Many African countries are progressively embracing the blue economy. African islands of the western Indian Ocean, however, have been involved in it for more than twenty years through the exploitation of their 'blue gold': tuna. In this article, we use Ribot and Peluso's (2003) "theory of access" to map the different ways actors access tuna under diverse socio-economic contexts and how power relations are created through different mechanisms of access. We show that rights-based mechanisms such as fishing access agreements are highly questionable for their fairness and sustainability but bring benefits such as funding for fisheries-related infrastructures and projects. We also show that access to the resource is dependent on knowledge held by fishers, on technological advances as well as on diverse labor relations. These mechanisms significantly impact the quantity of fish that can be accessed by artisanal versus industrial market sectors, and generate narratives of unequal access to tuna. Furthermore, we take into consideration the materiality of tuna as a highly mobile resource in a space of fluid boundaries, to show how the fish can be an actant in shaping access but also how fishing practices can produce new materialities. Based on the above evidence, we propose an enhancement of the theory of access to consider the role of materiality of the resources and the sea. We conclude that to ensure that tuna fisheries continue to contribute to the blue economy of African islands, stakeholders need to balance between the diverse benefits produced by the fisheries and the uneven power relations that can arise, and to integrate the impact of a material sea and fish in this reflection.