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Diseases and mortality
Comparatively few diseases and mortalities due to infection have been reported in tilapia farms in the tropics. Many of the pathogenic organisms described from the wild stocks only indicate possible infections under culture conditions. Besides a possible natural resistance to disease, the low-density culture practices may have helped to reduce stress and consequent susceptibility to diseases. However, even in low-density culture the high organic loads create conditions suitable for significant bacterial populations to flourish and infect the fish. Several pathogenic protozoans and bacteria have been observed in species of tilapia, but very few of them have been reported to cause major concern. Some of the known diseases seem to occur only in sub-tropical and temperate regions, where over-wintering of fry causes greater stress.
Among mortalities caused by environmental factors, the most important are anoxia following blooms of algae such as Microcystis, Anabaena, Oscillatoria, etc. A sudden loweringof the temperature as a result of environmental changes or the entry of very cold water at a temperature below the tolerance level of about 11°C can create problems including mortalities.
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