Effects of Age Structure and Spatial Diversity on Resilience in Networks of MPAs

Fishing reduces the resilience of fish populations to environmental variability. This occurs in part due to ‘cohort resonance’: as older fish are removed, generation time shrinks, and the population becomes more sensitive to variability at frequencies occurring on time scales near the generation time. This leaves populations more vulnerable to increases in environmental variability, as well as changes in environmental frequencies (e.g., more frequent ENSO events). Single MPAs can restore lost resilience by protecting older fish, allowing reproduction to be spread out over longer fish lifetimes and thus reducing sensitivity to environmental variability. Additional resilience is provided when there is a network of multiple MPAs. This additional diversity arises from a portfolio effect in which multiple MPA populations fluctuate separately, reducing the overall variation in the entire meta-population. Here we show how the truncation due to fishing has increased the frequency-dependent sensitivity of 18 species of California near-shore fishes; the results vary depending on life history and current level of exploitation. This indicates how much resilience can be recovered by MPAs, and how that resilience interacts with specific frequencies in the environment. This illustrates how MPAs could guard against negative impacts on populations caused by increased environmental variability due to climate change. We then show how much networks of MPAs with various size and spacing increase resilience via a portfolio effect. Together these results are important steps in setting expectations for when MPAs will – and will not – improve resilience to climatic disturbances.