An extensive deep reef terrace on the Tortugas Bank, Florida Keys National Marine Sanctuary

Abstract

While most of the mapped area (137.5 km 2 ) of the Tortugas Bank consists of low-relief hard-bottom (105.5 km 2 or 77%) and scattered, rocky outcrops (16.6 km 2 or 12%), a sizeable portion of the western rim or platform edge (24 o 42.30’ N, 83 o 02.64’ W) is a well-developed reef terrace community (top panel). The topography of the substratum is very complex, owing to the numerous undercuts and caverns, as well as mushroom-shaped and plating corals up to 2 m in height. Coral cover is high (26.3 to 28.3% among three sites visited) relative to offshore reefs in the Florida Keys and is dominated by Montastraea faveolata, M. franksi, M. cavernosa, and Siderastrea siderea. The terrace community is a deeper version (22-27 m) of the reef terraces near Loggerhead Key 15 km to the southeast in Dry Tortugas National Park (Davis 1982). Anecdotal observations suggested that the western edge of the Tortugas Bank, locally named Sherwood Forest because of the predominance of mushroom-shaped corals (bottom panel), was spatially extensive. Using a combination of side-scan sonar, diver surveys, and bathymetry data, we estimate that the reef terrace is approximately 15 km 2 or about 10% of the mapped area of the Tortugas Bank. The “discovery” of this area by our science team was clearly preceded by the knowledgeable and active fishers of the region. Despite the remoteness of the Dry Tortugas relative to the Florida Keys, and the well developed reef structure, preliminary data indicate evidence of overfishing by a general absence of large species and individuals among exploited species, especially grouper (Serranidae) and snapper (Lutjanidae). Also, surprisingly few shark and barracuda were observed, and there was evidence of shrimp trawl damage to hard-bottom habitat. Because remoteness does not guarantee protection, the implementation of marine reserves in this region holds promise for restoration of fish stocks and protection of one of the largest areas of well-developed and previously undescribed coral reefs in Florida.