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Costs and economic impacts of expanding marine protected area systems to 30%

International proposals for marine biodiversity seek to expand marine protected area (MPA) coverage from 8% to 30%, known as 30×30. Quadrupling MPA coverage implies considerably higher MPA system costs and governments need early knowledge of these to inform debate. Ambitious MPA expansion also implies large potential losses or “opportunity costs” for fishers, putting pressure on governments to compromise and permit some fishing inside protected areas (a mixed high/low protection system). Crafting a balanced compromise needs to be informed by model projections of future fisheries outcomes under different protection regimes, climate change scenarios and behavioural adaptations. Here, we develop the first models for management costs at national MPA-system scale. We create scenarios of 30×30 at different compromises around protection strictness. We then examine how both MPA costs and opportunity costs vary with strictness, by simultaneously applying our management cost models and two Marine Ecosystem Models. We find that a no-take (high protection) MPA system could cost just $2 billion/year for the developing world and ~$8 billion overall, but would also create opportunity costs several times larger. A compromise mix of high and medium protection would have much higher MPA costs (e.g. $4.5 billion for the developing world) but much lower opportunity costs, to the point of fisheries actually benefiting in the future. Since lower protection also compromises on biodiversity goals, our results show the trade-offs that political decisions need to consider beyond COP15. More generally, the unusually large opportunity costs show how marine contexts generate very different economic issues from terrestrial ones, by attempting to protect a common pool resource area that envisages no automatic market compensation for income lost to conservation.