Global efforts to protect 30% of the ocean may substantially overestimate conservation success if outcomes are assumed rather than measured. Current assessments emphasize protected area coverage and stated protection levels instead of whether marine protected areas (MPAs) deliver ecological benefits. By comparing observed fish biomass on shallow reefs with counterfactual predictions, we found that at most one quarter of MPAs are achieving meaningful conservation outcomes, with success strongly dependent on compliance with fishing restrictions. If shallow reefs reflect broader trends, less than 2% of the ocean currently receives meaningful biodiversity protection. Measuring management effectiveness, compliance, and ecological outcomes is essential for achieving global conservation goals.
