Assessment of anchor damage and carrying capacity of seagrass beds in Francis and Maho Bays for green sea turtles
Williams SL. 1988. Assessment of anchor damage and carrying capacity of seagrass beds in Francis and Maho Bays for green sea turtles. Biosphere Reserve Research Report No. 25. National Park Service and Virgin Islands Resource Management Cooperative. St. Thomas, Virgin Islands
Over the past two decades, there has been a serious decline in the seagrass beds in Francis and Maho Bays, St. John, U.S. Virgin Islands. These beds provide habitat and food for one of the largest populations of the endangered species of sea turtles, Chelonia mydas L. , the green sea turtle, in U.S. jurisdiction. The population size was estimated at 50 subadult turtles. These turtles exert heavy grazing pressure on the leaves of their preferred food, the seagrass Thalassia, in the bays. In addition to grazing pressure, the seagrass beds are seriously disturbed by the increasing number of boats that anchor in the seagrass, damaging roots and rhizomes. Up to 10% of the seagrass beds in the bays is presently damaged by anchors. The effects of the grazing and anchor damage were evident in the extremely low productivity of the fragile, achlorotic, short and narrow Thalassia leaves. A carrying capacity for the turtle population was calculated based on this productivity and the feeding requirements of the turtles, estimated by observing turtles directly and remotely using radiotelemetry. The carrying capacity was estimated at 11-31 subadult turtles, indicating that the population may be in danger of decline. The grazing behavior of the turtles differed from the patterns reported previously in that the turtles fed throughout the day without taking characteristic midday breaks in feeding. This difference may be in response to the poor quality and quantity of food provided by the seagrasses in the bays. Although anchor scars in the bay recovered little during the course of the study. some recovery of Thalassia occurred inside fences that excluded anchor damage and turtles. After 3 months, Thalassia blades inside the fences were significantly longer than outside and the productivity per shoot increased. In order to protect the turtle population from further habitat degradation, it will be necessary to reduce the damage done by boats in anchoring the seagrass beds in the bays.
- Type
- Published Report
- Authors
- Williams, Susan
- Date of Issue
- 1988
- Publisher
- National Park Service and Virgin Islands Resource Management Cooperative
- Units
- VIIS
- Keywords
- Anchoring, Animal Studies, Carrying Capacity, Exclosures, Francis Bay St. John US Virgin Islands, Green Sea Turtle (Chelonia mydas), Halodule (seagrass), Maho Bay St. John US Virgin Islands, Map, Plant Studies, Productivity, Seagrass Beds, Syringodium (seagrass), Thalassia (seagrass)