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The impact of organic wastes released from shrimp aquaculture on the surrounding environment has been shown (Biao et al. 2004; Das et al. 2004; Sara 2007). It depends not only on the level of its confinement, but also on the quantity of wastes introduced within this environment. This quantity of wastes depends mainly on the intensification of the rearing system. As suggested by Fuchs et al. 1998 there are three possible ways to increase the waste fluxes:

1. The size of the shrimp aquaculture farm: The impact on an ecosystem increases when an important surface of rearing ponds is located near the same ecosystem.

2. The annual number of rearing cycles: for instance, in inter-tropical countries, because of the temperature of the water, three cycles can take place each year).

3. The stocking density (according to the different rearing systems: it can reach up to 100 shrimps m-2 pond). The quantity of generated wastes depends on two main factors: the quantity of pellets added into the ponds and the food conversion ratio (FCR).

The FCR indicates the quantity of feed pellets used to produce 1 kg shrimp. Thus, for FCR = 2, for example, 2 kg feed pellets are used to produce 1 kg shrimp. This means that FCR = 2 leads to the formation of 1 kg organic wastes per kg shrimps harvested. Thus, in some sites hosting 1,000 ha ponds with a production of 6-10 t shrimp ha-1 year-1 and a FCR = 2, the quantity of waste rejected into the surrounding environment ranges from 6,000 to 10,000 t of organic wastes. Some sites can host up to 16,000 ha ponds (Fuchs et al. 1998).

The quality of management of the rearing system may have a huge influence on the formation of wastes (Biao and Kaijin 2007). Thus, the quality of the food (Chim et al. 2001), the rhythm of feed pellet distribution, once or several times per day (Della Patrona and Brun 2007), the water renewal (Lemonnier and Faninoz 2006), and the pond water aeration (Della Patrona and Brun 2007) make a huge difference in the FCR performance (Della Patrona and Brun 2007) and consequently in the formation of wastes. One of the main parameters controlling the FCR is the stocking density. Figure 1a, shows (Martin et al. 1998) that, for ponds of the same zootechnic conditions, the weight of the shrimp at harvest decreased when the stocking density increased. At the same time, FCR increased along with an increase of the stocking density (Fig. 1b). In fact, it has been shown that in earthen ponds, the natural food chain participates to a large extent in the nutrition of the shrimp. Thus, for example, natural prey (copepods, nematods, harpacticoids, etc.) may represents 42% of the ingested food for Metapenaeus macieayi (Maguire and Bell 1981), from 37% to 43% for Penaeus japonicus (Reymond and Lagardere 1990), more than 50% for Penaeus monodon (Focken et al. 1998), and up to 84% for Penaeus subtilis (Nunes et al. 1997).

Fig. 1 Relationship between the stocking density and (a) the weight of shrimps at harvest, and (b) the food conversion ratio (FCR)

For a same species, the variation of the proportion of natural food in the total food intake is inversely proportional to the stocking density (Della Patrona and Brun 2007). This explains the increase of the FCR along with the socking density (Fig. 1b). Before being removed from the pond, with the effluent or through mechanical means, the organic wastes accumulate in the pond at the water-sediment boundary layer level. This accumulation can lead to dystrophy in the pond ecology. Thus, it has been shown (Martin et al. 1998; Lemonnier et al. 2002) that the one of the consequences of the accumulation of organic matter is an increase in the concentration of nutriments in the sediment (Burford and Lorenzen 2004). Figure 2 shows the relationship between the concentrations of N-NH3-4 in the 1-cm top layer of the sediment in ponds with increasing shrimp instant biomass. Thus, from very low to very high biomass (up to 800 g m-2), the concentration of N-NH3-4 can reach up to 7,000 mM mL-1 in pore water. Furthermore, the accumulation of organic matter in the pond leads to a very high consumption of oxygen. Figure 3 shows, as a consequence of this consumption, the redox potential measured in sediment cores sampled in ponds with increasing shrimp instant biomass. A decrease in the redox potential occurs from the lowest instant biomass to the highest one. Thus, for the highest instant biomass, the redox potential ranged from »-100 to »-300. This very low value explains the high concentrations of nutriments measured in highly intensified rearing ponds (Suplee and Cotner 1996).

Parameters relevant to the quality of the management of the ponds may explain the variations of the quantity of wastes released from the ponds to the surrounding environment. Nevertheless, some parameters that are not dependent on management may have an influence on the rearing performances, and, as a consequence, on the quantities of wastes produced. Thus, for similar management, variations in yield have been noted in relation to the season and are mainly attributed to the difference in salinity and/or temperature and to natural food availability (Scura 1995). A decrease in daylight intensity for several days because of seasonal cloud cover has been shown to lead to a decrease in shrimp production, an increase in the FCR ratio, and an increase in waste formation (Garen and Martin 2002).

