Cayman Islands Coral Reef Health and Resilience Assessments
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Cayman Islands Coral Reef Health and Resilience Assessments
During June 2010, a team of 5-7 marine scientists surveyed 41 shallow reefs off Little Cayman, Cayman Brac and Grand Cayman. These surveys assessed benthic cover, population dynamics (size structure and condition) of reef building corals, and community structure of about 100 species of reef fishes, along with a number of factors that confer resilience. At the time of these surveys, the reefs were recovering from a recent (2009) bleaching event and an ongoing outbreak of white plague was threatening the vitality of these communities. In addition, an infestation by a pest species of macroalgae (Microdictyon spp.) carpeted northern Cayman Brac reefs, and the algae was overgrowing many of the corals.
Coral cover, as estimated from a minimum of 600 points per reef collected in hardground areas from 5-15 m depth, ranged from about 12% to 35%, with island-wide cover (mean value, pooled for all reefs) of 21% for Little Cayman, 22% for Cayman Brac and 26% for Grand Cayman. The dominant space occupier on these reefs was macroalgae, which occupied about 50% of the bottom. Other important colonizers included non-coral invertebrates (especially branching gorgonians and sponges), pest species (bioeroding and encrusting sponges, gorgonians, hydrozoan corals, colonial anemones and tunicates), and crustose coralline algae. Montastraea, Agaricia and Porites, respectively, were the dominant genera of corals in terms of cover. Evidence of past storms (rubble), residual coral bleaching (pale, but recovering corals), and old coral mortality was observed in most transects.
Recruits and juvenile colonies of 23 of scleractinian coral species were observed on these reefs, although P. astreoides and A. agaricites dominated in terms of total numbers of recruits (30% and 26% respectively of the 943 recruits identified). Low abundances of a number of important broadcast spawning species, including isolated colonies of M. annularis (complex), Diploria spp., Colpophyllia, M. cavernosa, and S. siderea. Two isolated A. cervicornis recruits were identified, but these corals remain rare throughout all islands. Recruitment appears to be higher at Little Cayman, with up to 4 recruits per square meter of reef substrate. Conversely, crustose coralline algae (CCA), an important cue for recruitment, was more abundant on Grand Cayman. Larger (4 cm+) reef building corals were dominated by Agaricia agaricites (approx. 25% of the population), M. annularis complex (25%), P. astreoides (13%), and Siderastrea siderea (10%), with 26 species making up the remainder of the population. M. annularis complex colonies were significantly larger than all other species (mean diameter >40 cm vs. 16-19 cm for other species). Most colonies of M. annularis were from 20-70 cm diameter, with very few colonies in the 4-10 cm size class, while most other species were 4-30 cm diameter. The M. annularis complex also exhibited significantly higher levels of transitional mortality (>32%) and old mortality (>25%), while other species were missing from 20-25% of their tissue overall.
The most significant cause of tissue loss was white plague, an extremely virulent coral disease that denudes tissue in a linear manner at a mean rate of 1 cm per day. This disease affects most (>90%) of the corals found on Cayman Island reefs, but it was most common and was having the greatest impact to M. annularis (complex). Other conditions of concern included 1) dark spots disease, primarily because this affected species (A. agaricites; D. stokesii) that have not been reported with this condition elsewhere; 2) snail predation, which was minor but a potential factor preventing recovery of acroporid populations; 3) overgrowth by clionid sponges and Trididemnum tunicates, which were observed killing corals and colonizing denuded skeletal surfaces and exposed substrates; and 4) Stegastes planifrons damselfish, which established algal lawns on massive corals that eventually led to their demise. Most other coral diseases known from the western Atlantic were seen on these reefs, but they were relatively uncommon. One disease, Caribbean yellow band disease (CYBD) has severely impacted M. annularis population on many other islands; this condition was seen in the Cayman Islands, but it was fairly rare. Given the extensive amount of old mortality observed on these corals, and low but widespread occurrence of CYBD, it is possible that an outbreak of this disease occurred in the past.
Reef fishes were dominated by parrotfishes (38%), surgeonfishes (22%), grunts (9%), and wrasse (8%). Important predators (groupers, snappers, jacks, barracuda) were present but at relatively low abundances. Most fish recorded in transects were in the 6-10 cm size class, followed by 11-20 cm and 0-5 cm with very few fish (<2%) over 40 cm in total length. Most of the parrotfish recorded in surveys were 0-20 cm, while most groupers were in the 11-20 cm size class. Interestingly, some juvenile groupers (0-5 cm) were seen on reefs, while the smallest size classes of snappers were absent. Reefs showed both positive and negative signs of resilience. Biotic factors that degrade resilience included the high cover of macroalgae, coral diseases and pest species. Human impacts were fairly minimal, with possible exception of fishing pressure (grouper, snapper, conch, lobster); and land-based pollution and sedimentation that may be associated with coastal development, dredging and canal development primarily on Grand Cayman. Shallow sites showed signs of diver/snorkeler impacts, especially shallow Acropora reefs off Grand Cayman. Positive factors included the presence of recruits of broadcast spawning species, the presence of large Montastraea colonies and other species that appear to be resistant to recent disease outbreaks, and the widespread occurrence of tissue remnants on colonies that have died back. Reef structure, presence of deep water near to reef communities, high relief sites with multiple canopy layers and high wave action/currents in certain locations also help enhance resilience.
In general, reefs of all three islands are in better shape than many other locations in the Caribbean, including the Bahamas, Florida, Puerto Rico, and Jamaica. There have been large, historic declines of acroporids, like that seen in other locations. The most important framework corals, M. annularis complex, have also declined precipitously, but many healthy colonies remain and even in those that have suffered large scale tissue loss, living tissue remnants are surviving and continuing to grow. Reefs also still contained groupers and other important predators, as well as other commercially and ecologically important invertebrates (conch, lobster) and fishes (herbivores and omnivores) at higher abundances and of a larger size than that seen in many other Caribbean localities.
Distinct differences were seen in all aspects of these surveys between islands and within individual reefs. A more detailed analysis will be presented in a subsequent report that incorporates additional data not yet included in the present analysis (e.g. algal composition, Borneman’s coral data) to better describe the current status of these reefs. The trends observed in 2010 will also be compared to previous studies to identify the changes that have occurred since these studies, potential future trends, and options to enhance protection and conservation of the reefs.