Marine protected areas are more effective but less reliable in protecting fish biomass than fish diversity.

Marine protected areas (MPAs) provide multiple conservation benefits, thus raising the question of how good and consistent they are at their roles. Here, we quantified three components, namely, diversity, biomass, and other relevant variables, in numerous protected and unprotected areas across four marine ecoregions in south-western Europe. We created a "global conservation status index" (CSI) as the sum of CSI, CSI, and CSI. We then tested whether CSI and its three components varied as a function of protection and marine ecoregion. MPA efficiency, defined as the effect size of protection on CSI, was unreliable and varied with geography. CSI and CSI contributed to the unreliability of MPA efficiency, while CSI was reliable. CSI showed the major efficiency in protected areas (60%). Biomass of threatened species was the single largest variable that contributed to MPA efficiency. Our easy-to-use approach can identify high- and low-efficient MPAs and help to clarify their actual roles.