|a longsnout flathead (Thysanophrys chiltonae)|
11th Jan. – 2nd Feb., 2006
Sagharughombe reef is located 10 kilometers east of Gizo, the provincial capital of Western Province, Solomon Islands. The reef lies adjacent to Kennedy (Plum Pudding) Island, a small islet that gained its fame during WWII. In a heroic rescue, John F Kennedy pulled his fellow soldiers ashore after the PT109 sank in the area.
Nowadays Sagharughombe and the vast reef area around Sagharughombe are visited regularly by dive tourism operators. Hence Dive Gizo, WWF and two other parties have placed dive buoys on Sagharughombe reef as well as on other reefs in the area to relieve environmental pressure on this popular dive site.
Mbambanga is an island situated on the other side of the lagoon. Although the closest settlement to Sagharughombe reef is a Gilbertese community on the island of Mbambanga, the villagers do not claim the fishing rights to the reef, however, they frequently fish it. The Gilbertese community were relocated to Mbambanga by the British government in the 1950s after a food and water shortage occurred in the Phoenix Islands. Apparently, there are no claims to the reef, which is surprising in Melanesia, where ownership to reefs is commonplace. It is common territory for fishermen from Mbambanga and other places in the Gizo area.
Lever’s Pacific Timber Company were responsible for clear cutting the forest in central Gizo, beginning in the 1960’s. This same company was also involved in the logging of Kolombangara Island, an extinct volcano located across the Blackett Strait, 4.2 nm north of Sagharughombe reef. Lever’s Pacific Timber pulled out of the area in 1986 leaving behind muddy rivers, bare earth, and massive erosion problems. The destruction of the Kolombangara forest has led to severe environmental and social problems on the island. It is also possible that runoff from the logging activity could have been impacting nearby reefs like Sagharughombe reef.
Today ecological friendly logging on Kolombangara leaves a line of forest around the coast to reduce run off from the land into the sea.
The Sagharughombe reef system is part of a chain of platform reefs and small islands with fringing reefs, which protect the islands of Nusatupe, Mbambanga, Epangga and Olasana south-east of Gizo. Each of these islands also have their own fringing reefs. A lagoon lies between these fringing reefs and the outer chain of fringing reefs and platform reefs of which the Sagharughombe reef system forms a part. This outer reef chain is broken up by several passages, through which water masses draw during tide changes.
|this map shows the outer platform reef system surrounding Mbambanga and |
Olasana Islands, the location of the Sagharughombe Reef Tip and Lagoon Slope and
the sites of observational dives conducted during January/February 2006
When we studied Sagharughombe reef in 2000, we described it as a fringing reef around the small island of Sagharughombe which we nicknamed ‘One Tree Island’ because there was one large tree shooting up from this very small landmass. The tree was already gone by our return in 2002 and all that remains today is a small pile of coral rock and sand (~8 m2). Since this landmass is so small and located at the tip of this 0.4 km2 reef system, we shall now describe Sagarughombe reef as a platform reef.
Sagharughombe reef runs in a strip from north west to south east. The western side of the reef slopes to the lagoon, beyond which lie Olasana and Mbambanga Islands. Corals grow here on a steep slope down to about 20 meters depth. From here on sand substrate drops down to the bottom of the lagoon, reaching about 70 meters maximum depth. The eastern outer slope is a steep drop off, first to about 50 metres then reaching about 300 meters in the Blacket Strait. In the north, the reef is separated from Kennedy Island’s fringing reef by a channel, 35 meters deep. In the south Sagharughombe is also separated from its neighbouring reef by a 33 meter channel.
The most northwestern tip of Sagharughombe reef is marked by a navigational marker and a dive bouy. It is also separated from the rest of the entire Sagharughombe reef by a narrow channel, 15 meters deep. The reef flat of the northwestern tip rises to 3 meters below the surface, while the reef flat south of the small channel rises up to 1 meter below the surface. For the purposes of our study, we covered both parts of Sagharughombe reef.
Vitareef data collection and transects were conducted on the reef’s western slope inside the lagoon (which we shall call the ‘lagoon slope’) and its northwestern tip (which we shall call the ‘reef tip’). Within these areas two depth zones were studied:
We conducted observational dives all along the western inner and eastern outer slopes of Sagharughombe Reef, at the outside wall of the fringing reef surrounding Leorava Island and in the channels north and south of the Sagharughombe reef system.
