For the first time since Chagos became a no-take marine protected area, a scientific expedition of 12 scientists visited the archipelago. We collected data to inform management, and conducted projects to help determine how reef systems function, in a way which now cannot be done in most over-exploited areas of this tropical ocean.
What we did in just 18 days, and why, follows. The linked pdf gives a fuller account.
Coral cover monitoring
The amount of coral reef surface covered by coral has long been used as a simple method to assess the health of a coral reef and in Chagos, it has been used since the 1970s. It recorded the crash seen in 1998 caused by high temperatures, and the subsequent amazing recovery and, it also recorded earlier smaller warming events which were unnoticed at the time. Measurements have been frequently repeated at the same places, at intervals of 5 m depth down to 25 m, to map a spectacular recovery. The kinds of corals most vulnerable to temperature rise are branching corals, and it is good to see that a large abundance of these have returned to Chagos today.
‘Hidden’ species
Raised sea temperature is an important cause of coral mortality and was responsible for massive mortality in 1998. Since 2006, numerous recorders placed at many sites and depths have logged temperatures at 2-hour intervals, and 10 new ones were deployed in 2012. Results show that on seaward reefs cool water rises up the reef during the hottest periods of the year, which may have a bearing on the excellent recovery shown by coral reefs here.
At the same places, several corals were cored. Isotope ratios in their annual growth rings (like tree rings) show temperature and fresh water inputs for the year in which that growth ring was laid down. This tells us about past temperatures and rainfall, which is of interest to global climate programmes.
Juvenile corals
Juvenile corals will provide the reef fabric and habitats of the future. In 2001, their numbers were the highest recorded anywhere to that date. In 2012 Diego Garcia was focussed on, and results again show high and very healthy recruitment in this atoll also.
Remote underwater video sampling for fish
There is a need to expand monitoring for fish, and a system of Baited Remote Underwater Video cameras (BRUVS) were developed to study these, including shark assemblages. These extend both the taxa sampled and the depth range over which sampling occurs, compared to diving surveys. This allows us to determine how fish species assemblages and densities vary across the Chagos shelf and whether depth ranges of many fish species are in fact greater than previously assumed from diver surveys. BRUVs were deployed at 208 stations, yielding over 400 hours of video imagery at depths from 6 to 84 m deep. Large numbers of predatory fish including sharks were counted as well as a rich diversity of smaller reef fish at all sites.
Particularly interesting were submerged reefs and a sea mount whose upper surface was 85 m deep, which similarly showed very high numbers of predatory fish species including deep water species. The video will also be a valuable communications and outreach tool.
Reef fish biomass in Diego Garcia
Reef fish biomass is known to be 1 to 2 orders of magnitude greater than from other measured locations in the Indian Ocean. This information was used in 2012 around Diego Garcia to detect whether there are any detrimental effects of the recreational fishery or other activites. Preliminary results indicate the biomass in Diego Garcia is lower than that in the northern atolls, but many large bodied fish of species typically vulnerable to fishing were still abundant, suggesting that recreational fishing there is not too detrimental and, furthermore, biomass was still higher than most locations across the Ocean. Work will now try and determine what differences between Diego Garcia and northern atolls of Chagos are due to fishing and what are due to factors such as its relatively distant location.
Monitoring of shark populations.
The number of sharks seen per scientific dive has been documented in Chagos since 1975. By 1996, population numbers had declined by about 90%, and had not recovered by 2006. Data from 2012 may warrant cautious optimism that populations are showing a weak recovery, but it will be important to continue monitoring the populations to be sure about this.
Flight initiation distance of reef fishes
Flight initiation distance (FID) is the distance to which a human can approach an animal before it leaves, and is a widely used technique. For fish, FID varies in response to fishing; specifically, fish can be approached much more closely when fishing is light or when fish are within a protected area. FID of parrotfishes and surgeonfishes was assessed at 12 sites where reef fish biomass data were collected previously, thus allowing assessment of predation risks. FID of fish in Chagos was between 40 to 65% less than has been recorded elsewhere, reflecting how tame, or naïve to humans the fish are.
Size sex change in parrotfishes
Some fish families, such as parrotfishes, show sequential sex change through life. Parrotfishes are initially females, and latterly males with usually much more vibrant coloration. Males generally live in harems with many females. Fish size at which sex change occurs can vary according to fishing pressure and the influence of other predators, which has implications for the number of larvae produced and the functions which these important fish play on the reef. The size at sex change of 17 species of parrotfish was estimated at 16 sites where reef fish biomass, including that of predators, is also available. Again, similar data are available for a range of other locations across the Indo Pacific which will allow Chagos to be compared with them.
Genetic analysis
Over 50 scientists have participated in research using materials from Chagos collected for them. Again in 2012, samples were taken of several groups for others in several countries. One programme uses genetic analysis to try and determine why Chagos corals have been particularly resistant to sea water warming, or, why they have recovered so quickly. One result has been to show how symbiotic algae in coral tissues have developed resistance to bleaching. Other genetic work is designed to help determine the amount of genetic exchange between Chagos and elsewhere in the Indian Ocean, including the genetics of the important coral predator, the Crown of Thorns starfish….
Crown of Thorns starfish
These are coral predators, whose outbreaks kill most corals on a reef. They are usually an uncommon component on reefs, but occasionally appear in plague numbers. In Chagos, past records show only one plague recorded before, on Speakers Bank in the late 1970s. This time, however, an outbreak was found on Eagle Island, and this was mapped to see how the trend continues in the future. The cause remains a mystery for now.
Shoreline erosion
In several places erosion of islands by the sea continues, and is being mapped in some places. In Diego Garcia too it is causing increasing concern. This is caused mainly by the highest tides which are now causing seawater incursions in several islands throughout the archipelago, and in Diego Garcia water in several places has now broached the narrow strips of raised land between beaches and road or grass areas, allowing flooding. In one area small sharks now can be seen swimming over the grass.
Islands and birds
Nesting seabirds are enormously abundant on several islands, especially those of the Great Chagos Bank. These nest very poorly on rat infested islands dominated by coconut trees, and birds are clearly limited by habitat and not by food resources. Tens of thousands of pairs of birds were nesting at the time of the resurvey in 2012. Plans are being developed to restore native vegetation on some small, manageable islands, and a priority list has been drawn up. In Diego Garcia, three large experimental restoration plots have shown the viability of this.
Professor Charles Sheppard