Extreme events delay the detection of marine protected area effects: Implications for monitoring and management

The establishment of protected natural areas worldwide has surged in the last decade, with more protected area coverage expected in the future. In the case of marine protected areas (MPAs) that prohibit fishing, there are ongoing efforts to evaluate their effectiveness in conserving fish populations. Accurate assessments of MPA effects (namely, their ability to increase fish biomass within their boundaries) are crucial to support these evaluations. However, marine ecosystems are increasingly affected by extreme events including marine heatwaves, hypoxia, storms, and epizootic outbreaks. These events can disrupt population increases within MPAs, leading to delays in expected conservation outcomes and the misperception that protection may not be effective. However, it is currently unclear to what extent disturbances could bias MPA assessments and for how long. To address this gap, we used a population model based on a common nearshore fishery species from the USA west coast (blue rockfish, Sebastes mystinus) to evaluate the detectability of MPA effects when an acute disturbance occurs within the first decade of protection. We found that detecting MPA effects with high confidence can be delayed up to 10 years following an extreme disturbance. Furthermore, the before-after sampling design (as opposed to an insideoutside or before-after-control-impact, or BACI design) is most biased by a widespread disturbance event, especially with long (>5 years) and severe disturbances (>30 % population reduction). These findings indicate that extreme events can profoundly impact MPA assessments, causing delays in recovery and detectability. Our results provide a benchmark to calibrate monitoring and management evaluations accordingly.