Additionally using a handheld global positioning unit (GPS) we spent one day tracking the entire Sagharughombe reef and the fringing reef around Kennedy Island.
Inside the lagoon we observed visibilities between 23 and 30 meters measured by Secchi disk through most of January. At the end of January/beginning of February we went through a bad weather period with a lot of rain and wind blowing up to 25 knots from northwest. Tropical Cyclone Jim was passing about 600 nautical miles south of us during this time. During these days visibility was noticeably reduced, but due to rough conditions we could not take Secchi disk readings.
The water temperature, measured from the RV Heraclitus anchored 3 cables off Mbambanga Island, ranged from 30.3°C to 30.7°C for most of January. At the beginning of February during the bad weather period, the water temperature dropped to 29.1°C.
On Sagharughombe reef itself a water temperature of 31°C was encountered during most days of January, but it reached 32°C at times. On the outside of the reef we measured 30°C at 30 meters depth. While making our last dive at the end of January the temperature on the reef had dropped to 30°C.
We checked the NOAA/NESDIS sea surface temperature monitoring website on 6th January 2006 and found that a hot spot was beginning to form in an area of ocean including the Sagharughombe reef. The sea surface temperature was more than a degree above the maximum expected summer time temperature. According to the NOAA/NESDS methodologies, this temperature anomaly usually indicates potential coral bleaching.
Coral bleaching is a process in which the symbiotic algae of corals (zooxanthellae) become increasing vulnerable to damage by light at higher than normal water temperatures. The resulting damage leads to the expulsion of these important organisms from the coral host. Since zooxanthellae give the colorful appearance to corals, their expulsion results in the coral’s loss of color.
|completely bleached Acropora colonie|
In January 2000 local divers had noticed the start of a bleaching epidemic at Sagharughombe reef and other reefs in the area. In May 2000 PCRF conducted its first study at Sagharughombe and observed on some reef slopes that up to 30% of all corals were bleached. In some colonies filamentous and macroalgae had taken root on the bleached corals smothering and overgrowing the weakened colonies, leaving no chance of recovery. The genera most affected were large table Acroporas, some of the large branching Acroporas, the foliose Pachyseris and Seriatopora, Favites and Pocillopora. There also appeared to be bleaching in soft coral and many anemones. Some areas were definitely worse affected by bleaching than others and corals were affected all the way down to 25 meters depth.
In September/October 2002 bleaching was a minor impact on the reef, but the skeletons remained of the corals that had bleached two years before.
Now in January 2006 Sagharughombe reef is suffering again from another bleaching event. Our Vitareef data show that the deeper zone, where 14 % of all coral colonies showed signs of bleaching, is more affected than the shallow zone with 11 % bleached colonies. In addition to the Vitareef data collection we also counted the numbers of bleached colonies in each transect area (80 m2). In each transect area we counted between 21 and 114 bleached colonies. Many coral genera are affected but most heavily bleached are Stylophora, Seriatopora, Fungia, Porites, Goniastrea and Acropora.
|a small colony of Pocillopora, with bleaching in evidence||partially bleached Porites colonie|
Almost all Stylophora and Seriatopora colonies encountered are completely or almost completely bleached. The massive Goniastrea colonies are mostly bleached on the upper part of the colonies. Acropora plates are often entirely bleached, while in branching Acroporas mostly the upper surfaces are affected. Many of the Fungia and Porites colonies are either completely or partially bleached. Most of the encrusting Porites are entirely bleached, while the massive Porites are often only partially bleached. These partially bleached Porites have either lost their color completely in the affected areas with translucent tissue showing the white skeleton beneath, or the affected areas have become very pale. In the partially bleached colony it is the areas that are most exposed to the sunlight that show the greatest effects of bleaching. Some bleached colonies still retain their color in parts of the coral which are shaded by overhanging macroalgae mats. The bleaching patterns observed here at Sagharughombe reflect the fact that in a bleaching phenomenon, the symbiotic algae of the corals become vulnerable to light and are then expelled.
Just as observed in 2000 in a few colonies, filamentous and macroalgae have taken root on the bleached corals, smothering and overgrowing the weakened colonies, leaving no chance of recovery. Throughout the reef we also found small areas (~2 m2) which were totally covered by the macroalgae Halimeda with dead colonies underneath. These colonies must have died off a while ago and could perhaps be the colonies we witnessed being smothered during the 2000 bleaching event.
