Evaluation du potentiel biologique, économique et social de la coralliculture dans le sud-ouest de Madagascar

Abstract

In the context of the global decline of coral reef biodiversity and considering the development of live corals marketing, the coral aquaculture is a promising technique to develop in a near future. In Madagascar, community-based aquacultures are alternative to fishing and to wild organisms harvesting. The present study was conducted within the PIC (“Programme Interuniversitaire Ciblé”) ARES-CCD program titled “Development of community-based coastal polyaquaculture in the Southwest and the North of Madagascar”. The main objectives of the study consisted (i) to identify the biodiversity of the most common scleractinians, to evaluate their recruitment potential and to choose the first best candidates for coral farming (ii) to find out the best method of transport for exporting live farmed corals and (iii) to evaluate the potential of coral farming in the villages of the South-West of Madagascar. The scleractinian biodiversity of Madagascar is mainly known from one study performed in the Bay of Toliara (SW of Madagascar) in the seventies. In the present study, we reinvestigated this biodiversity 40 years later and we initiated the implementation of a molecular database based on 18S rDNA fragments as an easy tool for identification of adults and recruits. Results showed lower species diversity compared to the previous studies and to similar sites in the Indian Ocean region. However, most of the well-represented genera were recorded. The appearance of previously unrecorded species suggests that the scleractinian communities are changing instead of only declining. In addition, we obtained 18S rDNA sequence fragments for 20 of the most dominant species. Fifteen groups are distinguishable by pairwise comparison: 13 are monospecific, 1 includes 5 species and the last includes 2 species. Sequence fragment of 13 recruits were also obtained and BLAST searches against the adult genotypic database permitted to increase the resolution of recruit’s identification from the level of family (with the traditional taxonomic key) to the level of species or genus. Identified recruits were Pocillopora sp., Porites profundus, P. rus, Pavona clavus and Lobophyllia corymbosa. We finally discuss about the use of the genetic database. The reproduction period of scleractinians in the SW region was evaluated by a weekly monitoring of the presence of coral larvae (planula) in plankton, by a monthly evaluation of the new settled corals (1 year < Recruits) and a monitoring of the juveniles’ recruitment (1< Juveniles < 2 years). Planula was present in plankton 9 months a year. It was abundant from the beginning of the warm wet season (September to November). The annual recorded density of planula varied from 1.30 planula m-3 to 16.17 planula m-3 depending on the studied stations, with a peak larval density in November and December suggesting that most of corals release their fertilized eggs few days or weeks before November/December. Compared to other regions, the recruitment rate observed in the SW of Madagascar was high (100 to > 1000 recruits m-2 year-1). It also varied from seasons and peaks of recruitment was observed between October and December. We observed a huge difference of recruit records and juvenile records between the stations. Results of juveniles monitoring revealed high rates (> 10 juveniles m-2) compared to other regions and to the threshold, but it revealed high mortality of the recruits. The coral species Acropora nasuta and Seriatopora caliendrum were used to experiment community-based coral aquaculture in Madagascar. Suitable rearing techniques were experienced. Survival and growth rate of the coral nubbins were monitored during wet, warm and dry, cold seasons. Coral nubbins were reared at a depth of 1m, with a temperature of 26.31±2.07°C, a salinity of 32.67±1.19 psu, and a sedimentation rate of 0.55±0.28 mg cm-2 d-1. Coral nubbins reared during the wet, warm season showed a final survival rate of 67±6% and 57±4% respectively for A. nasuta and S. caliendrum, while in the dry, cold season, the survival rates were respectively of 85±7% and 69±1%. A. nasuta had a significantly higher survival rate than S. caliendrum during both seasons. During the wet, warm season, growth rates were respectively 0.46±0.16%d-1 and 0.54±0.16%d-1 for A. nasuta and S. caliendrum. In the dry, cold season, A. nasuta had 0.63±0.18%d-1 of growth rate, while S. caliendrum grew 0.65±0.15%d-1. Significant difference was observed between both species during the wet, warm season, but not during the dry, cold season. Furthermore, both species grew faster during the dry, cold season. These results are in the range of reference values for corals. Transporting live coral nubbins is one of the most important constraints in coral economy. Nowadays, they must be transported by air, and preferably in less than 20h using concentrated oxygen, if not, post-transportation coral mortality drastically increases. In order to better understand the effect of the transportation conditioning the physiology of the nubbins and their mortality after transportation, we performed series of transportation simulations on Seriatopora hystrix and tested different volumes of water (including dry method, 125 ml, 190 ml and 325 ml), different oxygen concentrations in the gas phase of the container (21%, 40% and 85%) and enlightened transportation box (with 24 LED Handy Lamp which provided light irradiance of >100 μmol.m-2.s-1 in the box during the first 12h). Nubbins continued to grow during transportation and pumped calcium carbonates, which rapidly reduced the alkalinity of the water and therefore the pH. Dissolved oxygen was also rapidly reduced. The concentration of inorganic nitrogen, particularly the ammonium ions increased after 24h and abruptly reached maximum and lethal values at 48h. The dry method was inappropriate for small coral nubbins rather than wet methods. In less than 48h, the higher the water volume was, the better the nubbin grow after transportation. Yet, in less than 48h, the higher the oxygen concentration was, the better the nubbin grew after transportation. Beyond 48h, in all cases, the water conditions became abruptly critical for the nubbins and can induce their death. To determine economical feasibility, the coral market was investigated and the yields were also calculated using the technical, biological and social parameters of the production. Then, the appropriate business model and appropriate clients were determined. The activity is profitable from less than 76 coral nubbins sold per month. Profit can already be perceivable from the second year and a total of more than EUR 27,000 is earned after 5 years of developing project, for an initial investment of EUR 1,978. Marine animals retailer companies and biodiversity conservation NGOs are the appropriate clients and partners for coralliculture farming in Madagascar. The findings of the present study proved that community-based coral aquaculture is technically and biologically feasible using very low coast materials and spending negligible working time for the community compared to fishing and practicing other types of aquaculture like sea cucumber or algae farming. Also, the economical evaluation of a coral farming project also proved that it could be fully profitable at a small-scale production, notably a community scale. Thereby, it could be an alternative source of revenue to the fishermen communities and contribute to reduce the fishing pressures to the coral reefs. But the integration of the coral farming within existing and well- established community-based polyaquaculture is suggested to ensure its viability.