Biological and technological basis
The rationale of aquaculture is not limited merely to socioeconomic and marketing advantages. There are also scientific principles that weigh very much in favour of aquatic farming of fish and shellfish. It is a relatively efficient means of producing animal protein which can compare very favourably with poultry, pork and beef in the economies of production, when appropriate species and techniques are adopted. Poikilothermic (cold-blooded) animals, especially fish, have relatively low energy requirements, as they do not spend any energy for the maintenance of a constant body temperature and the energy spent for routine locomotory activity is normally low. Since the specific gravity of their bodies is nearly the same as that of the water they inhabit, loss of energy in supporting them-selves is minimal. These advantages result in higher growth rates and greater production per unit area, taking full benefit of the three-dimen-sional nature of water bodies. Filter-feeding sessile shellfish, such as oysters and mussels, spend very little energy in obtaining their food. Fish are highest on the comparative list in terms of gross body weight gain and high in terms of protein gain per unit of feed intake (Hastings and Dickie, 1972). When fed balanced diets under favourable environmental conditions, the feed conversion ratio (wet weight gain per unit of dry feed intake) has been found to be in the range 1:1 to 1:1.25. The protein efficiency ratio (weight gain per unit of protein intake) is either equal to or higher than that for poultry and higher than for swine, sheep and steers (Hastings and Dickie, 1972). Fish are able to utilize high levels of protein in the diet, whereas in poultry almost one-half of the amino acids are deaminated and lost for protein synthesis. A weanling pig may lose as much as two-thirds of the amino acids through deamination.
The absolute economics of a culture system depend very much on the species, production technology and market conditions. Basically, low trophic feeders can generally be raised at lower costs than those which are high in the food chain and which thus require a higher proportion of proteins, particularly animal proteins. However, the latter species usually fetchhigher prices in the market place and compensate for the higher production costs. Aquaculture offers the option to produce low- or high-cost products, and it is up to the farmer to decide which. However, it has to be remembered that many types of proteins that are not consumed by man can be upgraded through aquaculture to produce highly acceptable and well-relished products. Very often, waste products of capture fisheries and animal and crop farming form the main basis of aquaculture feeds. Also, much of present-day aquaculture is based on the natural fertility of soil and water, supplemented by organic or inorganic fertilizers and the plentiful energy of the sun.
In certain situations, the application of aquaculture technologies is an inevitable necessity and not a matter of choice. The case in point is of species or populations that have been decimated by overfishing or environmental perturbations. Culture techniques have to be used to prevent the extinction of species that are ecologically or economically important to the environment. The diminishing salmon stocks in river systems of countries in the northern hemisphere and their slow rehabilitation through environmental improvements and repopulation with hatchery-produced smolts are probably a good example of the role of fish propagation. Similarly, recreational fisheries and aquaria are largely dependent on the application of culture techniques.
Irrigation and hydropower development projects, as well as land reclamation, have seriously affected fishery resources in many areas. At the same time, some of these projects have resulted in the creation of vast reservoirs that require the development of new fishery resources to compensate for the losses incurred. The potential for the application of culture techniques in developing fishery resources has been clearly demonstrated in many countries such as the former USSR (Volgogradskoya and Tzimljanskoye reservoirs), China (Taihu Lake), India (Damodar Valley Corporation and Mettur reservoirs) and the USA (TVA reservoirs).
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