Marine protected areas (MPAs) are the cornerstone of global marine conservation strategies. Yet, their capacity to achieve conservation gains (i.e., conservation additionality) hinges on the decrease in human threats to biodiversity within their boundary, a feature rarely measured or reported. Here, we test a central but underexamined question in global conservation policy: Have MPA implementations actually reduced within-boundary fishing pressure, currently the main direct human threat to marine biodiversity? Using a stratified before-after-control-impact approach, we investigated 861 MPAs of varying levels of protection across all ocean basins, representing half of global MPA coverage, and found that most implementations were not followed by decreases in industrial fishing effort. Protection levels strongly predicted the direction and magnitude of relative changes in fishing effort after implementation. Only fully protected areas were associated with significant decreases in fishing effort within boundaries. Highly protected areas were only associated with lower rates of increase in fishing effort within compared to outside boundaries. Both fully and highly protected areas were implemented in areas experiencing initial low fishing effort, reducing the overall abatement in fishing effort achieved. Conversely, areas with higher initial fishing effort tended to be attributed to MPAs with low levels of protection, also limiting abatement. Larger MPAs were associated with greater decreases in fishing effort, while time since implementation had no significant effect. Complementing targets of protection coverage and protection levels with conservation additionality targets, such as pressure abatement, could shift incentives from quantity to effectiveness of interventions and ensure positive biodiversity conservation outcomes.
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