5.1 Background
5.2 Research Needs
Control of tilapia reproduction can be attained when they are raised in well-designed cages where no access to the natural breeding grounds is allowed. The eggs and sperms of the ripe fish which spawn pass through the cage netting and are lost and therefore no fertilization occurs. Besides impeding reproduction, cages also limit the space available for each fish to move, thus saving energy for growth purposes; a build-up of waste metabolites does not occur since there is a continuous water exchange. This allows the production of fish of more uniform sizes compared with tilapia of both sexes raised free in ponds, where the population density is controlled through use of predatory species. A better control of the growth rates also allows for a more precise adjustment of feeding rates, with consequent advantages in the costs of production.
On the other hand, the main drawback of this system is that fish living in confined cages do not have access to natural foods, unless a filter-feeding species is raised in cages located in a plankton-rich environment. Fish require a more or less complete feed, bringing with it all the problems associated with costs and availability. Another constraint refers to the cost of construction and relatively short life span of the cages, depending on materials being used. If the cages are used in natural water bodies or in artificial multi-purpose reservoirs, the cost of the cages may be offset by the fact that no expenses are incurred in pond construction. Other constraints deserving consideration are water quality changes brought about by the introduction of many cages into a particular environment, parasites and diseases which attack fishes held in a very confined space, and the accessibility of caged fish to poachers.
Small-scale cage experiments have been conducted in El Salvador using T. aurea, in Puerto Rico (Pagán-Font, 1977) with the same species, and in Brazil and Colombia using T. rendalli. In the first two countries, the economic viability of this system with the utilization of a complete feed has been proven. In Colombia, T. rendalli, the herbivorous tilapia, has been raised to commercial size in cages using leaves of a terrestrial macrophyte (Alocasia macrorhiza, Araceae) and other plant material, with acceptable results in terms of fish growth and yield; no particular problems arising from the use of fresh low-protein leaves as the sole source of food were confronted, although growth was enhanced through the addition of a cereal by-product.
There appears to be a definite potential for cage culture of the tilapias in many Latin American countries, both on a commercial scale and as a subsistence-type of fish culture for people living adjacent to water bodies. Therefore, investigations at the Centre probably should include some experimentation on this culture system from the first phase on.
Species selected for this purpose should be the same as those chosen for pond culture. Tilapia rendalli also deserves to be included because of its likelihood of being used by small farmers who cannot afford to buy fish feeds for omnivorous tilapias, but could grow Alocasia macrorhiza or other plants as food for the herbivorous T. rendalli. Studies identified as relevant to this subject include the following:
(i) size and design of cages from the functional and economical viewpoints;(ii) stocking densities;
(iii) feed preparation and feeding techniques;
(iv) fish growth, and yield per unit size;
(v) placement and securing methods for cage units in lakes or reservoirs;
(vi) water quality and other changes occurring in the water body consequential to the introduction of cage culture;
(vii) economics of the system.
