Akumal and Xcalak, Mexico

Robert Steneck & Judith Lang

Akumal Field Report Photos

The AGRRA protocols for corals, algae and fishes were applied at 6 fore reef habitats (averaging about 13m), in each of two regions 200 km apart in the eastern Yucatan coast. The Akumal region has a higher population density and higher degree of tourism than does Xcalak which is a small fishing community. Rapid assessments conducted March 1997 and 1999 compared the condition of the reefs in these two regions relative to each other and before and after the recent mass bleaching event and hurricane Mitch (both of which occurred in October 1998). In 1999 a total of 170 transects, 865 coral colonies and 798 algal quadrats were quantified.

Overall there was no regional differences between Akumal and Xcalak in the percent live coral (17.3, 16.3% respectively), percent old-dead (28.2, 28.4%), percent recently dead (4.0, 4.7%). However these reefs had already changed significantly by the time of the surveys. Although the dominant live coral species in the fore reefs in 1999 were Montastrea annularis and M. faviolata (comprising more than 50% of the total live coral cover), it was obvious that Acropora palmata, now ranked third in skeletal abundance, had once dominated this zone throughout the Yucatan coast. In 1999 there was little living A. palmata ( 62% of all A. palmata surveyed was standing dead skeletons and only about 3% of which was determined to have died recently).

Coral mortality and macroalgal abundance both increased recently in Akumal relative to Xcalak. Macroalgal abundances were high in both regions (43.8, 21.3% in Akumal and Xcalak, respectively). The significantly higher percent macroalgal cover in Akumal (p < 0.05 T-test) was mirrored by its higher canopy height (22.8, 15.8 cm) and thus algal biomass was much higher in Akumal than in Xcalak. The macroalgal cover between the two regions in 1997 was similar (28, 24%) so the recent increase is limited to Akumal. This observation is consistent with the increasing trends in coral mortality in Akumal. The percent of both old and recently dead coral increased in Akumal between 1997 and 1999 (27, 38% old, 3, 4% new).

The Akumal reef may be at greater risk than Xcalak but both show troubling signs. Reef fish communities are depleted of large carnivores and herbivore bite rates are well below those published elsewhere in the Caribbean. Fishing pressure may be greater in Akumal than it is in Xcalak. Fish surveys revealed higher densities of snappers, grunts, groupers and triggerfish at Xcalak. Grazing surgeon fish densities are lower (1.8 , 3.5/100 m2 for Akumal and Xcalak respectively) but parrotfish higher in Akumal than Xcalak (3.0, 1.8 / 100 m2). At all 12 fore reefs surveyed, the grazing sea urchin Diadema antillarum was below detectable levels.

The incidence of both diseases and bleaching were low in both regions. The highest incidence of disease was attributed to White Plague which was found on 5.0 % of all colonies in Akumal but only 0.63% of the colonies in Xcalak. Bleaching was subdivided into light, moderate and heavy bleaching. In Akumal most of the bleaching was moderate (9.2%) where as in Xcalak most of the bleaching was said to be moderate (11.5%). Mortality from all sources increased dramatically toward shallower depths. Whereas all mortality from all sources were below 10% at all the reefs at depths greater than or equal to 10 m, the mortality percent jumped above 25% in the shallow patch reefs we studied. Itıs possible that thermal stress resulting from the strong 1998 El Nino resulted in mortality only at the shallowest depths.

Overall, the Yucatan reefs appear to be at risk. Reduced fish fauna, low rates of herbivory and possibly eutrophication from high nutrient groundwater leave the reefs with dangerously high levels of macroalgal biomass. Where algal biomass has increased most rapidly (Akumal), the incidence of disease and mortality is higher.

For more information contact Robert Steneck at Steneck@maine.maine.edu or Judith Lang at JandL@rivnet.net.

İ 2000 - AGRRA