In 2006, as already observed during the bleaching event in 2000, bleaching also affects anemones and soft corals. Many anemones have lost their color and are totally white. One Leather soft coral was observed to have a very pale yellow color.
During the entire study time we found only one crown of thorns starfish in the shallow zone of the reef tip, and two individuals on the lagoon slope.
|crown of thorns starfish,|
observed on the outer reef at Kennedy Island
Diving we spotted several times predators of the crown of thorns starfish such as the yellowmargin triggerfish (Pseudobalistes flavimarginatus) and the moustache triggerfish (Balistapus viridescens). We also sighted small groups of up to four juvenile napoleon wrasses (Cheilinus undulates) swimming along the reef slopes. While Manta towing we found that these three fish species are present all around Kennedy Island reef and the inside of Sagharughombe reef. These predatory fish should keep the crown of thorns starfish population down at depths where we observed them.
Nevertheless Dive Gizo (a dive shop based in the town of Gizo) conducted a crown of thorns starfish collection at the very shallow areas (~1 m) around Kennedy Island. They collected over 100 small individuals in one morning. According to their local knowledge only the very shallow areas are affected by high numbers of crown of thorns starfish.
In the two previous study years no commonly known coral disease had been observed. This year we spotted at the inside of Sagharughombe Reef a few Acropora table colonies, which appeared to be suffering from white-band disease.
The shallow reef flat (~1m) south of the narrow channel separating the reef tip from the rest of the reef consists of a few scattered coral colonies alternating with areas of high coral coverage and rubble/sand areas. In some of these very shallow areas on the reef flat the most abundant genus is branching Acropora while in other areas densely packed Porites heads are most abundant. Apart from bleaching, the corals here are in good condition.
All along the upper edge of the western lagoon slope of Sagharughombe reef in 2-5 meters depth, just below the crest we found a protective shroud of closely packed massive Porites heads. They seem to provide enough protection for more delicate and branching colonies to grow safely on the reef flat. Porites clearly dominates the shallow zone and coral coverage is very high.
Descending this slope to 7 meters and deeper, the diversity of coral species increases, while coral coverage decreases. Porites is still the most abundant genus, but also Acropora, Goniastrea, Montipora, Fungia, Stylophora, Seriatopora and Pocillopora become abundant in the deeper zone.
|Halimeda overgrowing dead coral colonies|
The condition most affecting corals on the lagoon slope is the current bleaching event as described above. Overgrowth by the macroalgae Halimedais still in process in a few areas, especially on currently bleached colonies. But in most cases where we found macroalgae mats, they were smothering corals which died a long time ago, possibly during the previous bleaching event. Many of these macroalgae mats are themselves now being overgrown by filamentous algae.
Halimeda is not yet clearly identified as a eutrophication indicator, while the macroalgae Caulerpa racemose is a known pollution indicator, indicating high Concentrations of nutrients which can have an industrial or domestic source. Caulerpa racemose was found in small and non-threatening amounts on the reef, usually growing below the base of corals without seeming to affect the colony.
In general, corals along the lagoon slope of Sagharughombe seem to have suffered less from previous bleaching events and other destructive conditions then the reef tip. In 2000 the reef tip was a beautiful reef with high coral coverage and many large Acropora tables on the reef flat. Many of these large table Acropora colonies suffered during the 2000 bleaching event but this area was still intact in 2002 with many large table colonies flourishing. Unfortunately over the past four years the large Acropora tables have disappeared and we found only their skeletons scattered all over the reef tip. Coral coverage has also been reduced at the reef tip leaving a lot of bare coral rock on the reef flat as well as the slopes of the reef tip.
Apart from bleaching effects, many corals on the lagoon slope still seem to be in good condition and coral coverage in the shallow zone is especially high. It appears that here on the lagoon slope less corals suffered from bleaching or had increased chances of recovery than the corals at the reef tip.
Many corals all over the Sagharughombe reef system carry scars from former damage which are now healed. These colonies are a patchwork of healthy live tissue and old bare skeleton. The large Porites heads often carry these scars and are also often affected by invertebrates like bioeroding mussels, snails, and Christmas tree worms although are still in perfect condition.
In a few places along the lagoon slope the reef is broken up by sand wedges. Near these areas shifting sand is causing a little sedimentation on the corals, but the currents appear to move the sands quickly so little of the coral is permanently damaged.
The western lagoon slope of Sagharughombe has a high coral coverage and corals are general in a good state, while on the eastern outer slope we found vast areas of pure coral rock with many old Acropora table skeletons and less than 5% live coral coverage. On the eastern side of the reef tip we also observed a few very large Acropora colonies which were all in the process of dying off.
