Considérations sur la recherche agronomique en matière de fertilisation en conditions tropicales

Considerations in agricultural research with special reference to the use of fertilizers under tropical conditions. Although since long known by African fishermen it is only in the last 40 years that Tilapia has been recognized as one of the most promising groups of fish species for culture. The initial successes for culture in Central Africa were followed by several failures mainly because of excessive breeding and early sexual maturity in shallow waterbodies as ponds. From the present knowledge it appears that tilapia has a great future for increasing the productivity in unmanaged environments as man-made lakes and reservoirs primarily destined for the production of hydro-electricity. Careful stocking of paddies and irrigation canals can solve a number of biological problems associated with them and provide an additional though valuable high-protein food source. Great future offers also the culture of tilapia in traditional pond culture especially in polyculture with members of the carp family, mullets and waterfowl in areas of the tropical and subtropical belt. In coastal ponds T, mossambica is a valuable species for sanitary reasons. The culture of tilapia in small farm ponds often meets with failure owing to excessive breeding and stunting unless the all-male technique can be applied through government input and encouragement. As a rule this type of production will be the least attractive. Although Tilapia spp. do not achieve the largest individu al growth their tolerance towards adverse conditions and their acceptance of a wide variety of foodstuffs, primarily waste products from agriculture, their resistance to diseases and (at least in some species) their tolerance of crowded environments make them suitable subject for cultures in raceways, circular tanks and cages. Through heavy inputs of water and pelletized feeds nearly incredible annual yields as 2 000 tonnes per ha of water surface (1) and more were realized. This means that this type of production surpasses by far any other known form of animal husbandry but it needs high technological input (thus capital) and skiII which are seldom available in developing countries where the need for fish is the greatest. Tilapias gained rapid popularity as cultured species in many parts of the world but their often indiscriminate transplantation and the fact that cross-breedings between species occur very easily resulted in hybrids which are often impossible to identify. In view of the genetic work for producing fast growing and late maturing stock lines it is necessary to conserve what pure stocks still remain in the world. This can only be achieved by an international institute