Bases teóricas que sustentan el PEA del diseño de software

In general, cultured species in the case study farms correlate with species typically produced in the region with the exception of a few catfish raised in farm SV1 and pirapitinga in two farms that were not considered in the study on the micro level. Pirapitinga are omnivorous fish that belong to the subfamily Serrasalminae; they originate from the Amazonian basin (FAO, 2006d) and are produced in ponds in China (FAO, 2006d) as well as Vietnam (Leschen, 2003; Vietnam News, 2003). These fish adapt well to any tropical environment, but in the case that they are starving, they are known to attack other living fish (see Vietnam News, 2003) and cause serious bites with their powerful dentition (Robins et al. quoted in Fishbase, 2006c). Similarly, parts of small fish were recovered from the intestines of the pirapitinga caught in the case study ponds. It is likely that the low survival rate of fish in the nursery pond of SV3 (chapter 6.3.1) was partly caused by the presence of this fish

7 Discussion ₁₃₇

species. Farmers stocked this “exotic breed” without being aware of these typical properties or considering an adequate combination of fish with suitable sizes in order to avoid fish killing.

The stocking of different fish species was usually based on the farmers’ preferences and experience as well as the availability and prices of the individual species rather than the creation of an ideal fish combination. It appeared that farmers were usually not aware of the feed base of the individual fish (typically with the exception of the grass carp). However, an adequate combination of fish species at appropriate densities is required to fully exploit the advantages of a polyculture system (e.g. Milstein, 1992). The combination of species in the case study ponds is discussed in more detail in chapter 7.3.4.

In the interviews (meso level), farmers reported stocking 1.6 fish m-2 on average, but nothing is known about the real stocking densities in these ponds. In the case study ponds, the average stocking density was only 1 fish m-2 _{with considerable differences among the ponds,}

ranging from 0.4 to 1.8 fish m-2_{. Also, stocking densities in IAA systems in different regions}

of the Red River Delta varied enormously and ranged from 0.04 to 14 fish m-2, which correlated significantly with fish yields (Luu et al., 2002). Here, the average stocking density was 2.2 fish m-2; thus, it was more than double the amount of the fish stocked in the case study ponds. Also in the case study ponds, higher yields tend to correspond with higher masses stocked; however, there was no clear correlation between those parameters.

Fish densities have to be adapted to the individual pond conditions in order to find the “optimal stocking densities”, which results in a production that is highest in the quantity and quality of fish and is most profitable (Kumar, 1992). Higher stocking densities typically result in higher total production, but at stocking densities above the optimum, fish compete for food, space, dissolved oxygen and are stressed due to aggressive interaction. These factors may all result in decreased fish growth and a higher susceptibility to infection. Normally, the proper density of fish is directly related to the abundance of food (Little and Muir, 1987), which is discussed in the following chapters.

The amount of fish stocked was, to some extent, a result of farmers’ liquidity during the major stocking procedure, e.g. farmers of SV1 had a higher income in the year 2004 compared to the other farmers, which allowed these farmers to purchase more (and partly larger) fish for stocking. Similarly, some of the interviewed farmers reported that they could not buy additional fish because of their lack of money. Also, Martinez Cordero et al. (1999) reported that suboptimal stocking densities were related to farmers’ limited financial resources in Sulawesi.

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Besides the financial aspects, the general seed availability in the local hatcheries also influenced the stocking densities. An instable seed supply is also known to be the case in other areas (Edwards, 2000). Appleford et al. (2003) stated that a reliable source of fish seed in adequate quality and quantity is fundamental to all aquaculture ventures.

When comparing advances achieved in breeding technology in aquaculture with terrestrial animals, relatively speaking, aquaculture is still in its infancy (Delgado et al., 2003). Chinese carps, such as grass carp, are highly fecund river spawners and do not breed naturally in ponds (Rottmann and Shireman, 1992; Beveridge and Haylor, 1998); thus, they need to be produced in hatcheries. The local hatchery operators as well as farmers observed a decrease in the quality of fish seed over the last several decades. Factors that are associated with the decreasing quality are a low growth performance of fish as well as the susceptibility of grass carp to diseases. There is some evidence that the inappropriate management in the local hatcheries (e.g. crossing of fish without considering the degrees of relationship) as well as use of a restricted gene pool led to a genetic degeneration of local fish. Now, the hatchery operators are seeking funds in order to improve their genetic resources.

