Environmental variability introduces time lags into marine protected area responses

Implementation of marine protected areas (MPAs) to benefit biodiversity and rebuild populations is becoming more widespread globally, and there is now a network of MPAs spanning California and Oregon. One of the anticipated benefits of MPAs is increases in previously harvested populations, which may increase resilience of marine populations as well as provide subsidies to nearshore fisheries, as larvae produced inside MPAs “spill over” into fished areas. Within an adaptive management framework, a first step in assessing whether MPAs are or will produce these desired effects is to quantify the magnitude of population increase, and the time scale over which it could be expected to manifest. We use age- and size-structured models of rockfish populations to build expected responses of harvested populations to MPA implementation. We show that the degree of harvest prior to MPA implementation shapes the expected magnitude of change and that there is a decade-scale time lag in the expected full realization of benefits to populations, corresponding to the time taken for fish inside MPAs to mature and for their (dispersed) offspring to enter the population. The timing of any large benefit to populations is further affected by nearshore environmental variability (e.g., ENSO, NPGO) that affects the magnitude of annual cohorts of larval recruits. Our analysis shows that the timing of MPA implementation relative to the phase of environmental variability (i.e., was the MPA preceded by a period of very high or very low larval recruitment) has a major effect on the timing of realized MPA benefits. This effect must be accounted for in the adaptive management of these populations and fisheries, as managers will need this information to plan timelines of sampling and decision-making.