COS 82-6
The time scale of transient population dynamics hampers the utility of rotational schemes for marine reserves
No-take marine reserves that rotate over space have recently come in vogue as a proposed way to preserve fish biomass without forever “locking away” the resource. However, implementation of periodically harvested or rotational schemes for the purpose of fisheries or conservation has been met with mixed results. Earlier theoretical results suggest optimal rotation schemes based on expected rate of biomass recovery after protection – essentially taking advantage of the transient dynamics of a non-equilibrium system. Unfortunately, those models did not accurately represent the likely time scale of recovery transients in age-structured fishery populations. We re-examined this topic using a more realistic, spatially explicit age-structured model of a generic fish population. We examined two different rotation schemes: 1) “rotational closure” (the full reserve is opened to fishing after each rotation period and a new area is closed) and 2) “periodically harvested” (after each rotation period, only a portion of the reserve is opened and an equivalent area of fished coastline is closed. We used the model to compare the long-term average biomass and fishery yield of the two different rotation schemes (under a range of reserve sizes and rotation frequencies) to a static reserve system.
Results/Conclusions
The two rotational reserve schemes always faired worse than the static reserve system in preserving biomass, but approached the biomass level of the static reserve as the rotation period length increased to infinity (i.e., “effectively static”). Mean fishery yield for the rotational schemes was only nominally larger than that obtained under the static reserve system. Thus the advantages of rotation predicted by earlier models disappear when more realistic transient dynamics are included in the model. In the rotational schemes the yield was unevenly distributed over time with a few high peak years due to the sudden shifting of fishing effort in response to a reserve opening, followed by low yield for the rest of the rotation period. Life history, and in particular, the natural mortality rate strongly determined the optimal rotation period for yield. This influence is a strike against using the rotational reserve strategy as a management tool for multi-species fisheries since it would be difficult to find a rotation period to suit different life histories. For the purpose of conservation goals, a rotational reserve system should only be considered over a static system if the latter cannot be enforced for socioeconomic reasons (e.g., subsistence fishing).