Size Matters
Design Tip
MPAs that are larger in size will capture the adult movement ranges and larval dispersal distances of more species than small MPAs. Although small sized reserves can have positive impacts, larger MPAs provide a benefit to a wider diversity of species.
The size of an MPA should take into consideration the need for large populations to insure against catastrophes, as well as the patterns of connectivity. Even small sized MPAs can provide positive benefits, in terms of fish biomass, size and abundance, but a single, small MPA provides insufficient protection to large populations of many species. In general, bigger MPAs can protect more habitat types, more habitat area, larger populations of species, and a greater number of species in the ecosystem.
The size criteria of MPAs within a network to maximize catch in surrounding waters are different from those used to design an MPA for conservation. The ideal size of MPAs for biodiversity conservation will generally be larger than those planned for fish stock protection and enhanced recruitment. In terms of fisheries, as MPA size increases, the potential fisheries benefit from spillover and larval recruitment will increase, but only to a certain point, and only if those targeted species are protected. If the MPA becomes too large, spillover and export will not offset the losses to fisheries due to the reduction in fishing grounds.1,2
General MPA size principles that apply to the entire MPA network are provided in the following recommendations (these may depend on how many MPAs or what percentage of total area is protected wtihin the network):
- Aim for MPA sizes that are 10-20 km in diameter, across their minimum width. Ultimately, the size of the MPAs within the network will be dependent on the management objectives and the specific social, economic, and ecological characteristics of the region.
- In terms of biodiversity protection, fewer large MPAs are preferable to a greater number of smaller ones. However, small MPAs that are embedded in a larger management area may have distinct advantages in that they protect pockets of high resistance and resilience to bleaching (and other valuable assets, such as fish spawning aggregation sites).
- To meet both fishery and conservation goals, intermediate sizes of MPAs, and a variation of sizes within a network may be ideal.3
- If the design of the MPA is focused on target species, optimal sizing may differ depending on the particular species characteristics.
- Another consideration in terms of the optimal size of an MPA is the feasibility of management. A smaller MPA is easier to enforce, and the monitoring efforts are less demanding. Larger MPAs take longer to establish and implement, and require greater financial support.
Strictly protected, no take areas are critical components of a resilient MPA network. Photo © James Oliver (reefbase.org)
No-take Areas
Coral bleaching events have demonstrated that replenishment is an important consideration for reef survival, regardless of the management objective. The effects of bleaching cannot be lessened by MPA zones, boundaries, regulations, or management efforts. Therefore, MPAs should be designed specifically to meet the requirements for reef survival. MPAs need to be large enough to be self-replenishing and sustainable. The optimal size of an MPA is designed around a strictly protected, no-take zone, or ‘core zone’, which encompasses sufficient target coral areas to be self-replenishing.
To support self-replenishing MPAs, the following no-take zone guidelines are recommended:
- The no-take area should be selected to encompass a diverse range of reef habitats.
- The no-take area should be as large as possible to preserve a high diversity of reef biota. At a critical minimum reef size the diversity of coral, and presumably other reef taxa, begins to decrease.
- Large reefs may be self-replenishing, because their size allows portions of reefs damaged by bleaching, slumping (collapse of the reef slope), storm surges, freshwater flooding, crown-of-thorns starfish invasions, or other stresses, to be replenished by recruits from undamaged parts of the same reef. Such large reefs are mosaics in different stages of community recovery and development.
- To ensure self-seeding, the MPA should be as large as the mean larval dispersal distance of the target species.