Abstract
Marine protected areas (MPAs) are effective at employing ecosystem-based management as a conservation tool, however these networks of protected zones must be carefully chosen. Therefore, strategic area placement and size is crucial for the best conservation outcome. While the benefits of implementing clear protected zones are obvious for species that live sedentary lifestyles, the proportion of habitat protection that they offer to wide-ranging species is less clear. Here, we analyze satellite telemetry data in order to determine key habitat areas for three wide-ranging shark species in both the south Florida and Bahamas regions, and then quantify the level of potential habitat protection that is offered to these species by both management zones within Florida and The Bahamas EEZ. We further assess their movements and habitat use with regards to habitat preference, specifically water column depth and distance from land. The three species selected for this study are the great hammerhead (Sphyrna mokarran), tiger shark (Galeocerdo cuvier), and bull shark (Carcharhinus leucas). These species were selected primarily because, as a group, they are the apex predators in the region, are representative of the region’s marine megafauna, and they rank as a conservation priority. The study area encompasses a number of different MPAs, each differing in qualities such as purpose, management agency, level of protection, and restrictions on human uses. However, for this study we are most concerned with locational boundaries and level of protection offered. More specifically, we primarily look at those areas that restrict fishing, which is a major anthropogenic threat to the study species. To accomplish this analysis, we use ArcGIS, a geographical information systems program, to overlay MPA data in the study region with analysis of satellite telemetry data. Core activity space was determined using kernel density estimates, and the 50% kernel density estimate isolpleth was considered “critical habitat.” The proportion of critical habitat that is protected for each target species was then calculated by determining the percent overlap of the critical habitat area with protected areas relevant to each species. To supplement this assessment, we then evaluate habitat preference based on overlap of critical habitat use areas and physical factors, where bathymetry data is overlaid with kernel density estimate isopleths. Mean and max distance from land was also calculated for all interpolated points for each species, and all interpolated points within defined critical habitat boundaries. This analysis allowed us to quantify habitat use with regards to water column depth and distance from shore. Finally, we created separate seasonal kernel density estimates for all species for the wet and dry seasons, in order to evaluate seasonal habitat preference. Results from 26 tagged bull sharks demonstrate that Biscayne and Everglades National Parks are high use areas for these sharks, and that none of their critical habitat is currently within areas that protect the species from fishing pressure, more specifically prohibiting the landing of the species. Analysis of data from 22 tagged great hammerhead sharks illustrates that 17.88% of their critical habitat is currently protected, almost all of which is due to regulations that prohibit the landing of the species within Florida state waters. Finally, 34.74% of tagged tiger shark (n=44) critical habitat is currently protected, due to regulations that prohibit the landing of the species within Florida state waters and regulations prohibiting shark fishing within The Bahamas EEZ. Habitat preference results are consistent with our previous analysis, and show that bull sharks prefer depths of less than 10 meters and are, on average, very close to land. We found that mean distance to shore for all interpolated bull shark points was 6.4 km, and mean distance to shore for all interpolated bull shark points that lie within their defined critical habitat area was 4.3 km. Data from our 44 tagged tiger sharks demonstrated that 78.25% of their critical habitat consists of depths greater than 100 meters and mean distance from land for all points was furthest for this species, at 176.6. Average distance to shore for all points within the tiger shark critical habitat was 71.9. We found that great hammerhead critical habitat was split relatively evenly between each depth range, and their mean distance to shore for all points and for only those points within their critical habitat was 82.8km and 16.6km, respectively. Finally, seasonal kernel density estimates created for the wet and dry seasons showed that habitat use does indeed seem to vary by season. Many wide-ranging marine top predators are experiencing population declines globally (Pauly et al. 1998, Myers & Worm 2003, Hampton et al. 2005, Dulvy et al. 2008). Marine reserves are the first step toward evaluating how effective these MPAs are as conservation tools and understanding how we can work to improve them. The results from this study have valuable implications for marine conservation planning and help to develop an understanding of the current and potential level of protection for the top predator species that play integral roles in south Florida’s and The Bahamas’ ecosystems.