As already observed in the previous years of 2000 and 2002, fish life on Sagharughombe reef is still diverse and abundant. Unlike most other areas in the Solomon Islands, this reef is not owned by the nearest village. The nearby relocated Gilbertese community does not claim the fishing rights for the reef, hence fish stock has the chance to flourish here. We found a healthy population of commercial fish. Schools of trevallies were regularly patrolling the reef. One school of small bluefin trevallies (Caranx melampygus), numbering at least 200 individuals, passed us twice on different transect dives on the lagoon slope. Other trevallies, snappers, groupers and enormous sized schools of fusiliers were a common sighting at the study site. Juvenile fusiliers schooled in numbers of around 1000 individuals. Small reef fish like damsel fish, wrasses and anthiases are all over the reef. Parrotfish, surgeonfish and butterflyfish are present but not in extraordinarily high numbers.
As already mentioned above we found a healthy population of the crown of thorns starfish predatory fish - yellowmargin triggerfish (Pseudobalistes flavimarginatus), moustache triggerfish (Balistapus viridescens) and napoleon wrasse (Cheilinus undulates) of which we only saw juveniles and females.
The current system in the area is very unpredictable showing no real pattern with the tides. During a few dives at the study site we experienced pretty strong currents. During these dives many very large black snappers (Macolor niger) and twinspot snappers (Lutjanus bohar) along with thousands of little reef fish were feeding in the current.
Several times we spotted within the study site single spotted eagle rays (Aetobatus narinari), white tip (Triaenodon obesus) and black tip (Carcharhinus melanopterus) reef shark. Both these species were also observed at other sites during observational dives along with gray reef sharks (Carcharhinus amblyrhynchos) and devil rays (Mobula japonica). But compared with the many encounters of sharks in 2000 it appears that the numbers have decreased as already observed in 2002. Shark finning has become a profitable business in this part of the world and has become one of the few sources of income for many fishermen since the national banning of the beche-de-mer fishery.
While manta towing it became clear that the channel between Sagharughombe reef and Leorava fringing reef is a hot spot for fish abundance. Huge schools of mackerels, fusiliers and a school of bat fish was sighted.
Danny Kennedy who runs a dive operation on Gizo told us that Gerald Allen had recently dived at Jari Island, 11 miles northwest of Sagharughombe reef which resulted in the second highest fish diversity count in the world.
This section describes the populations of invertebrates in the shallow and deep zones of the western lagoon slope and the reef tip of the Sagharughombe reef system.
|corallivorous snails eating the live tissue of an Acropora colony|
Throughout the studied area we found feeding trails on corals made by the corallivorous snail Coralliophilia neritoidea, most notably on Porites heads and branching Acropora colonies. Several branching Acroporas had fresh damage to the tissue of their bases leaving the base of corals with bare white skeleton. During the day we spotted these snails sitting inactively at the bases of the affected corals, waiting for the night to start feeding again.
The presence of the highly valued trochus, Tectus niloticus, was limited to the deep zone of the reef tip where one specimen only was observed. This low presence indicates that numbers are in decline through over fishing.
Throughout the reef we found a good presence of Giant Clams, with Tridacna squamosa observed on every transect dive taken. Juvenile clams appeared to be faring well as all the majority of animals sighted measured between 10 – 15 cm in length and all were less than 25cm. The fact that there were no mature Giant Clams indicates a history of over fishing. The high number of young giant clams is most likely due to the nearby giant clam garden. The World Fish Center (formerly ICLARM) set up a giant clam garden next to Nusatupe before 2000 but the management was severely interrupted during the political tensions in the early 2000s.
The scallop, Pedum spondyloideum, and the horse mussel, Modiolus philippinarum, had a strong presence throughout the reef boring into the massive Porites heads.
Bio eroding invertebrates were prolific at the slope of the lagoon slope while they were hardly present on the reef tip.
|horse mussel (Modiolus philippinarum) in a Porites colonie||Christmas tree worms, Spirobranchus giganteus,|
on a massive Porites colony
The shallow zones of the reef slope have a very strong presence of bio eroding snails, Dendropoma maxima, while the deep zone has a stronger presence of bio eroding mussels, Lithophaga zittelliana. High populations of Christmas tree worms Spirobranchus giganteus, were observed throughout the reef especially on massive Porites heads again.
