Jean-Louis Martin and Jacques Fuchs

The goals of the program were to define ecological “integrating indicators” allowing (i) to improve the process of site selection for shrimp aquaculture (ii) a more objective representation of the site quality in terms of potential production capacity (expressed as tons of shrimps.ha^-1 ponds^-1-year^-1); (iii) the assessment of the activity impact on the environment.

The ecological investigation of several coastal ecosystems has led to identify three main types of sites hosting aquaculture, mostly as a function of the continental influence. It went from deltaïc zones with highest continental pressure (in terms of freshwater and detritic organic matter) to coralline type coastlines with virtually no influence at all. The aquaculture impact could be best summarised by increased concentrations in total suspended matter, particulate organic matter and sulfate reducing bacteria. The organic matter either comes from the ponds or is from telluric origin, due to mangrove eradication when building the ponds. The results have provided a way of advising the best possible location for aquaculture farms within a given ecosystem and furthermore a way of building a reference state in order to determine the impact of the activity on the coastal environment.

An evaluation of the production capacity of the study sites has been conducted by correlating production data (in tons per hectare) and environmental data, namely selected ecological integrators such as total suspended matter, nature of particulate organic matter (detritic or phytoplanktonic), phytoplankton populations diversity (percentage of cyanobacteria and sulfate reducing bacteria). A preliminary scale integrating the range of these environmental parameters with shrimp production levels has been built aiming at: (i) assess the potential production capacity of a site, (ii) facilitate the positioning of shrimp farms in each kind of ecosystem, in relation with its structure and the level of the selected ecological indicators, (iii) anticipate the production trend of a site according to environmental changes induced by waste discharge.

Jean Blanchot (IRD)

The Takapoto atoll lagoon is largely dominated by picophytoplankton, which is poorly ingested by black-pearl oyster Pinctada margaritifera. The cyanobacteria (Prochlorococcus and Synechococcus) are not ingested and only picoeukaryotes are partly ingested. In the water column, the picophytoplankton is cropped by heterotrophic protozoan which constitute a potential trophic link between the dominant primary producers and the oysters. To check this hypothesis the heterotrophic protists community of Takapoto Atoll was investigated, and grazing experiment were done. The abundance and carbon biomass of protists >35 µm (ciliates and dinoflagellates) were ca. 1600 cell l-1. The ciliates ingested efficiently reared picocyanobacteria and had a growth rate up to several generations per day. Pinctada margaritifera exhibited a high clearance rate (ca 20 l h-1 g-1). Grazing experiments showed high retention efficiencies of ciliates (>90 %) and dinoflagellates (99%) by the pearl oyster. On the contrary, in the field, the low retention efficiencies of Synechococcus (0.2%) and pico/nanoeukaryotes (30%) resulted in a poor contribution to the diet of the pearl oyster. A ciliate, Protocruzia, was maintained in culture and used as a picoplanktivorous protozoan model. After being biolabelled with autofluorescent Synechococcus and offered as a prey to pearl oysters, its presence in stomach contents demonstrated the ingestion of heterotrophic protists by the bivalve. The estimation of the energy transfer between picoplankton and phagotrophic protists was obtained from the growth characteristics of Protocruzia (generation time 4 h). This kind of trophic link between picoplankton and pearl oysters could provide an additional available food resource which contributes to explain the vigorous growth of Pinctada margaritifera in an oligotrophic atoll lagoon dominated by very small sized primary producers and devoted to aquaculture.

Yves Auda (UPS Toulouse)  

It is well known that one of the main applications of high resolution satellite products is related to cartographic inventories. The various approaches that can be adopted according to the scale and the aim of the map have been detailed in an atlas recently published (Spadling & al.). Some studies related only to the quantification of a few parameters (specific measurement of a leaf index, estimation of biomass, ...) without approaching the aspect of spatial distribution. In our case, mapping is an invaluable help for many topics such the distribution of natural vegetation, crops rotations, physiological status and hydric stresses, etc 

To map an area, three data types are generally used :

Collection of field data is absolutely essential. Several recommendations can be given :

The aims of a study governs the choice of the spatial scale. For Gambas, we suggest to work on two different spacial scales. The smallest scale 1/50 000 will lead to the establishment of a general map while the largest scale will be appropriate for the production of maps of the sites selected after the field survey.