The Bermuda Platform consists of a variety of tropical marine habitats – coral reef, sandy bottoms, algal beds, mangroves and seagrass. These interconnected habitats are vital to fish, crustaceans, such as spiny lobsters, and sea turtles, in particular green turtles and hawksbills.
Since 2006 the Department of Conservation Services’ Marine Ecology Section has been monitoring the health of benthic habitats on the Bermuda Platform with a Benthic Mapping, Monitoring and Assessment Program . These programs were initiated in response to a reported decline in seagrass around Bermuda (Murdoch et al 2007). This decline may have significant effects on populations of green turtles, fish and crustaceans in Bermuda.
BENTHIC - relating to the bottom of the sea
BENTHOS - the animals and plants living on
Green turtles in Bermuda waters have been protected by law since 1978 (Fisheries (Protected Species) Order 1978) and more recently seagrasses were added to Bermuda’s list of protected species (Protected Species Order 2012). We realize that a better understanding of the dynamic relationship between green turtles and their essential habitats in Bermuda is pivotal for developing a conservation strategy that protects both the turtles and their habitat.
In addition to our benthic monitoring program, we have conducted research addressing the effects of sea turtle grazing on the condition and health of seagrass in Bermuda. Initially, we placed exclusion cages to prevent turtles from feeding in three 2 m x 2 m plots in a heavily grazed seagrass meadow to determine if the seagrass under the cages would recover.
After one year the seagrass leaves were as long as those in the nearby ungrazed meadow, indicating that over the short term seagrass can recover from heavy grazing (Fourqurean et al 2010). This summer, four years after the exclusion cages were installed, we removed them and positioned video cameras underwater to observe if turtles would return to eat the grass in the plots that had been protected. Twelve days later, there was no evidence of turtle grazing in the previously caged plots but the video footage documented turtles grazing outside the formerly protected seagrass plots. Previous studies have suggested that green turtles prefer the new growth on grazed shoots (Bjorndal 1997) and it appears that turtles in Bermuda may too.
This summer we also extended our studies to look at how green turtles are using the Bermuda Platform. To do this we have attached satellite tags to 3 green turtles (http://www.tourdeturtles.org/Bermuda/) at three locations on the Platform. We also deployed small video cameras and time-depth recorders on pop off tags temporarily attached to 13 large green turtles. The video footage will provide information about turtle behaviour, showing the various habitats the turtles are using and what they are eating. The time depth recorders will tell us how long the turtles are at the surface, under the water, and to what depths they are going. The dive and surface interval data in association with boat surveys that count turtles will be used to help us estimate the number of green turtles on the Bermuda Platform. This is part of a longer term study designed to estimate the carrying capacity of the Bermuda Platform for green turtles.
In addition to studying where green turtles live and move about on the Bermuda Platform, we have been collecting small samples of skin and carapace from the animals. These samples will be analyzed for nitrogen and carbon stable isotopes to determine the turtles’ diet. We are interested in knowing whether the green turtles on the Bermuda Platform are primarily eating seagrass or if they are supplementing their diet with other food items like they do in Shark Bay, Australia (Burkholder et al 2011). In Shark Bay it appears that green turtles derive most of their energy from macroalgae and gelatinous macroplankton (e.g. ctenophores and cnidarians) rather than seagrass. Knowing the full scope of the turtles’ diet, their behaviour, density and utilization of the different habitats will provide valuable information that will be incorporated into management strategies for the conservation of green turtles and their essential resources in Bermuda.
These studies are a collaborative effort between;
Sarah Manuel, Kathy Coates, Anson Nash, Jan Locke, Katie Dilke and Sarah Gosling, Department of Conservation Services
Mark Outerbridge, Bermuda Zoological Society
Jim Fourqurean, Derek Burkholder and Mike Heithaus, Florida International University
Jud Kenworthy, NOAA (retired)
Anne and Peter Meylan, Bermuda Turtle Project
Robert Hardy, Florida Fish and Wildlife Conservation Commission
Burkholder DA, Heithaus MR, Thomson JA, Fouqurean JW (2011) Diversity in trophic interactions of green sea turtles Chelonia mydas on a relatively pristine coastal foraging ground. Mar Ecol Prog Ser 439:277-293
Fourqurean JW, Manuel S, Coates KA, Kenworthy WJ, Smith SR (2010) Effects of excluding sea turtle herbivores from a seagrass bed: Overgrazing may have led to loss of seagrass meadows in Bermuda. Mar Ecol Prog Ser 419: 223–232