Implementation of no-take reserves is ideally followed by long-term monitoring to ensure that a reserve meets its intended goal, such as increasing the abundance of exploited species. This process is especially important for marine reserves, which are growing in popularity as a management tool, but which have shown mixed success in meeting conservation goals. Moreover, the time scale over which increases in fish abundance should be observed is unclear; some meta-analyses show quick results while others reveal slow increases. The factors determining whether abundance should increase within a reserve are well characterized, but have been predicted primarily by analyses of long-term equilibria in population models. A better understanding of the time scale over which abundance should increase (or decrease) is needed. Here we use age-structured models of a generic fish population to examine the factors affecting shorter-term, transient responses to no-take marine reserves.
Results/Conclusions
Our analysis reveals that it may take decades for a fished population to converge on its long-term dynamic behavior. In the meantime, short-term transient dynamics dominate. During the transient, post-reserve population abundance could remain unchanged, decrease, or oscillate, even when the long-term result will be increased abundance. We show that such transient dynamics are longer and more oscillatory for populations with heavier fishing, higher natural mortality rates, older ages at maturity, and low connectivity with neighboring populations. We also provide several metrics derived from the theory of linear population models to quantify transient behavior, including the distance to the stable age distribution, the rate of convergence on the stable age distribution, and the initial trajectory of the transient. Extreme transient dynamics could complicate short-term (e.g,. < 5 yr) empirical assessments of reserve “success,” so using these metrics could be critical to setting expectations for the likely time scale and variability of responses of fished populations.