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International audience; 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.

A simple, cheaply-constructed corer is described for operation by SCUBA divers on marine sediments at shallow depths. It proved effective in the collection of bioclastic, reef-associated sediments to a depth of 1 m, retaining their integrity for palaeoclimatological research.

A preliminary reef survey was conducted in July 2015 at Baixo São João, southern Mozambique, to investigate the merits of protecting it within a sanctuary. This involved point intercept analysis of photo-quadrat transects recorded in the northern, central and southern parts of the reef, on the reef top and its inshore and offshore slopes. Visual techniques were used to superficially describe the ichthyofauna. The coral community proved to be relatively rich and uniform within all reef zones, but with no unique or over-vulnerable species. Hard corals were predominant (mean cover = 32.3%), followed by soft corals (mean cover = 12.8%). Little coral damage was evident. A total of 97 species of reef fish, belonging to 30 families was recorded. Large specimens were common, including species of commercial importance, as well as top predators. The reef would warrant protection based on the following premises: i) it is remote, rendering human interference unlikely; ii) it is located offshore in deeper water, protecting it from many of the potential effects of climate change; iii) it could provide a coral breeding refuge for replenishment of more southern reefs; and iv) it would provide a useful reference site for baseline and comparative studies.

Reef coral populations in the western Indian Ocean are neglected in terms of research and management. Very little is known about coral population connectivity and dynamics at regional scales. Platygyra daedalea was collected from Indian Ocean coral reefs, mainly from the east African coast between Mombasa Marine Park (Kenya) in the north and Maputaland (South Africa) in the South. Simple-sequence repeats from five independent loci in the nuclear genome were used to measure differentiation between populations of P. daedalea. Of 350 specimens successfully amplified for one or more simple-sequence repeats, 231 amplified at three or more loci. The remaining specimens comprised a data set including null alleles. Overall heterozygosity was high, He = 0.8, and the mean number of alleles across loci, per population was 4.31. The large numbers of null alleles encountered across loci may be attributed to possible parapatric divergence of the nuclear genome in this genus. Ten populations of the nineteen sampled in this study showed signs of heterozygote excesses and deviation from expectations of Hardy-Weinberg equilibrium. Populations in this study conformed closely to expectations of a panmictic metapopulation with fine-scale structure amongst sub populations (FST = 0.049), although admixture was evident.

Sexual reproduction in Pocillopora damicornis was studied off Durban in South Africa (29°S), close to its southernmost distribution in Africa. This was compared with P. damicornis reproduction on tropical reefs to ascertain how corals may adapt their reproduction on marginal reefs. Monthly gametogenesis was correlated with SST, light intensity, and day length. P. damicornis proved to be a hermaphroditic broadcast spawner in South Africa and had a higher fecundity than its tropical counterparts. However, its gamete development was slow (6-7 months) and confined to the warmer months of the year at this high latitude. Its reproductive pattern was similar to that found in this species in south Western Australia (32°S). Spawning in P. damicornis off South Africa occurred at the end of the summer and was inferred from the disappearance of mature gamete in April 2008. No correlation was found between monthly mean gamete diameter and light intensity or day length, but the increase in gamete size was correlated with a rise in SST, suggesting that this parameter regulates its spawning. The lower SST at high latitude may account for its extended gametogenesis and late spawning.