Biological characteristics of aquaculture species
A major characteristic that determines the suit-ability of a species for aquaculture is the rate of growth and production under culture conditions. Although certain slow-growing species may be candidates for culture because of their market value, it is often difficult to make their culture economical. But recent events have shown at least one clear instance of a slow-growing and small-sized species (86mm maximum size) of freshwater prawn, namely
Macrobrachium nipponense, that has beenrecruited into farming in China where the consumer demand and aptness to local environmental conditions have led to its production reaching 15000 tons in 1998 (Wang and Qianhong, 1999). It is also of interest to note that the much smaller local prawn is likely toreplace the introduced M. rosenbergii, which has problems of adjusting to winter temperatures in China as well as genetic deterioration owing to inbreeding (New and Valenti, 2000). Growth rates of many species can be improved through the use of heated water, but commercial growout using such methods has not yet proved very successful. In principle, a faster growth rate, as obtained in many tropical species, allows them to grow to marketable size in a shorter time, making it possible to have more frequent harvests. The size and age at first maturity is also an important consideration, as it will be preferable to have them reach mar-ketable size before they attain first maturity so that most of the feed and energy are used for somatic growth. Early maturity would ensure easier availability of breeders for hatchery operations, but early maturity before the species reaches marketable size will also be a great handicap, as in the case of tilapia species.
It is certainly preferable to culture a species that can be bred easily under captive conditions. This permits hatchery production of seed in adequate quantities. If it is a speices that matures more than once a year, it should be possible to have several crops of seed and possible adults, if other conditions are suitable. High fecundity can be an advantage, as can frequency of spawning; however, small-sized eggs and small larvae make hatching operationsmore difficult. A shorter incubation period and larval cycle often contribute to lower mortality of larvae and greater survival in hatcheries.
Larvae that would accept artificial feeds would be easier to rear in hatcheries. The raising of live foods is comparatively more difficult and often expensive.
In cases where controlled breeding techniques have not been perfected, the aquaculturist may have to depend on seed available from the wild. But as has been experienced in many situations, it proves to be an unreliable source in large-scale farming as their abundance in nature depends on a number of unpre-dictable factors. Further, large-scale collection of wild spawn and fry has given rise to conflicts with commercial fishermen, who ascribe the decline in catches of the species concerned to the removal of early stages, despite the lack of any scientific evidence. So, even from a public relations and biodiversity point of view, it is better to select species that can be propagated in hatcheries and to start hatchery production as early as possible.
In modern aquaculture, feeding is one of the major elements of production cost and may amount to 50 per cent or more. Nutritional requirements of aquaculture species.. In most traditional aqua-culture practices, herbivorous or omnivorous species have been preferred as they feed on natural food organisms in water, the growth of which can be enhanced through fertilization and water management. In such cases, the cost of feeding will be relatively low and, because of this, species low in the food chain are preferable for the production of low-priced products. However, even with such species, supplementary feeding with artificial feedstuffs has to be adopted in intensive culture systems. The feed efficiency in relation to growth and productivity then becomes an important criterion. Some of the low trophic level feeders can also be highly selective in their feeding habits, as in the case of filter-feeders that require plankton of a particular size and shape. The need to grow the species to market size within a limited season or period often makes it necessary to resort to artificial feeding. Further, with improved feed conversion efficiency, manipulable through N and P reduction in artificial feeding, a reduction in nutrient loading can be achieved, thus leading to more ecofriendly and sustainable culture systems, as shown for example in salmon (Makinen, 1991; Bergheim, 2000) and carp (Jhan et al., 2001).
Carnivorous species generally need a high-protein diet and are therefore considered to be more expensive to produce, even though the costs will depend largely on local availability and price of the required feedstuffs. To compensate for feeding costs, most carnivorous species command higher market prices. Such species generally have greater export markets and therefore attract substantial investments.
Species that are hardy and can tolerate unfavourable conditions will have the advantage of better survival in relatively poor environmental conditions that may occur occasionally in culture situations. The temperature and oxygen concentration can fluctuate in ponds and other enclosures and deterioration of the water quality may unavoidably occur. In such situations, hardier species will obviously fare better. Besides the possible effects of poor water quality on the candidate species, it is also necessary to consider the influence of the species on the environment. Soil erosion that may be caused by the feeding habits of carp has been referred. Species that easily escape into natural bodies of water and upset their ecology would need special protective measures, leading to higher costs and environmental concern.
In intensive and semi-intensive culture, dense populations are confined in a limited space. In such cases, behaviour patterns of species in confinement are of special significance. Increases in transmission of disease, cannibalism in the early stages and accumulation of waste products are related to overcrowding. Species that have better resistance to such unfavourable conditions are better candidates for culture.