Cartographie de l’habitat benthique des récifs coralliens et distribution spatiale de la biomasse de poisson dans l’Aire Marine Protégée SOARIAKE, au Sud-Ouest de Madagascar.

Abstract

Coral reefs are globally threatened ecosystems, making the precise monitoring of their
health status essential for improving conservation. In Madagascar, where these marine
habitats cover a vast area, a remote sensing approach was developed to assess their
condition, using the SOARIAKE Marine Protected Area (MPA) as a case study. The
objective of the study is to provide a benthic map and the spatial distribution of fish biomass
for the MPA, as well as to document the evolution of the marine habitat over 36 years (1985-
2021). The methodology combines satellite imagery and field verification. In-situ work
(March-October 2021) within the SOARIAKE MPA, across 28 transects of 500m, allowed
for the collection of 4,833 georeferenced photos for analysis and validation. Fish biomass
was interpolated using the inverse distance weighting (IDW) method. The analysis of
habitat changes over 36 years used multitemporal Landsat imagery, corroborated by the
2021 data. The classification accuracy achieved for five habitat classes is 82% for Sentinel-
2 and 74% for Landsat 8. The results reveal a major decline in live coral cover, dropping
from 9,292 hectares in 1985 to 3,172 hectares in 2021. The analysis of spatial fish biomass
distribution also shows a marked heterogeneity, where the highest values, ranging from 10
kg/ha to a maximum of 176 kg/ha, are specifically concentrated on the outer reef slope,
confirming its key ecological role for marine fauna. This study demonstrates the feasibility
of creating high-precision maps for reef conservation in developing countries using free
satellite data such as Sentinel-2 and Landsat 8. These results provide NGOs and the state
with a precise decision-support tool, enabling targeted surveillance of high-biomass areas
and guiding restoration actions toward the most degraded habitats. This work thus
constitutes a fundamental support for adaptive and informed management of marine
resources.