Coral Reef Monitoring Project

Reef Care's Coral Monitoring Project is part of a Caribbean-wide monitoring program coordinated and supported by ReefKeeper International. It started in May 1997 with the survey by Reef Care Curacao volunteers of four sites, evenly distributed along the coast of our island. Since then, every three months these same sites have been surveyed by the dedicated volunteers of Reef Care Curacao's Monitoring Group. The aim of this monitoring project is to keep track of what is happening on the reef and get a measure of the state of the reef. Keeping a finger on the pulse of the reef is particularly important, as it appears to be deteriorating rapidly, especially over the past decade or so. A monitoring program will alert us to sudden changes or new threats, hopefully in time to do something about them, but in any case enabling us to quantify the deterioration and make people aware of what is happening.

Here we present you the latest results. All data were collected using triplicate 50 m long, 50 cm point-intersect transects. For a full description of the survey method used see Reef Care News Vol. 5(2/3). For more information or to join the monitoring group contact Reef Care Curacao, e-mail: reefcare@cura.net, or call coordinator Theo v/d Giessen at Uniek Curacao: 462-8989

Two years of monitoring

For over two years Reef Care Curacao's volunteer divers group has been monitoring four reef sites along the leeward coast of Curacao. Eight times over the past two years this group of dedicated people has sacrificed two entire weekends to collect data on the amount of coral cover, the state of health of the reef and the amount and type of algae cover present at the sites. These sites are Playa Wachi (Slangenbaai, just east of Boka San Michiel), Portomari, Pos Spañó and Watamula Reef. When possible, the group also surveys Oostpunt, but boat transportation is hard to come by. It is time to take a look at the accumulated data and see what they tell us.

Fig. 1. Map of the island of Curacao and the five survey sites monitored every quarter.

In graph number 1 at the top of next page we see the mean coral cover percentages for the different locations and depths. It also shows the standard deviation as a straight vertical line bracketing the top of each column in the graph. The standard deviation is a measure for the amount of variation between the three different transect-lines that are surveyed for each depth and location. Because the coral reef is so highly variable, no two spots are exactly alike so the coral cover varies between the transects lines surveyed, even if they are only 1 m apart. That variability is also the reason why we do not use the same permanently set line at every survey; that would give no measure of the reefs variability. We do go to the exact same area every survey, so the variation is small, and the mean of three transect lines is a good measure of the situation.

Graph 1. Hard Coral cover at 20 ft and 40 ft, at the four monitoring sites on Curacao. Standard deviation indicated bracketing the top of each bar

Looking at graph 1 we immediately see that the coral cover (the part of the bottom that is covered by coral) for each site is not the same, nor is it the same for the different depths at each location. Slangenbaai at 20 feet has a decidedly lower coral cover than at 40 feet. At Portomari both depths show about the same coral cover. As you can see the average coral cover for all sites is around 40%, but there are some sites that have a much higher coral cover at 40 feet depth, i.e. Oostpunt, Slangenbaai and especially Watamula Reef. They have a cover of more than 70%, almost 80% at Watamula. This is exceptionally high, not only for Curacao but for the whole Caribbean. The average coral cover for all sites of around 40% is also higher than in many other places in the Caribbean. Besides some variation in amount of coral cover between surveys which can be attributed to different positioning of the transect lines and the variability within the surveyed area, it can be seen that the coral cover did not significantly change over time.

Graph 2. Percentage of corals manifesting signs of disease

Looking at graph nr. 2, which shows the percentages diseased corals found it is immediately apparent that in the first few surveys of 1997 a lot of disease (10-20%) was present in most sites, but that the percentages drop markedly in later surveys, reaching a more or less steady level of around 1-2% in the most recent surveys. It appears that the incidence of yellow-blotch disease responsible for most of the corals marked as diseased, has dropped rapidly over the last two years, and the reefs are healthy now.

However the effects of the epidemic remain and can be recognized in graph 3, which shows the numbers of corals with recently dead areas. They reach a peak right after the highest incidence of disease was found, and presumably reflect the effects of the disease which kills off parts of the corals. As the epidemic dies down, so does the amount of recently dead coral, corresponding fairly accurately with the low level of disease now present.

Graph 3. Percentages of corals with 10 percent or more of their surface area dead.

Although the reefs are pretty healthy and the incidence of disease is at a low level now, it is not quite gone yet and parts of corals are still dying off, probably faster than they can re-grow. Hopefully the decline of the disease will continue until it is completely gone. The epidemic may be a natural event and the corals may be becoming immune to it, but it does drive home again the importance of the good condition of the corals so they will have their maximum natural resistance against diseases. We cannot change the natural course of things, but we can try to remove all human induced stress from impacting on corals. We will continue monitoring and keep our fingers crossed.

This Coral Reef Monitoring Program would not have been possible without the help, not only of the dedicated volunteers, but also because of the support of a number of dive shops on Curacao, who provided us with tanks, diving equipment if needed, and boat trips out to the survey locations where necessary.