As noted during the studies from 2000 and 2002, there is a marked absence of sea urchins. During our latest study, their presence was noted only on the reef tip with Diadema and Echinometra being the only genera observed.
There were hardly any sea cucumbers (Beche-de-mer) anywhere on the reef, with only one low value species, Bohadshia graeffei, seen during just one of the transect dives taken. This extremely low presence leads one to conclude that also over fishing of sea cucumbers continues to be a problem here, as previously noted in the 2000 and 2002 studies undertaken, despite the fact that beche-de-mer fishery was banned years ago. There were many colonies of ascidians throughout the reef in particular Atriolum robustum and the solitary tunicate, Polycarpa aurata.
|the tunicate, Atriolum robustum|
Also noteworthy is the strong presence of the encrusting grey sponge, Dysidea granulosa that grows on the rocky substrata of coral heads and frequently overgrows the live coral. Characteristic for Sagharughombe and other reefs in the area is also the strong presence of elephant ear sponges Ianthella basta. At Sagharughombe they start growing all over the reef at a depth of about 8 meters and deeper. In 2000 we observed that most of the elephant ear sponges were suffering from a condition, which created big holes in them but in 2006 the sponges looked reasonably healthy.
The entire reef is littered with beautiful Crinoids, feather stars, which feed in the currents running along the reef due to tide changes.
In the area between our anchorage north of Mbambanga and the channel that runs past Kennedy Island and west towards Sagharughombe reef we encountered pods of Spinner and Bottlenose dolphins on several occasions. The pods we observed were usually comprised of 10 to 30 dolphins. Several times Spinner dolphins were bow riding our ship, the RV Heraclitus, or our small boat. Within one pod of more than 10 Bottlenose Dolphins that we observed here on Jan 27, 2006, there was one young calf bow riding the small boat alongside its mother.
During the study that was made here in 2000 there was an almost daily encounter with a large school of Spinner Dolphins whereas in 2002 there were very few observations made and noted as a change in the area. Fortunately, the number of sightings for 2006 has apparently increased since 2002, with two species of Dolphins observed, and several sightings logged.
No sea turtles were observed during our VitaReef and transect dives on Sagharughombe reef, but two were observed off Kennedy Island, the neighbouring reef during recreational dives. One was a green turtle and the other was unidentified. During manta towing around the entire Sagharughombe reef system and Kennedy Island’s fringing reef only one sea turtle was spotted on Sagharughombe reef. It was either a green or a hawksbill turtle. The visibility of the water and the short time during which it was visible before it quickly swam away made identification impossible.
Sea turtles seem to be an important food source for the village of Mbambanga, a Gilbertese community. Turtle meat is a traditional food source for the Kiribas people. During the Christmas dance competition held between three Gilbertese communities in the Gizo area on the island of Mbambanga, three green sea turtles were seen on their backs in the village, most likely being kept alive for later meals. On a later date, other members of our crew observed another green turtle in the village in the same condition. A green sea turtle shell was seen displayed up in a tree at the entrance of the village. Although no sea turtles were seen around our anchorage at Mbambanga, the area seems to be an ideal feeding site for sea turtles with a lush seagrass bed adjacent to Mbambanga island.
Bleaching definitely seems to be a major threat for the Sagharughombe reef and other reefs in the area during the past six years. Two of the three studies conducted by PCRF during the last six years have been during bleaching events. It appears that those events have decreased coral coverage at the reef tip. Bleaching leaves corals vulnerable to other stress like overgrowth by algae which has also been in evidence at this site. The algae mats that we found throughout the reef in 2006 might indicate a nutrification of the marine area. Nutrification can either originate from domestic sewage from nearby villages or from land run off enhanced by logging.
From many islands in the Solomons we heard of extensive logging causing a high level of sedimentation on the reef, which suffocates corals. Since logging on the nearby volcanic island of Kolombangara is conducted in an ecological friendly way, sedimentation from land runoff does not seem to be a big problem in this area.
Many reef invertebrates seem to have not yet recovered from over fishing although giant clams have spread all over the reef, perhaps showing good results from the nearby giant clam garden set up by the World Fish Center at nearby Nusatupe.
The eastern side of Sagharoghombe reef facing the Blackett Strait seems to have suffered a lot in the past and experienced a high die off in previous years. It is not possible to cite a cause.
Considering fish life this area is so far untouched by the live reef fish trade in Hong Kong etc., and also large fish are still caught for local consumption.