Accounting for non-equilibrium transient dynamics in the design and assessment of marine reserves

Background/Question/Methods

Non-equilibrium dynamics are an inherent problem in marine fisheries management, because fishing truncates the natural age structure of a fish population.  When fishing is removed, the time required for those age classes to fill back in produces demographic lags in abundance and recruitment.  Furthermore, larval recruitment is itself a fundamentally stochastic process driven by oceanographic variability.  Nonetheless until recently much of the theory underpinning marine reserve design and the subsequent adaptive management of reserves was grounded in a fundamentally equilibrium-centric worldview.  We describe the theory of transient dynamics post-reserve implementation and how it can be applied to the design and adaptive management of marine reserves.

Results/Conclusions

In the simple case of a single reserve that receives most of its larval supply from elsewhere, the post-reserve increase in fish abundance is due purely to ‘filling in’ the age structure.  There is a simple expression for the expected post reserve increase (expressed as the ratio of population density after:before): 1 + F/M, where F is the prior fishing rate and M is the natural mortality rate.  The time scale of filling in is also determined by M.  The variability around this expectation can be large, particularly if there is high variation in larval supply, and we provide an analytical expression for that variance.  The theoretical predictions for the mean and variance match observations from several species in recently implemented marine reserves in coastal California.   That example shows the necessity of appreciating the potentially slow time scale of recovery and the high variability (due to larval recruitment) relative to the small increase predicted for species that were not heavily fished.  We also show how the basic theoretical framework for post-reserve transients could be extended to several special cases, such as rotating marine reserves and reserves for sex-changing species.