Traditional fishery management schemes have gained increasing recognition worldwide. It can be explained by a better compliance to ancient cultural practices, still rooted in present-day coastal communities despite globalization and modern livelihoods. This revival is widespread and welcome by policy makers, scientists, and the communities themselves. However, current environmental and socio-economic contexts are often not conform to ancient-time situations. Baselines are different. Effective adjustments of traditional practices may be advocated. Re-establishment of traditional schemes ‘as such’ warrants further investigations and modern quantitative assessment and management approaches can help. A demonstration is provided here for a rural Polynesian island that faces declining marine resources. Recently, local fishers discussed the implementation of a traditional system (called rāhui) to preserve the island lagoon resources, based on the rotational closure of an arbitrary 50% of each lagoon subdivision. Upon the fishers’ request who questioned a traditional scheme that has not been applied for decades and seeked some scientific approval, we used systematic conservation planning (SCP) tools to explore potential optimisation pathways. All quantitative conservation objectives being equal, SCP suggested reserve sizes and opportunity costs on average 7 and 5 times lower than the traditional design. Traditional management federates communities and is strongly encouraged, but fishers are now aware that effective alternative designs are possible. A hybrid design mixing traditional practices and data-based optimizations is advocated. Similar findings and recommendations can be expected in other regions.