Genetic degeneration of carp has also been observed in neighbouring China. Ye (2001) stated that most of the brooders of the main cultivated fish in China are derived from wild strains, that little mass selection is applied and that almost no genetically improved strains have been obtained so far. Inbreeding and unsuitable genetic manipulation led to the appearance of genetic degeneration, such as slower growth, poor resistance to disease and earlier maturation.

The quality of the young fish may also be influenced by suboptimal feeding in the hatcheries. After the yolk sac of grass carp larvae has been depleted (1-3 days), larvae start feeding on exogenous food (Rottmann and Shireman, 1992). As in the early developmental stages of almost all fish, the larvae and fry of grass carp feed on small invertebrates, which primarily consist of zooplankton (see 2.3.2). In order to improve the availability of zooplankton, ponds require certain amounts of manure. In rather intensive fry rearing, live food (e.g. freshwater rotifers and brine shrimp (Artemia)) is cultured separately and then fed to the fry (Rottmann and Shireman, 1992; Southgate, 2002).

In the Son La hatchery, however, the first feed given to the larvae following the consumption of the yolk sac is the ground yolk of duck eggs. The feeding of suspended material to fry is usually associated with a number of unwanted effects: the material is not buoyant; thus, the availability of food to the (not yet free swimming) larvae is reduced, the food particles may settle on the tank bottom and pollute the water quality as well as increase

7 Discussion ₁₃₉

bacterial activity (Southgate, 2002). After two days of feeding, fry are transferred to ponds and supplied with germinated rice, soybean and grass and leaf material as well as small amounts of manure. It is questionable whether the applied grass and leaf material functions as fry feed. Grass carp change their feeding habits and exhibit herbivorous tendency at a larger body size (see 2.3.2). However, the applied green material probably acts as a substrate for zooplankton production in the ponds. Hay, for example, has the advantage of encouraging the long-term production of zooplankton (Rottmann and Shireman, 1992). The practice of supplementing natural food with applied feed has been recommended when fry are stocked at high densities (Rottmann and Shireman, 1992). The feeding of egg yolk paste or soybean milk and peanut cake to grass carp and bighead carp up to the age of 30 days has also been reported from China (Jhingran and Pullin quoted in De Silva, 2003a).

The stocking of low quality fish probably influences the whole fish production in farmers’ ponds and might negate farmers’ efforts through better pond management. However, farmers do not only obtain fish from the local hatcheries, lowland fish suppliers also deliver fish seed to the region. In addition, tilapia and common carp reproduce naturally in farmers’ ponds and therefore do not need to be purchased on a regular basis.

The early and uncontrolled reproduction of Nile tilapia may lead to overpopulation in ponds. Normally, this is accompanied by competition for space and food resources among the offspring and a considerable quantum of the energy intake is utilized for reproduction purposes and not for growth. The result of this is “stunting”, where there is a large number of fish of limited size (Suresh, 2003). This was also observed in the case study ponds, where a large number of tilapia with body sizes rarely exceeding 50 g were caught and the small fish then fetch comparatively low fish prices. One method for controlling the reproduction of tilapia is the use of synthetic testosterone to produce sex-reversed, (phenotypic) all-male tilapia, a technique that is used by the Son La hatchery and which may be associated with a number of environmental effects as well as health concerns (see review of Pandian and Sheela, 1995). It is surprising, since all tilapia sold from the main fish supplier in the region are supposed to be males, that the tilapia reproduction in farmers’ ponds is still obviously high. However, the effectiveness of hormone treatment is quite variable and depends on various factors, which makes it difficult to achieve the desired 100% male stock. However, even a small proportion of females may result in significant levels of recruitment (Mair and Little, 1991).

The above-mentioned factors, including the low quality of fish seed, the instable access to stocking resources and the stocking densities that are potentially too low, might

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have contributed to the relatively low yields in the study area. Veerina et al. (1999) found that besides the stocking density, the application of protein and organic fertilizer also has a large impact on carp yields in Andhra Pradesh (India).

7.3.3 Rating the growth of grass carp and the quantity and quality of the applied feed