Towards sustainable seaweed aquaculture in Nosy Boraha, Madagascar: Insights from social and ecological sciences

Cottonii seaweed ( Kappaphycus alvarezii) farming is a growing activity in Madagascar, promoted as part of the country’s blue economy strategy for sustainable development. Despite its expansion, few interdisciplinary studies have simultaneously examined its environmental, social, and legal dimensions. In 2024, we conducted an interdisciplinary study in Nosy Boraha, where seaweed aquaculture is rapidly developing. Combining anthropology, marine ecology, and environmental law, the research aimed to assess the effects of seaweed farming on local communities, describe the lagoon’s environmental characteristics and benthic habitats, and analyse the legal framework governing the activity, including national legislation and local customary law (Dina). The results indicate that seaweed farming provides an important supplementary livelihood for residents of Ilampy and nearby villages, particularly as fisheries resources decline. However, farmers reported challenging working conditions and expressed concerns about potential impacts on coral reefs and traditional fishing grounds. Remote sensing analyses using satellite and drone imagery estimated the north reef lagoon at approximately 600 ha. This shallow environment (<5 m) is dominated by sandy substrates and macrophytes, we have identified four seagrass and three macroalgal genera in the seaweed cultivation areas. Coral communities, comprising roughly four genera, are mainly concentrated near the reef crest. The company Nosy Boraha Seaweed operates across about 300 ha of the lagoon, although cultivation plots cover only 8.3% of this area. Nutrient analyses showed uptake of dissolved inorganic nitrogen and phosphates both within farming zones and in control areas without seaweed cultivation. Finally, the local Dina, grounded in the Fihavanana principle of solidarity and reciprocity, plays a central role in marine resource management. It operates alongside formal governance structures led by local fisheries committees and Madagascar’s Environmental and Blue-Economy Ministries.