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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