Article - Towards Sustainable Seaweed Aquaculture in Nosy Boraha, Madagascar: Insights from Social and Ecological Sciences

DATA: Orthomosaics x3 radials of the study; and Maps: (i) Habitat maps; (ii) Seaweed farms; (iii) Current tracks, (iv) Study sites. * correspondence: [e-mail]; [e-mail] Towards Sustainable Seaweed Aquaculture in Nosy Boraha, Madagascar: Insights from Social and Ecological Sciences Isabel Urbina-Barreto1 *; Rindra Razandriarison1,2; Oly Ny Aina Ralison Andrianantoandro1,3; Anne Chauvin1; Hambinintsoa Solofoharimanana1,3; Elsa Lagoutte4; Georgeta Stoica5; Victor David6; Pascale Cuet7; Dina Andrianaivoravelona1; Manon Museux8; Sébastien Jan8; Aline Tribollet1 1 French National Research Institute for Sustainable Development – IRD, LOCEAN-IPSL laboratory, Sorbonne University-IRD-CNRS-MNHN. Parc Technologique Universitaire, 2 Rue Joseph Wetzell, Sainte-Clotilde 97490, La Réunion, France. 2 Institute of Fisheries and Marine Sciences IH SM. 48B042 Rue Dr Rabesandratana, PO Box 141, Toliara - 601, Madagascar 3 University of Antananarivo. BP 566 Antananarivo, 101, Madagascar. 4 French National Research Institute for Sustainable Development –IRD, Sorbonne Université-CNRS-MNHN, UMR LOCEAN-IPSL, Sorbonne Université, 4 Place Jussieu, 75005, Paris, France. 5 ICARE Unity- Mayotte University. Iloni - Mayotte, France. 6 MBE Laboratory - Aix-Marseille University – CNRS – IRD Avignon University, France. 7 ENTROPIE laboratory, Reunion Island University-–97490 La Réunion, France. 8 Nosy Boraha Seaweed company. Ilot Madame BP04 - Ambodifotatra, Sainte-Marie, Madagascar. Author Summary Our research aims to establish a baseline for assessing the sustainability of seaweed aquaculture at Nosy Boraha (Sainte-Marie Island), Madagascar. An interdisciplinary team of anthropologists, reef ecologists, chemists, and environmental law specialists collaborated to characterise the reef socio-ecosystem in which seaweed farming operates and to examine the legal frameworks governing the activity, including both national regulations and customary law (Dina). This study draws on field investigations conducted in 2024. We provide new insights into the effects of seaweed farming in Ilampy and nearby villages, highlighting its contribution to local livelihoods amid declining fisheries. At the same time, we describe the environmental setting of the north-east lagoon, where cultivation occurs, including its physico-chemical conditions and benthic communities. The ecosystem is dominated by seagrass and macroalgae, while nutrient concentrations are low, consistent with oligotrophic tropical environments. These findings suggest that seaweed farming currently operates within a relatively nutrient-poor but ecologically sensitive system. Overall, our results show that the governance of seaweed aquaculture combines national legislation with the locally enforced Dina, reflecting a hybrid legal framework. This integrated approach is particularly significant in the context of accelerating blue-economy development in coastal regions, especially in low- and middle-income countries such as Madagascar. Keywords: seaweed aquaculture, Cottonii, Kappaphycus alvarezii, reef socio-ecosystems, socio-environmental characterization, drone, interdisciplinary, sustainable science, Madagascar.