Pond fertilization for production of live foods
The nutritient value of live foods for aquaculture species and the need for essential nutrients in the media for the growth of food organisms. The traditional practice of fertilizing fish ponds is based on this knowledge and the relative economics of fertilizing compared with the use of processed or unprocessed artificial feeds. Even today, the majority of pond culture of herbivorous and omnivorous species is based on food production through fertilization, sometimes combined with supplementary feeding with easily available feedstuffs. In many tropical developing countries, where the priority is to produce less-expensive species to feed low-income populations, use of artificial feeds may not be feasible as it is likely to raise the cost of production, making the product beyond the reach of the majority of the population. In most of these countries manufacture of aquaculture feeds is not well developed and import of large quantities of feed is not practical. Furthermore, the design and operation of pond farms in tropical Asia are also best suited for the growth of natural food organisms. Viewed from the point of view of input/output ratios, such types of farming have to be considered as intensive, even when no supplementary feedstuffs are used.
The production and maintenance of a crop of live food organisms in an aquaculture pond or similar enclosure is rather complex, even though fish farmers in Asia have been practising it for centuries and have made it into an art. There are many factors that affect the growth of live food which can be controlled only to a limited extent in large ponds and enclosures.
The interaction between the underlying soil and the pond water is one of the factors that distinguish algae and zooplankton culture methods described earlier from pond raising of live food. Fertilizers introduced
in a pond ecosystem are intended to support and modify the food chain or food web, where each link depends for its food supply on the lower trophic levels. The first links are generally bacteria and algae. Phytoplankton is the autotrophic link which produces organic matter, whereas the other organisms are heterotrophs that consume organic matter. Besides the fertilizer that is added by the farmer, the pond usually also receives a certain amount of additional fertilization through water introduced from outside sources, for replenishment or maintenance of water circulation in the ponds and through rainfall. A third important factor is that the aquaculture species is raised in the same media where its food organisms are grown, and so continuous grazing by a progressively increasing biomass of the culture species takes place. Besides these factors, the seasonal variations of temperature, photoperiods, pH and density-related interactions influence the growth of live food in ponds. Consequently, the application of fertilizers has to be adjusted according to needs determined on the basis of regular monitoring. Often rough and ready methods are followed by farmers, which sometimes result in the development of unfavourable conditions.
The fertilizers used in aquaculture are inor-ganic or organic in nature, or a combination of both. Probably because of the large bulk to be handled when using organic manures and the variability of its chemical composition, industrially advanced countries have not adopted its use very widely except in a few instances. On the other hand, there is greater reliance on organics in developing tropical countries.
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