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Coral bleaching and various human stressors have degraded the coral reefs of the Comoros Archipelago in the past 40 years and rising atmospheric CO levels are predicted to further impact marine habitats. The condition of reefs in the Comoros is poorly known; using SCUBA based methods we surveyed reef condition and resilience to bleaching at sites in Grande Comore and Mohéli in 2010 and 2016. The condition of reefs was highly variable, with a range in live coral cover between 6% and 60% and target fishery species biomass between 20 and 500 kg per ha. The vulnerability assessment of reefs to future coral bleaching and their exposure to fishing, soil erosion and river pollution in Mohéli Marine Park found that offshore sites around the islets south of the island were least likely to be impacted by these negative pressures. The high variability in both reef condition and vulnerability across reefs in the Park lends itself to spatially explicit conservation actions. However, it is noteworthy that climate impacts to date appear moderate and that local human pressures are not having a major impact on components of reef health and recovery, suggesting these reefs are relatively resilient to the current anthropogenic stresses that they are experiencing.

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This paper presents the first quantitative assessment of the rocky reef ecology of St Helena, a remote island in the central tropical Atlantic. Monitoring data were used to characterise different habitat types found around St Helena. These findings were compared with 9 other locations in the tropical Atlantic, in different biogeographic, oceanic and reef settings, along with the environmental variables known to limit coral reef formation. St Helena’s rocky and boulder reefs had ~50% cover dominated by turf and other filamentous algae, with lower levels of sessile invertebrates (15%) and macroalgae (4%). Both coral and rocky reef comparison locations also showed a dominance of turf and filamentous algae, with higher levels of macroalgae and sessile invertebrates in areas with higher nutrient concentrations (e.g. south-east continental Brazil). Coral growth in St Helena appeared to be limited by cool average sea temperatures of 22°C, which is near, but not below, accepted lower thresholds for reef formation. The main trophic groups of fish found on rocky reefs in St Helena were comparable to other Atlantic rocky and coral reefs, with a dominance of planktivores, mobile invertivores and roving herbivores, with the major difference in trophic structure being driven by more planktivores in oceanic vs. continental versus locations. St Helena’s narrow rocky coastal strip varied little in terms of reef geomorphology, resulting in high homogeneity around the island. However, endemic fish were numerous, demonstrating that isolation has produced a unique tropical Atlantic marine assemblage.