Craig P. Dahlgren and Jack Sobel. ABSTRACT ... big must such a reserve be to provide specific benefits, and how can one predict this size without having one?
BULLETIN OF MARINE SCIENCE, 66(3): 707–719, 2000
DESIGNING A DRY TORTUGAS ECOLOGICAL RESERVE: HOW BIG IS BIG ENOUGH? …TO DO WHAT? Craig P. Dahlgren and Jack Sobel ABSTRACT A review of the global experience with no-take marine reserves strongly suggests that they are important tools for marine conservation and fishery management, capable of providing benefits in the form of ecosystem protection, improved fishery yields, expanded understanding of marine systems, and improved nonconsumptive opportunities. The degree to which a reserve will provide certain benefits or achieve specific goals will vary with species, depending on life-history characteristics and various aspects of reserve design. The Florida Keys National Marine Sanctuary management plan created a network of no-take reserves encompassing just 0.5% of the sanctuary’s total area but provided for the creation of a Dry Tortugas Ecological Reserve, an additional marine reserve in and adjacent to the Dry Tortugas region of the sanctuary. The existing reserves are small because they were intended to provide only limited fishery benefits, but additional fishery benefits are being considered among the objectives of a Dry Tortugas reserve. How big must such a reserve be to provide specific benefits, and how can one predict this size without having one? We use a simple model based on the percent of virgin biomass (%B0) in fished and unfished areas to provide managers with a quick and easy way of estimating the reserve size required to meet specific management objectives. Analyses of %B0 for populations in fished and reserve areas suggests that a Dry Tortugas reserve encompassing at least 30–40% of the region is required to elevate all stocks from current levels to overfishing-threshold %B0 levels, but smaller reserves might be used to complement conventional fisheries-management practices as a buffer against some level of overfishing and insurance against complete stock collapse.
Recent reviews have discussed the potential for no-take marine reserves to provide over 50 specific benefits grouped into four broad categories: (1) protection of ecosystem structure, function, and integrity; (2) improvement of fishery yields; (3) expansion of knowledge and understanding of marine systems; and (4) enhancement of nonconsumptive opportunities (Sobel, 1996; Bohnsack, 1998). Although a reserve’s ability to provide many benefits may be most influenced by the level of protection and enforcement within it (Russ and Alcala, 1996; Roberts, this issue), reserve function and efficacy will also be influenced by characteristics of reserve design, such as location, shape, size, and proximity to other reserves (Carr and Reed, 1993; Rowley, 1994; Allison et al., 1998). The appropriateness of a particular reserve design depends on both the management objectives and characteristics of the species being protected. Unfortunately, little empirical evidence from field studies suggests how large a reserve must be to provide specific benefits, particularly those related to managing fisheries. A very small reserve (30% of the region of influence may be necessary. In general, reserves smaller than 30% of the region were only useful for raising SSB of the most threatened stocks enough to help prevent against complete collapse of the stock; they were too small to achieve most other management objectives when reserves were somewhat leaky. Because reserve size depends not only on status of fished stocks and management objectives but also on the buildup of SSB within the reserve, it is important to understand how the level of reserve protection affects SSB. Although population density, age, and size structure, as well as SSB, are expected to increase within the reserve and even approach unexploited levels in parts of the reserve, they are unlikely to do so to the same extent in all parts of the reserve. Because mobile individuals near the edge of a reserve are more likely to move out and be caught in the fishery than those nearer the center, populations in the center may approach unexploited levels, but SSB is expected to decrease with increasing distance from the center (Rakitin and Kramer, 1996). BR is therefore expected to be somewhat less than 100% B0. Similar BR values may result if poaching occurs within the reserve or if the reserve includes disproportionately high amounts of suboptimal habitats (Roberts, this issue). Although the Dry Tortugas region contains appropriate habitats and is expected to be a source area for downstream areas in the Florida Keys, there will be some leakage, and poaching may be problematic (E. Proulx, Southeast Region, Head law enforcement officer, National Marine Fisheries Service, pers. comm.). The effectiveness of a reserve for conserving SSB is also expected to be influenced by movement rates and life-history characteristics of species within the reserve and by reserve design characteristics. Simply put, species that have large home ranges, or large daily, seasonal, or ontogenetic migrations, are more likely to leave a reserve and be caught in the fishery than are sedentary species. They are therefore less likely to experience as large an increase in SSB within a reserve as sedentary species. The effect of movement rates on the buildup of SSB is also related to reserve size; smaller reserves are expected to result in lower BR values than are large reserves for mobile species. Thus, the effect of a reserve on BR is difficult to predict, and reserve size and BR may be somewhat confounded. Because our model does not take the interaction between reserve size and BR into account, minimum estimates of reserve size required to meet various management objectives may be slightly underestimated at small reserve sizes. This bias may be particularly important for hermaphroditic species, such as many of the grouper species found in the Florida Keys. For these species, reproductive output is related not only to BR but also to the sex ratio of the population. Thus, reserves must be large enough to preserve the size structure and sex ratio of the population in addition to increasing BR if they are to achieve fishery-management objectives. Other aspects of reserve design, such as the shape or perimeter-to-area ratio, location of reserve in source or sink areas, and location of reserve boundaries relative to barriers to movement, will also influence BR and the area that must be protected to meet specific objectives. Similarly, the number of reserves used to protect an area will influence the
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total area that must be protected. Our model only predicts the total area that must be protected under various BR, BF, and BT combinations. If the total area protected is one large reserve, BR may be higher than if the same area consists of a network of smaller reserves, because of increased leakiness due to differences in the perimeter-to-area ratio. A network of smaller reserves would therefore have to encompass a larger total area than would a single large one to achieve a given management goal. The area that must be protected to meet a given objective is also influenced by changes in BF, which may change as a result of reserve protection. For example, it may decrease if reserve establishment displaces fishing effort from the reserve to other areas. Nevertheless, model results indicate that, when BR is high, even large decreases in BF, from 15 to 1%, require relatively small increases in reserve size (e.g., Fig. 4). In general, our results suggest a range of reserve sizes similar to those of other studies using more complex, mechanistic models (e.g., Polacheck, 1990; DeMartini, 1993; Sladek Nowlis and Roberts, 1999). For example, Sladek Nowlis and Roberts (1999) indicated reserve sizes of 75–80% of the total area were optimal for maintaining long-term sustainable yields of fish species with high fishing mortalities, but reserves encompassing only 40% of heavily fished areas may produce substantial benefits. Our estimates of reserve size required to prevent BT from falling below thresholds when BF does were also similar to those of other studies, but we examined a narrower range of variability in harvest than other studies. For example, Lauk et al. (1998) proposed that >50% of the area of interest must be protected to ensure a high probability of stock persistence under slightly variable harvests, and even larger reserves were required for persistence under more variable harvests. Although models used in other studies may include more biological detail, results provided by our simple model are expected to meet the needs of reserve and fishery managers, without requiring collection of detailed biological information. Nevertheless, information related to buildup of SSB within the reserve and the establishment of threshold and target levels of biomass for reef-fish species in the Florida Keys may improve the model as a predictive tool for fisheries management. Our model sugest that a large reserve (or large network of reserves) in the Tortugas region may be vital for protecting fish stocks in the Florida Keys. The FKNMS now contains a network of small no-take areas (Fig. 1) that total only 0.5% of the sanctuary area. Although such small reserves may provide some benefits, our model suggests they will do little to increase SSB enough to achieve regional fisheries management objectives. Similarly, they will provide little insurance against fluctuations in fished stocks on the scale of the entire sanctuary (i.e., will only buffer BT for the entire sanctuary against a drop in BF
