Mémoire

Abstract

Seas and oceans provide vital animal protein and support global food security. In Madagascar, small-scale fisheries are crucial for livelihoods and income due to high fish demand, coastal migration, and limited alternative jobs. The Great Reef of Toliara (GRT – SW Madagascar) is one of the largest coral reef ecosystems in the Western Indian Ocean, offering an immense fishing ground. In the 1970-1980’s, the GRT was considered as one of the most biodiverse reef ecosystems of the Indian Ocean. However, the GRT now faces multiple stressors, including population growth leading to overfishing, deforestation and changes in rainfall contributing to an increase of suspended sediment in the water. These pressures have caused coral loss and macroalgal overgrowth but the impact on reef fish communities has not yet been evaluated. The present project aimed to assess variations of fish biodiversity over a period of 10 years, by analysing an unprecedented time series of fish catches from Vezo fishermen distributed in various areas of the lagoon. The 198 samples comprised 83,435 fish specimens belonging to 243 species and 51 families. The study of multiple indicators of fish biodiversity (i.e. number of specimens, biomass, mean fish size, species diversity and morphological functional diversity) revealed shifts in reef fish communities. A collapse of approximately 50% in the quantity of fishes (number of fishes, biomass) was detected between the period 2016-2017 and 2021-2022. In parallel, the diversity of species (alpha diversity and related indices) also decreased during the same period. If temporal variation in the mean size of fish was not evident, morphometric analysis revealed a decline in morphological functional disparity of fish communities suggesting that some ecological functions once fulfilled by fish are now lacking. Even if we are unable to identify the major factors driving this change of fish biodiversity, our results provide strong arguments for administrations to implement conservation policies.