-going form known as the steelhead and a land-locked fresh-water form. The steelhead grows very rapidly in salt water, reaching 6–9kg in the sea in about three years. The fresh-water form, which is the one usually used in commercial aquaculture today, attains a weight of 4.5kg or more under favourable conditions. Sedgwick (1985) reported that this fresh-water form attains weights of up to 9kg in the South American Andes, indicating the potential of this species under suitable conditions. Even under normal conditions, the rainbow grows faster than all other trouts and is more adaptable to higher temperatures than the others, which enables its cultivation under a wider range of climatic conditions and the utilization of higher water temperatures for rapid growth rates.
The optimum water temperature in a rainbow trout farm is below 21°C, although the lethal limit is in the region of 25–27°C, in which the animal may survive for short periods but may not grow and be active.
Fish culturists in Europe prefer to maintain temperatures ranging over 10–15°C for as long as possible in their farms. While higher temperatures would assist higher levels of metabolism and growth, it has to be remembered that the dissolved oxygen content decreases as water temperature increases. This situation can, however, be overcome by reducing the stock density or by using special measures to increase the oxygen concentration of the water. The minimum concentration needed for both rainbow and brown trout is 6ppm, but it is not allowed to fall to that level and is maintained in a fully saturated state.
The potential for expansion of trout farming, utilizing the advances of culture technology and product development, is much greater than is generally appreciated. In the vast areas of the South American Andes, where there are no other comparable cold-water species available for aquaculture, there appear to be considerable possibilities for developing large- as well as small-scale trout culture. The experience in trout farm development in the upland areas of Mexico clearly indicates its potential in creating rural self-employment and commercial production, in areas with adequate supplies of good quality water. In countries where commercial trout farming is already well developed, as in Europe, increases in harvesting size from portion size (170–230g) to 350–450g for the fresh fish market and to 1.5–3kg for fillets and smoked trout are expected to stimulate market demand. This can be expected to lead to more intensive production in existing farms and wider utilization of new technologies such as cage farming.