Fifty years of changes in reef flat habitats of the Grand Récif of Toliara (SW Madagascar) and the impact of gleaning
Abstract
The Grand Récif of Toliara (GRT) in Madagascar is a large (33 km2) barrier reef system of the SW Indian Ocean that had been well investigated in the 1960s and early 1970s. A massive degradation of the reef has been reported since at least the early 1980s, just a few years after research activities ceased in the area. Examination of historical aerial photographs and modern high-resolution remote sensing images confirms a continuous loss of coral habitat on GRT outer reef flats between 1962 and 2011, with an average loss of 65 % and a range of 37–79 % loss during this 50-year period. The usual suspects of coral community declines (cyclones, bleaching and sedimentation) may have contributed to the demise of the GRT. However, an independent study (Salimo 1997) suggests that the chronic pressure of fisherman gleaning on reef flats with destructive tools is the main driver of the observed changes. Salimo’s reported level of frequentation (6.8 fishermen per day and per km−2) and rates of destruction per fisherman (7.7 m2 of coral habitat h−1) yield a cumulated overall loss in agreement with the image-based rates of habitat loss. The GRT is unlikely to recover because this chronic stress is unlikely to decrease in the near future. Indeed, the GRT daily provides subsistence fishery resources for local Vezo people and to agriculturalist or pastoralist ethnic groups who have turned to exploiting coastal resources due to increasing aridity and dwindling agricultural and livestock production.
Keywords
Subsistence fishery Reef gleaning Remote sensing Disturbance Resilience Habitat mappingNotes
Acknowledgments
This study is a contribution of the MASMA-GRT programme (grant MASMA/OR/2007/02) funded by the Western Indian Ocean Marine Science Association (WIOMSA). We thank Man Wai Rebenevanana, Director of IH.SM, for facilitating research. We thank Alain Bidar, Dr. Bernard Thomassin and Olivier Gerriet of the Muséum d’Histoire Naturelle at Nice for making available and scanning most of the 1962 IGN aerial photographs. We further gratefully acknowledge the funding of the synthesis workshop at la Réunion (grant MASMA/Workshop/2010/04). The 2011 Worldview-2 image was provided through a partnership between IRD and Collecte Localisation Satellites (CLS) when evaluating Earth Observation Information Products/Services for World Bank projects (grant ESRIN/Contract No. 4000103101/11/I-IW). We thank two anonymous reviewers for their comments and edits.
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