The relationship between aquaculture and environment is not only related to the level of confinement of the environment hosting the activity, and to the quantity of wastes introduced within this hosting environment, but also to the presence or not of mangroves. Needing warm water, shrimp aquaculture is mainly located within or near the tropical or equatorial areas. Furthermore, the activity needs huge surfaces to build the ponds. In order to minimize the functioning costs (pumping), the ponds are located very close to sea level. This is why during the huge development of shrimp aquaculture in the 1980s, the easiest and less expensive method for pond implantation was to eradicate the mangroves to build the ponds

Fig. 2 Relationship between the concentration of N-NH3 4 in pore water and the instant biomass in ponds

Fig. 3 Relationship between the potential redox values and the instant biomass in ponds

(Primavera 2005; Thu and Populus 2007). It is now well known that mangroves play an important role in the equilibrium of the coastal ecosystem (Blasco 2002).

It is not only a very productive ecosystem (De Graaf and Xuan 1998) showing a very high diversity (Carvajal and Alava 2007), but also a very efficient filter for a large variety of elements, particulate or dissolved, mineral or organic, issued from the catchment basin (Thampanya et al. 2006), or from aquaculture activities (Rivera-Monroy et al. 1999; Gautier et al. 2002). Destruction of mangroves has a huge impact on the surrounding environment (Hong 1996), including soil acidification (Mitra and Bhattacharyya 2003). Mangrove clearance has also been shown to play a leading role in the decreasing performance of coastal marine fisheries (De Graaf and Xuan 1998) and biodiversity (Fondo and Martens 1998). The main environmental impact of shrimp aquaculture is the destruction of coastal mangroves and deterioration of water quality. Many aquaculture activities, located in areas where mangroves had been destroyed, collapsed after a few years of activity. This was mainly due to an increase of the trophic level up to dystrophic conditions in the coastal ecosystem (Populus et al. 2004) under the pressure of the input of organic matter from aquaculture and the input of particles from the catchment basin. The consequences of mangrove destruction on the performances of aquaculture and of the speed of the collapses are directly related to the level of confinement of the surrounding environment (Populus et al. 2004).

The relationship among (1) the two main kinds of aquaculture (extensive and intensive), (2) the capacity of the surrounding ecosystem to change its water (confinement) and, consequently, to accumulate wastes, and (3) as a consequence the trophic level of this surrounding ecosystem is shown in Fig. 4.

We see that there is an inverse relationship between the water change capacity and the potential accumulation of organic matter (wastes). These characteristics determine the trophic level of the hosting ecosystem. In low confined ecosystems with high water capacity

Ecosystem features

Organic matter accumulation capacity

Type of

Organic matter accumulation capacity

Water Renewal Capacity

Fig. 4 Positioning the different types of aquaculture and their potential performances, according to the trophic characteristics of the surrounding environment (maximum production is expected in the oligotrophic area for intensive aquaculture,

High confinement

- Dystro/hypertrophic

- Trapping of organic matter

- Oxygen deficit

- Sulphate Reducing Bacteria

- Denitrification

- Very high heterotrophic bacteria abundance.

- Sinking of small particles..

- Eutrophic

- High primary and secondary production

- High phytoplankton concent.

- High heterotrophic bacteria abundance ...

- Oligotrophic

- Low phytoplankton concent.

- Low bacterial abundance

- High oxygenation capacity

- Sinking of large particles.

Low confinement

Water Renewal Capacity

Fig. 4 Positioning the different types of aquaculture and their potential performances, according to the trophic characteristics of the surrounding environment (maximum production is expected in the oligotrophic area for intensive aquaculture,

Type of

Capacityof production

(The darker the colour, the higher the production)

Capacityof production

(The darker the colour, the higher the production)

while the maximum occurs in the eutrophic area in extensive production. In the dystro/hypertrophic area, except for some species, aquaculture is not possible)

change, oligotrophic features prevail (low phytoplankton concentration, low heterotrophic bacteria abundance, high oxygenation capacity, etc.).

In this kind of ecosystem, intensive aquaculture that does not need a productive environment may be highly successful and sustainable. In such an environment, when rational management is carried out, no degradation of the environmental characteristics occurs, insuring the sustainability of the activity. On the contrary, ecosystems with a high level of confinement show hyper/dystrophic features, either naturally or under the pressure of the organic wastes released from aquaculture activities. In this kind of ecosystem, aquaculture activity, either intensive or extensive, showed very poor or no performance. In some ecosystems, particularly confined and dystrophic, or those becoming such, no rentable aquaculture activity is possible at all. The degradation of the environmental characteristics because of the input of organic wastes in a confined environment explains the many collapses of shrimp aquaculture activities worldwide in the 1980s (Martin 2004). Concerning extensive aquaculture, the best positioning appears to be in eutrophic ecosystems, where the reared organisms can be fed small organisms, such as meiofauna, grown in the food web developing from the input of nutrients and phytoplankton into the ponds (Reymond and Lagardère 1990). Respecting these considerations must lead (1) to insuring the best choice for site selection, both for intensive and extensive aquaculture, (2) to minimizing the impact of wastes on the surrounding environment for intensive aquaculture, (3) to insuring a satisfactory development of the food web inside the ponds for extensive aquaculture, and (4) finally to promoting the best environmental conditions in order to obtain real sustainability.

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