Mapping bathymetric and hydrographic features of Glover’s Reef, Belize, with a REMUS autonomous underwater vehicle

We used an autonomous underwater vehicle (AUV) to survey bathymetry and water mass properties at Glover’s Reef atoll in Belize as part of a multidisciplinary study of population connectivity of Nassau grouper (Epinephelus striatus). Nassau grouper populations are declining dramatically throughout its range in the Caribbean because of overfishing. In response, Marine Protected Areas have been established at several spawning aggregation (SPAG) sites. However, effectiveness of these measures is presently unclear, as the regional patterns of transport and recruitment of grouper larvae have not been measured in extensive field studies. As part of a pilot study to relate oceanographic processes with transport and recruitment of Nassau grouper larvae, we used a Remote Environmental Monitoring Unit(S) (REMUS) AUV to measure the bathymetry, stratification, current velocity, and chlorophyll a fluorescence around Glover’s Reef. The portability of the REMUS system enabled comprehensive oceanographic data collection in a remote location with minimal infrastructure. Using REMUS, we were able to effectively map the bathymetry of the reef and the shelfbreak. These data were merged with historical bathymetric data to develop a new chart of the atoll margin. A distinct contrast in the width of the shelves on the eastern and western sides of the atoll was demonstrated, with the widest shelf coinciding with the SPAG site. Water mass structure in the vicinity of SPAG site was also markedly different from other locations on the reef, suggesting an offshore advective influence on eggs and early-stage larvae released there. Many species of coral reef fishes aggregate to spawn at specific times and locations (Claydon 2004). This behavioral strategy makes species that aggregate to spawn particularly susceptible to overfishing. Nassau grouper (Epinephelus striatus) is perhaps the best-known example. It historically formed particularly large aggregations of up to 100,000 individuals at locations throughout the Caribbean (Smith 1972), but declined rapidly in the 1990s and was assigned an ‘‘endangered’’ status by the World Conservation Union (IUCN) in 1996. At least nine Nassau grouper aggregations were present in Belize before 1990, each with at least 30,000 fish. Currently, only three of these aggregations remain, but at abundance levels at least an order of magnitude lower than pre-1990 numbers (Sala et al. 2001). Protection of these aggregations is consequently a critical conservation imperative. There is also an urgent need to determine the degree to which larval replenishment from the remaining aggregations is sufficient to maintain current population levels and supply individuals to other heavily fished areas. Glover’s Reef (Fig. 1a), an atoll located approximately 45 km east off the coast on the edge of the continental shelf, harbors one of the three remaining significant aggregations of Nassau grouper in Belize (Starr et al. 2007). West of the atoll the continental shelf is relatively flat, with a mean depth of 400 m. On the east side of the atoll, the continental slope starts just beyond the reef and drops to over 3,000 m into the southern Yucatan Basin. The spawning aggregation (SPAG) site of Nassau grouper is located approximately 1 km east of the northeastern corner of the reef (Sala et al. 2001; Starr et al. 2007). Glover’s Reef has been designated a marine reserve by the government of Belize in an attempt to protect populations of Nassau grouper and other threatened and endangered marine species. However, the effectiveness of these measures cannot be determined without demonstrating larval replenishment from the protected areas. Although several recent studies have used coupled biophysical models to examine larval fish dispersal from Belizean atolls and barrier reef (Heyman et al. 2005; Cowen et al. 2006; Tang et al. 2006), none of the physical models used had the requisite spatial resolution to capture submesoscale physical processes that significantly affect population connectivity (Gawarkiewicz et al. 2007). Moreover, there is little detailed bathymetric data on which a high-resolution model can be built, and little hydrographic observations with which it can be validated (Heyman et al. in press). The work presented here was part of a larger effort to provide basic bathymetric and hydrographic information necessary for studies of larval dispersal from Nassau grouper SPAGs in Belizean waters. We conducted comprehensive mapping of bathymetry and water mass distributions using an autonomous underwater vehicle 1 Present address: Applied Physics Laboratory, University of Washington, Seattle, Washington, 98105.