Résumé
BackgroundUnderstanding the functioning of coral reefs and their resilience to disturbances requires studyof sites subjected to contrasting anthropogenic pressure while undergoing similar climaticforcing. The SIREME program (Monitoring and inventory of coral reefs of Mayotte and theIles Eparses, 10e FED Indian Ocean) was conducted in the Indian Ocean to measure andelucidate the state of health of coral reefs at Mayotte, where the reefs are subject to highanthropogenic pressure, and Iles Eparses (Glorieuses Islands and Europa) where thesepressures are minimal.MethodsExpeditions in 2015 at Glorieuses, and in Mayotte and Europa in 2016 enabled monitoring ofvarious organisms: corals (and associated diseases), fish and microplankton. An inventory ofsoft corals and habitat mapping using satellite and field data were also undertaken. The degreeof connectivity between reef populations in the Mozambique Channel was assessed through 2genetic studies and current data between islands were generated using a hydrodynamic model.Finally, monitoring indicators have been proposed to managers to assess the health status ofcoral reefs and to estimate the effectiveness of management measures.ResultsConcerning the health of coral reefs in Mayotte, the levels of benthic cover and the taxonomicdistribution of corals varied widely according to locality and reef geomorphology, with thebest coral health observed on the inner and fringing reefs. The coral and fish diversities werehigh, as was the total species richness, due to the favourable geological and hydrodynamicconditions in the north of the Mozambique Channel. Nevertheless, fish biomass was clearlydeclining at all the stations over the last decade, a consequence of high fishing pressure, inparticular on the higher trophic level species. Coral bleaching monitoring in Mayotte revealedan average colony morality of 24% in 2016 but almost half of the coral colonies were notaffected.In the Iles Eparses, the coral reefs were in very good health, which was reflected either byvery high coral cover (e.g. Europa: CV 80% on average and >235 coral sp) or by thedominance of "living corals-crustose coralline algae" compared to soft algae (e.g. Glorieuses).A high prevalence of coral diseases (> 31%) was recorded at Glorieuses at some stations nearLys Island, which can be explained by the proximity of colonies of thousands of seabirds thatcan be vectors and/or reservoirs of diseases released via faecal contamination. In contrast, avery low disease prevalence was observed at Europa (2%), which confirmed the exceptionalstate of health of its coral reefs. Fish communities were characterized by high proportions ofpredators (groupers, snappers, snappers, jacks, sharks) and large adult individuals.Nevertheless, at Glorieuses, although fish biomasses remained higher relative toanthropogenically-affected sites elsewhere in the Indian Ocean, this has decreased specificallyin terms of pelagic species probably preferentially targeted by illegal fishing. Hydrologicalanalyses of water chemistry and biological indicators confirmed the ultra-oligotrophic statusof the Glorieuses, its waters being characterized by very low concentrations ofpicocyanobacteria and nitrogen and carbon nutrients. In terms of bacterial contamination,there was no evidence of faecal contamination (coliforms and streptococci) at Glorieuses,whereas some stations in Mayotte exhibited high contamination levels and poor water qualitycompared to WFD (EU Water Framework Directive) thresholds. Indices integrating theautotrophic (and not heterotrophic) trophic levels thus provided relevant information on thehealth status of the systems studied.ConclusionThe selection of priority sites for conservation (species and/or habitats) is now underdiscussion with managers and this needs to be balanced with other considerations that can betake into account such as the functional role or the particular biology of certain species ofinterest, as well as socio-economic concerns. The exceptional coral reef of Europa displayedreef communities that are close to “pristine”, suggesting that they could be reference areas forthe Southwest Indian Ocean. Long-term scientific studies of these environments is essential tomonitor changes in their condition and propose appropriate management measures in thecontext of climate change that is rapidly impacting populations and ecosystems.