10.1 Background
One way to intensify fish culture without an input of expensive feed is through polyculture. In this way the natural food produced in the culture environment is utilized to a greater extent through compatible or complementary feeding habits of fish which do not compete with each other. In order to utilize natural food to its maximum, and as fish may change their food if their regular food resources become exhausted, it is very important to ascertain the correct ratio between the different species in the polyculture according to the ecological conditions in the pond and to adjust them so that they do not compete with each other.
Yields obtained by polyculture are usually much higher than those obtained by monoculture, especially if the right species have been chosen. Other benefits also may be gained by polyculture, as, for example, quite often the ecological conditions in a pond are improved by polyculture. It has been found that Tilapia aurea in a polyculture system improves the oxygen balance by feeding on the detritus which would otherwise decompose and take up oxygen. Silver carp (Hypophthalmichthys molitrix) and bighead carp (Aristichthys nobilis) also improve the oxygen balance by feeding on excess plankton. These fish may also gain by feeding on the excreta of common carp (Cyprinus carpio) or other fish.
The major drawback of polyculture is its complexity. It requires a supply of fry of different species, some of which may not spawn naturally but must be induced to spawn by hormone administration. Larger hatcheries are required; larger pond areas for nursing of fry are needed and a more complicated distribution system must be used. Polyculture also complicates somewhat the culture techniques. It may happen that the growth of one species falls short of expectations and this may require changes in the other culture techniques. Feeding rates may differ, and cropping and sorting may become more difficult. Obviously, polyculture calls for greater farming skill and experience. It is therefore a question whether polyculture should be adopted at this time in Latin America where aquaculture is in its early stages of development. However, one should recognize that polyculture can be practised at different levels of complexity with varying numbers of species. Eventually, with further development of aquaculture and the acquirement of skills, there will certainly be a tendency to move toward polyculture, and tested polyculture techniques should then be ready, at least at the lower level of two species.
Several options are available for simple polyculture experiments, viz.; Tilapia hybrids as plankton-feeders and common carp as a bottom feeder; common carp and Prochilodus species that appear to feed on different strata of the mud; T. nilotica which is a plankton-feeder and Prochilodus; T. rendalli as a herbivore and Prochilodus, etc.
Other valuable components for polyculture are the bighead carp (A. nobilis) and the silver carp (H. molitrix) because they grow fast and do not require artificial feeding, but their introduction to the Centre is not considered advisable at the moment.
