In some countries, mangrove forests are being cleared for aquaculture ponds, as the development of shrimp aquaculture continues to meet increasing export requirements. Degradation of coastal environments, which is attributed to the conversion of the mangrove ecosystem into brackish water ponds, has caused a worldwide decrease in cultured shrimp production, especially in Southeast Asian countries. There are many problems associated with using mangrove areas for shrimp ponds, such as, abandonment of ponds and outbreaks of diseases.

In order to develop an environment-friendly aquaculture system utilizing the natural purification capabilities and high productivity of mangrove brackish waters, water was circulated between aquaculture ponds and mangrove enclosures ( Fig. 1).

Method

Water was circulated between shrimp aquaculture ponds stocked with 10,000 or 20,000 PL-15 stage Penaeus monodon, and mangrove enclosures were planted with 476 Rhizophora mucronata per enclosure ( Fig. 2). About 30% of the water in Ponds 2 and 5 was moved to the mangrove enclosures (Ponds 3 and 4, respectively) every Thursday, and the same amount was returned to the shrimp ponds every Monday.

Shrimp Harvest

The survival rate ( Table 1) was significantly higher (P < 0.001, Fisher's exact test) in Pond 5 where 10,000 larvae were stocked and water was exchanged with the mangrove enclosure compared to the control pond; the feed conversion ratio was lowest in Pond 5.

Phosphorus Budget

Only 11-15% of phosphorus was incorporated into the shrimp bodies; P content in the mud exceeded the amount provided in the feed in Pond 1 ( Table 2), and the P budget was negative. This suggests that extra P was supplied from another source. We believe that anoxia was responsible for the release of P from the lower depths of the ponds. In Pond 2, the budget results suggest that 1.55 kg of P flowed into the mangrove-planted pond. Although the budget results were negative in Pond 5, it was higher than that of Pond 1. If the same quantity of P in Pond 1 had been supplied in Pond 5 from the lower depths of the pond, about 0.43 kg of P should have flowed into the mangrove ecosystem.

Effectiveness of This System

Pond 5 achieved the most efficient level of aquacultural production; the deterioration in the lower pond environment was reduced in Ponds 2 and 5 via the circulation of water between the shrimp and mangrove ponds, in comparison to the control. Because the P budget was higher in Ponds 2 and 5 relative to the control, environmental burden in the ponds was reduced.

Cooperating agency for this topic:

Fisheries Division,

Japan International Research Center for Agricultural Sciences

and Ishigaki Tropical Station, Seikai National Fisheries Research Institute, Fisheries Research Agency

Index of Images

Table 1 Stocked Larvae, Total Weight of Shrimp, Number of Individuals at Harvest, Survival Rate, Average Shrimp Size, Amount of Feed during the Experiment, and Food Conversion Ratio (FCR)

Figure 1 Mangrove Enclosure

Figure 2 Schematic Outline of Shrimp Aquaculture Ponds and Mangrove Enclosure

Table 2 Phosphorus Budget (KG/Pond/Experimental Period) in Shrimp Aquaculture Ponds during the 136-Day Experimental Period

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