Open Access

R. J. Mrowicki, E. Fanning

• The Bay of Diego-Suarez, considered to be one of the finest and largest natural harbours in the world, is located towards the northernmost tip of Madagascar in the Antsiranana province. Despite its historical and current use as a port, much of its convoluted perimeter is still somewhat untouched, harbouring pristine shorelines and subtidal coral reefs. The position of the bay between other regions in which high marine biodiversity has already been revealed suggests that it may also harbour high biodiversity. However, the relatively long coastline and limited connectivity of the bay with the Indian Ocean, in combination with existing anthropogenic activities, potentially make its marine environments susceptible to a range of environmental impacts including sedimentation, nutrification and pollution. The Frontier-Madagascar Marine Research Programme (FMMRP) became involved in conducting marine ecological survey work in the Bay of Diego-Suarez, north Madagascar, in April 2005, having relocated from its previous base at Anakao in southwest Madagascar. The rationale for the survey programme stemmed from the affiliation of the FMMRP with the Malagasy organisations Association Nationale pour la Gestion des Aires Protégées (ANGAP) and Service d’Appui a la Gestion de l’Environnement (SAGE), who were interested in identifying areas of the bay with particularly healthy coral reef systems. Additional environmental interest in the bay has arisen as a result of its proximity to surrounding terrestrial protected areas such as the newly managed Ramena complex, incorporating Orangea and Montagne des Français, and also Montagne d’Ambre. Since its relocation to the Diego-Suarez area, the FMMRP has compiled over two years’ worth of marine ecological data relating to benthic community composition, fish species abundance and population size structure, frequency of algae and invertebrate indicator species, and physical environmental parameters. Thus there exists an extensive dataset for the Bay of Diego-Suarez, from which details of the current condition of its marine habitats can be investigated and a baseline for temporal monitoring can be established. The primary purpose of this report is to signify the initial detailed dissection of the dataset and demonstrate the conclusions that can be made regarding the ecological status of coral reef systems within the bay. This has mostly involved the examination of benthic data, focusing upon variations in percentage cover of substrata and coral community characteristics as useful structural indicators of reef condition. Additionally, the report includes an assessment of the abundance and distribution of sea urchins and their relation to benthic community patterns, as a demonstration of the ability to interrelate different aspect of the FMMRP dataset to enhance the conclusions that can be drawn. Benthic community data were obtained from 380 line intercept transects conducted in different sectors of the Bay of Diego-Suarez between October 2005 and December 2007, representing a combined distance of 7,600 m. Sediment occupied the greatest overall proportion of the benthos (around 38%), especially in the western areas of the bay. Overall mean hard coral cover was around 15%, and tended to co-vary with other ‘hard’ substrata such as rock and rubble. In total, 38 scleractinian coral genera were recorded during survey work, in addition to a number of unidentified genera. The coral communities of the bay were dominated by Acropora and Porites spp., which comprised around 33% and 20% of total recorded hard coral cover, respectively. Hard coral cover and generic diversity appeared to be positively related. These indicators were greatest in the northeast area opposite the mouth of the bay, reaching mean values of around 37% and 6.8 genera, respectively. Here, the hard coral community was dominated by Acropora spp. and comprised a relatively high proportional cover of Galaxea spp. In the northwest of the bay, coral cover was approximately half as great and consisted primarily of species belonging to the genera Porites and Millepora. Habitats in this area were highly similar in terms of their overall coral community composition. Hard coral cover and diversity were generally lower in the southern portion of the bay, especially in more immediate proximity to the population centre of Diego-Suarez (around 2% and 1.5- 5.5 genera, respectively). Coral community composition was considerably more variable than in the northern portion of the bay. v After sediment and ‘hard’ substrata, seagrass formed the next major interplaying component of the benthic environment (around 10% overall proportional cover). The easternmost areas adjacent to the mouth of the bay were characterised by high seagrass cover, whic h reached around 48%. Little or no seagrass was encountered elsewhere, except at one locality in the northwest (around 13% cover). Macroalgae cover was low and less variable, reaching a maximum value of around 10% adjacent to Diego-Suarez. There were no differences between island and mainland sites in terms of overall benthic substratum characteristics, yet soft coral cover was significantly greater amongst island sectors. Sea urchin abundance data were obtained from 498 belt transects conducted between April 2006 and December 2007, representing a total area of 49,800 m2. A total of 6 species were recorded, of which Diadema setosum comprised by far the greatest relative abundance (96%) and observation frequency (55%). The greatest population densities of this species were encountered in the more exposed areas in the west and northwest, reaching around 1.5 m-2, and very few individuals were recorded in the eastern reaches. Data suggest a possible seasonal increase in D. setosum densities, corresponding with an increase in water temperature towards the end of the year. No significant correlation existed between D. setosum population density and coral cover, although these seemed to be inversely related in the central northern area of the bay. There was also no significant correlation with macroalgae cover. However, D. setosum density was positively and negatively associated with rubble and seagrass cover, respectively. There was a lack of a clear pattern amongst sectors with respect to overall benthic community characteristics, let alone between the density of D. setosum and benthic substratum composition. In conclusion, a relatively detailed map of benthic community composition has been produced for the Bay of Diego-Suarez, which shall be useful in elucidating the primary factors determining the condition of marine environments within the bay and developing effective sustainable management strategies. Further analysis, incorporating additional components of the FMMRP dataset, is required in order to further clarify our understanding of the key issues surrounding the current status of these coral reef systems. It is hoped that continued survey work will enable important long-term ecological monitoring of the marine environment of the bay and assessment of the effectiveness of any management initiatives that may be implemented.