Phase III – Is there a Western Indian Ocean “Coral Triangle” ?

The goal of this project was to establish diversity-creating and maintaining mechanisms, and related patterns of coral reef ecosystems in the WIO region. The specific objectives in the proposal were: (i) To determine the basic oceanography of the study region and in particular the presence and dynamics of a ‘Comoros gyre’; (ii) To determine the biogeography of representative taxa in the region and in relation to peripheral regions; (iii) To assess the resilience and current ecological state of coral reefs in the region; (iv) To assess the possible influence of historical tectonic and sea level changes on the oceanography and evolutionary history behind current biogeographic patterns; (v) To assess current status and resilience of reefs in relation to oceanography, biogeography and human impacts to lay a platform for an integrated science and management agenda for the study region. The project targeted to benefit the scientific community and conservationists; and in the long run the fisheries industry and communities. The projected was implemented in ten countries in the WIO comprising Comoros, Djibouti, France (Mayotte), Kenya, Madagascar, Mauritius, Mozambique, Seychelles, Tanzania and Reunion.

During this project three main subjects were investigated including oceanography, biogeography of key taxa and coral reef status and evolutionary hypothesis for the IO centre of diversity. In oceanography, both empirical oceanography as well as modelling studies were undertaken. (i) Empirical oceanography was conducted in partnership with the ASCLME program in 133 hydrographic stations on which physical (temperature and salinity) and chemical (nutrients) measurements were taken to a depth of 3000m. Modelling was done to study the potential influence of the local winds, geometry and heat fluxes on the circulation of the Comoros Gyre. In addition, SCUBA/snorkel dives were performed to establish coral reef fish diversity. Separate surveys of fish abundance and size of selected taxa were done using a SCUBA based field protocol developed by Samoilys.

(ii) For biogeography subject; coral diversity and patterns, coral bleaching, fish diversity, abundance and biomass and resilience of coral reef were investigated. (iii) A synthesis on evolutionary patterns underlying the current species distributions and biogeography of the WIO was studied. The evolutionary influences that formed the basis of this hypothesis include: the plate tectonics of the Indian Ocean opening and Tethys Sea closing; age of the Mozambique channel and East African coasts; formation of mid-ocean banks by the Mascarene-Reunion hotspot; changes in habitat area, sea level and equatorial migration of Africa/Madagascar; changes in major currents and connectivity with the Tethys, Atlantic and Indo-Malayan regions; dynamics and history of present-day Mozambique channel eddies and their role in genetic connectivity; and for extant coral species, their likely origins .

This project came up with several manuscripts and reports in different levels of publication. This work has also been written up in two popular formats, as part of the Conservation International/CEMEX annual book series in the 2011 edition “Oceans”, and in the magazine of the East African Wildlife Society, “Swara” (see publications).

Among the important research outputs in oceanography include the circulation in the Comoros Basin was dominated by cyclonic and anti-cyclonic eddies passing through the basin from the north, there was no gyre circulation in the basin, surface water mass structure showed a strong tropical influence that probably originated from the North East Madagascar current, there is westward and southward transport of high salinity surface water that originate north of the East Madagascar; In terms of particle movement drifter tracks indicate surface water in the basin to come from the East Madagascar current, South Equatorial current and from the northern Mozambique channel. Once in the basin, observed tracks show drifters exiting the basin mainly southward. Results showed that whilst gyre circulation is a major mode of variability the most common circulation pattern is a double or triple eddy structure. Sea surface height in the Comoros basin was found to be highly variable and increased vorticity also occurred, indicating that eddy structures are forming in the basin. And also the circulation in the Comoros basin was noted to consist of an anti-cyclonic feature which is not permanent nor does it occur in exactly the same place every time.

With regard to biogeography of coral reefs and fish, the project has developed a substantial database on coral reef fish species diversity across the core region of the WIO with around 430 species identified. Fish species diversity showed significant differences at island/mainland level, with species responsible for driving the patterns in the diversity identified. Results also highlighted missing species. The coral bleaching event of 2010 was captured during field surveys for this project, resulting in two publications in 2011, one focused on a regional analysis, the other on the highest-impacted site, Mayotte. Furthermore, fish density strongly indicated the absence, or low densities, of highly vulnerable species such as sharks, bumphead parrotfish (Bolbometapon muricatum) and Napoleon wrasse (Cheilinus undulatus).

It was found that the evolutionary patterns underlying the current species distributions and biogeography of the WIO are influenced by the plate tectonics of the Indian Ocean opening and Tethys Sea closing; age of the Mozambique channel and East African coasts; formation of mid-ocean banks by the Mascarene-Reunion hotspot; changes in habitat area, sea level and equatorial migration of Africa/Madagascar; changes in major currents and connectivity with the Tethys, Atlantic and Indo-Malayan regions; dynamics and history of present-day Mozambique channel eddies and their role in genetic connectivity; and for extant coral species, their likely origins.

This project has managed to spin-off development of (i) a Regional ROMS model of the project area, which was to become available for use by other researchers and projects in keeping with current practice in the modelling community and (ii) online access of genera and species level resources of the Corals of the Western Indian Ocean.

The project recommended an increase in the time series to at least 20 year and to use self-organizing maps as a means of extracting the major mode of variability as this may capture most of the variability occurring in the area. The 10-year time series used in this project may however have been too short and may have not reflected all the variability in empirical oceanography of the studied area.