Impacts on population indices if scientific surveys are excluded from marine protected areas

Marine protected areas (MPAs) are increasingly common worldwide, typically restricting fishing activities. However, MPAs may also limit scientific surveys that impact benthic habitat. We combine a historical data degradation approach and simulation to investigate the effects on population indices of excluding surveys from MPAs. Our approach quantifies losses in precision, inter-annual accuracy, trend accuracy, and power to detect trends, as well as correlates of these effects. We apply this approach to a proposed MPA network off western Canada, examining 43 groundfish species observed by four surveys. Survey exclusion particularly impacted less precise indices, species well-represented in MPAs, and those whose density shifted in or out of MPAs. Redistributing survey effort outside MPAs consistently improved precision but not accuracy or trend detection—sometimes making estimates more precise about the ‘wrong’ index. While these changes may not qualitatively alter stock assessment for many species, in some cases, ∼30 percentage point reductions in power to detect simulated 50% population declines suggest meaningful impacts are possible. If survey restrictions continue expanding, index integrity could further degrade, eventually compromising the management of exploited populations. Regulating surveys within MPA boundaries therefore requires careful consideration to balance MPA objectives with the need for reliable monitoring.