What is the major problem associated with the pond culture of tilapia?
Semi-intensive culture (SIC) is simply the production of fish, and other aquatic animals, using natural food, through pond fertilization, or fertilization and supplemental feeding. In other words, SIC is a means of producing low-cost fish, through low production inputs, which contributes to hunger alleviation and food security, especially in rural areas in developing countries. It is no surprise, therefore, that SIC systems are adopted mainly for herbivorous and omnivorous fish that feed lowdown in the food chain, such as tilapia and carp.
The production of SIC systems is usually lower than that of intensive systems due to the lower stocking density and production inputs. Taking into account the input/output (cost/benefit) ratio, this system could be more convenient and cost effective for small-scale farmers than intensive farming systems.
Semi-intensive farming of tilapia has been practised for many years in different parts of the world, particularly in South-east Asia, in either monoculture or polyculture systems. Over the past two decades, SIC of tilapia with other herbivorous fish, such as carp, has experienced increased popularity in several countries in the world, particularly among small-scale farmers in Asia. This chapter discusses the different aspects of SIC of tilapia, including pond fertilization, monoculture, polyculture and integrated culture, under different culture and environmental conditions.
An Overview of Pond Fertilization
The main objective of pond fertilization is to stimulate the primary productivity in fish ponds and enhance autotrophic and heterotrophic food production.
Practically, nitrogen (N), phosphorus (P)and carbon (C) are the major inputs in pond fertilization, and the ratio between them is also of prime importance. Potassium (K) is generally not considered a fertilizer element, since earlier studies revealed that K fertilization did not affect pond production. However, K may be required if the K content and alkalinity of culture water are low. The average nutrient composition of phytoplankton in fish ponds is 45–50% C,8–10% N and 1% P, giving a ratio of about50 : 10 : 1. Efficient fertilization requires a number of measures that should be taken into account.
- Availability of considerable amounts of carbon.
- Alkalinity should be > 20 mg/l.
If the water is relatively soft and acidic, with low alkalinity (< 20 mg/l), liming may be necessary to raise alkalinity before the application of inorganic fertilizer.
- Fertilization history of the pond. Understanding the fertilization history, water quality, bottom composition and overall pond dynamics is extremely important for efficient and sustainable fertilization. For example, available P in fish ponds is rapidly removed by reaction with divalent cations, such asFe2+, Ca2+ and Mg2+, forming inactive precipitation.
Pond bottom mud also adsorbs and leads to its removal from culture water. The more mud the bottom contains, the more P it adsorbs.
Therefore, P requirement for pond fertilization depends on the type of bottom soils and their P saturation.
It has been reported that maximum N and P fertilization rates of ponds in acid sulphate soil with high P binding capacity were 4 and2 kg/ha/day, respectively (Asian Institute of Technology (AIT), unpublished data, cited in Yakupitiyage, 1995). Further increase in N loading may lead to ammonia toxicity. There was a negative relationship between the ability of pond sediments to remove soluble P from water and organic matter and P contents of pond bottoms.
- Season of the year also has a significant effect on fertilization effectiveness of fish ponds. Green et al. (1990) found that the production of Nile tilapia in ponds fertilized with chicken manure in Panama was better in dry seasons than in rainy seasons. Similarly, the production of blue tilapia (Oreochromis aureus) in ponds fertilized with chicken manure in addition to 0–20–0 inorganic fertilizer was 48% lessin the wet season than in the dry season(Ledgerwood et al., 1978). Low production during the rainy season was attributed to the increased turbidity resulting from bank erosion following heavy rains, which reduces light penetration and decreases the photosynthesis process.
- Cost-efficiency analysis. Fertilization strategies are often evaluated based on their ability to enhance fish growth and pond yield. Nevertheless, a fertilization regime that produces the best yield may not necessarily be the most cost-effective. It is therefore essential that the farmers understand the dynamic processes in the ponds and how fertilization efficiency is affected by light penetration, pond depth, bottom soil composition and nutrient contents of the water. Economic and ecological analyses of fertilization application are also appreciated, so that the farmer can choose the best fertilization strategy.