In this paper the authors examine the strengths and weaknesses of state-supported Customary Marine Tenure (CMT) systems in two independent Melanesian states (Solomon Islands and Papua New Guinea) in the context of the management of rapidly intensifying commercial and subsistence fisheries. We focus particularly on the proposed use of permanent no-take Marine Protected Areas (MPAs), which are at present strongly favoured by scientists and environmentalists around the world, as the most versatile marine fishery management tool, especially in poor developing countries. They argue that, with some exceptions, typical Melanesian CMT regimes make MPAs difficult to establish, primarily due to issues of scale. They look closely at the ecological rationale for no-take MPAs, for different coral-reef based species, and assess the likelihood that populations of these species are self-replacing on the same scale as CMT territories for most of coastal PNG and Solomon Islands. They argue that with some exceptions (mainly species with short-lived larvae), the dynamics and scale of population replacement processes for most fished species make no-take permanent closures largely incompatible with traditional CMT systems, and therefore unlikely to prove a successful management tool in this socio- political context.