Tilapia culture in trinidad and tobago: yet another update


MEASURING THE EFFECT OF FERTILIZATION



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MEASURING THE EFFECT OF FERTILIZATION

Fertilization can be measured by the abundance of phytoplankton. When phytoplankton is abundant, the water becomes a turbid green or brownish color. If the pond water is not very muddy, the turbidity caused by phytoplankton can serve as a measure of phytoplankton abundance. When using a disk and when it disappears from sight it is the Secchi disk reading. See figure 2. (Bocek, 2009)





Figure 2. Showing the use of a secchi disc (Bocek, 2009)

FOOD CHAINS

The nutrients in chemical fertilizers are "food" for green plants, and have no direct food value to fish. Chemical fertilizers when added to a pond cause the phytoplankton to become more abundant. It is then consumed directly by fish or by zooplankton and insects, which are subsequently eaten by fish. This step-by-step process is called a food chain. See figure 3 below. (Bocek, 2009)



Adding manure instead of chemical fertilizer to a pond eliminates a step in the food chain since many fish will consume manure directly. Manure is consumed by zooplankton or insects which are later eaten by fish or it may be decomposed by bacteria and other organisms. Assimilation by phytoplankton occurs when nutrients are decomposed. A simplified food chain illustrating direct and indirect consumption of fertilizer nutrients by fish follows. See figure 3 (Bocek, 2009)






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