OPS 5-9
Anchor damage negatively impacts coral reefs in the British Virgin Islands
Coral reefs are declining globally due to the integrative effects of natural and anthropogenic stressors. Boat anchoring has long been recognized as one of those stressors; however, its effects have been little studied. To assess the damage potential of anchoring, we took a two-pronged approach by determining the effects of both acute and chronic anchoring. We conducted our study in the British Virgin Islands because of the high density of yachts and the rising popularity of increasingly large vessels. First, we analyzed the impact of a single anchoring event that occurred in part of a long-term reef monitoring area, simulating a Before-after Control-impact (BACI) design. We were able to assess the changes in cover of benthic taxa over time and determine the magnitude of change corresponding to this one event. Second, we conducted a spatial survey of reefs experiencing differing levels of anchor damage from high to little or none. We collected data on cover of benthic taxa; coral size, density, abundance and richness; reef rugosity; as well as fish populations.
Results/Conclusions
Coral cover declined 12% because of the single anchoring event, a decline equivalent to that which occurred over 21 years at 7 other nearby sites. Chronic anchor damage reduced overall coral cover by an average of 7% to a level well below the Caribbean-wide average. Chronic anchoring also reduced the sizes of corals by 40%, the colony density by 60%, and coral species richness by 50%. Reefs experiencing high anchoring activity are 40% less structurally complex and thereby support fewer fish, less than 50% as many as sites with little or no anchoring. The magnitude of the impact of boat anchoring is dramatic. It is an impact worth studying in other locations and factoring more highly into reef conservation and management efforts.