|Nudibranches on Porites fingers in the marine protected area |
of the inside fringing reef
The study of this reef provides an interesting case study in a coral reef which has been exposed to a naturally occurring but severe change in its environmental conditions, due to plate tectonic movement 40 years ago.
Uri is a flat islet found off the east coast of Malakula in the Malampa Province of Vanuatu. Beyond the east side of the island is open ocean, and on the west side is a protected mangrove-fringed bay known as Port Stanley. Fringing reef surrounds the entire island but differs drastically from the ocean side to the bay side. The village chief has designated marine protected areas (MPAs) on reefs around the northern tip of Uri Island. These sanctuaries have been strict no-take zones for the past 15 years. On the bay side of the reef, the village chief maintains a giant clam conservation area.
Malakula and its surrounding islets, some of Vanuatu’s oldest islands, formed from volcanic uplifting over 22 million years ago. These islands, located on the western edge of the Pacific Plate, are in an active seismic zone, where the neighboring Indo-Australian Plate is subducting beneath them. Earthquakes and volcanic eruptions are frequent in the Vanuatu archipelago, and much of the new land formed here is due to tectonic uplifting associated with seismic activity. According to a locally-prepared report, there is an annual 0.9 centimeters east to west movement of landmass on Malakula. It is believed that the D’Entrecasteaux Fracture Zone, running perpendicular to the archipelago between Santo and Malakula, heightens the frequency of earthquakes in the Malakula region, causing both uplifting and subsiding events.
In 1965, an earthquake measuring force seven on the Richter scale caused an abrupt 0.23 meter uplifting at the Lakatoro wharf, nearby Uri Island, and 1.0 meter uplifting on the outer reef margin. Scientists have documented a shift in mangrove zonation patterns after the earthquake. Aerial photos show that mangrove coverage is continuing to increase in the area. Uplifting and subduction events affect more than just the diversity of the mangroves but seagrass and fringing reefs, as well. For instance, after the 1965 earthquake, Avincennia marina and Rhizophora stylosa mangroves began to colonize on the fringing reef between Uri Island and Malakula mainland. Before the earthquake this area was unsuitable for the growth of mangroves. As a result of this, the channel between Uri Island and Malakula has been drastically reduced to only a few meters of mangrove free shallow reef. On extreme spring tides people can now wade across the channel, whereas this was never possible before the earthquake.
The reduced channel size and continuously growing mangroves are responsible for reduced water flow between the bay and the ocean and a greater degree of sedimentation within Port Stanley Bay. Inside the bay Uri Island’s fringing reef must now cope with these new conditions.
The reefs around Uri Island are typical fringing reefs. The eastern fringing reef faces the ocean and extends towards the mainland of Malakula. The western fringing reef is situated inside Port Stanley Bay. Together with the island, the fringing reefs form the eastern border of the bay. The reef area north of Uri Island spreading from the eastern to the western fringing reef comprises the MPA.
Vitareef data collection and transects were conducted within the MPA in two different zones:
Additionally, we conducted observational dives all along the outer fringing reef within and outside the MPA and at the fringing reef of the neighboring island Uripiv.
The outer fringing reef has a shallow reef top with hardly any corals. The reef flat turns into a little drop-off, which changes at about seven to ten meters into a less steep reef slope. The reef slopes are densely covered with corals and drop down deeper than 20 meters. This coral reef is facing an open sea dropping down to 300-700 meters of depth.
The reef flat of the outer fringing reef is connected by the small channel through the mangroves between Uri Island and the Malakula mainland to the fringing reef inside Port Stanley Bay. The inside reefs are in very shallow water of about two to ten meters. The sandy bottom of the bay drops down to about 30-40 meters. Seagrass is scattered throughout the bay.
|Slope at the outer fringing reef within the MPA||Transect area in the inside fringing reef (MPA)|
At the inside fringing reef within Port Stanley Bay we observed low visibilities between 16 and 26 meters; at the outside fringing reef we observed visibilities around 25 meters as measured through Secchi disk readings. During the entire period of the study we had strong south-easterly winds blowing with 15 – 20 knots. Measurements could only be taken on the calmest days. In the days before our study periods we witnessed extremely clear waters at the outer fringing reef. Corals growing in about ten meters of water could be easily seen from above the surface suggesting much higher visibility during calmer weather.
The water temperature, measured from the RV Heraclitus anchored 1,3 cables off Uri Island inside Port Stanley Bay, ranged from 27.1°C to 27.5°C during the study period.
At Uri Island’s fringing reef we found no significant signs of current coral bleaching.
At the outside fringing reef of Uri Island we did not encounter a single crown of thorns starfish, although several Acropora table colonies throughout the outer fringing reef of Uri Island looked as if they were suffering from crown of thorns starfish grazing. Acropora table colonies showed areas of algae overgrown skeleton, areas of white bare fresh skeleton and healthy tissue within the same colony. Also, completely dead Acropora table corals covered with a thin layer of algae as well as small recently dead colonies of other coral genera were found scattered throughout the outer fringing reef. Usually we would find several affected corals together in close proximity. Nevertheless, even after searching the area around affected colonies we did not spot any crown of thorns starfish in this reef. Hence the damage to the corals may very well have had a different cause.
|Acropora table colony losing its tissue|
We found a healthy population of crown of thorns starfish predatory fish. On every dive we spotted several yellowmargin triggerfish (Pseudobalistes flavimarginatus) and moustache triggerfish (Balistapus viridescens). At our first transect site at the outside fringing reef we also saw two big male Napoleon wrasses (Cheilinus undulates) hovering in the area.
At a part of Uri Islands fringing reef inside Port Stanley Bay, which does not belong to the MPA, we spotted a single crown of thorns starfish. At the fringing reef of the neighboring island, Uripiv, we spotted two individuals during a snorkel trip.
|Acropora table colony resembling the typical
of white band disease
Since we could not find any crown of thorns starfish around the damaged corals at the outside fringing reef we cannot rule out the possibility of disease affecting those corals. However, the damaging condition does not resemble any common coral disease known to us. It did not clearly resemble white band disease, since dead skeleton and live tissue were not separated by a distinct white band. Dead areas within a colony had rather round shapes of the size of typical crown of thorns starfish grazing trails.
Nevertheless in one or two cases we found Acropora table colonies that did resemble the typical picture of the white band disease, indicating this disease may be present.
The coral coverage and diversity at Uri’s outer fringing reef was high and gave the reef an especially colorful and beautiful appearance. Most coral colonies were in very good condition. Only in very few spots there were small areas in which corals were affected by a filamentous algae mat smothering. The most outstanding condition, affecting corals throughout the reef, was the condition described previously where corals seemed to have lost big areas of their live tissue, as well as the completely dead Acropora tables that were scattered throughout the reef.
|Corals on the outer fringing reef||Healthy coral on the outer fringing reef|
Inside Port Stanley Bay, Uri’s fringing reef was clearly dominated by Porites coral. Large Porites heads and fields of Porites fingers formed the typical picture of the reef. Another common coral genus found in the reef was Astreopora forming massive colonies of about 50 centimeters in diameter. Apart from some Montipora colonies other coral genera were only present as very small colonies.
|Large Porites head and Porites finger colonies on the|
inside fringing reef
The number of large Porites heads was outstanding. They were in good condition apart from scars originating from bioeroding Scallop mussels, which left deep valleys in the skeleton without live tissue. Most Porites finger colonies had partly lost their live tissue starting from the colony bases. The tissue on the upper part of the colonies was still healthy and the borders between live tissue and dead skeleton looked clean as if the process of tissue loss had come to a stop. Several Porites finger colonies were broken apart, presumably since the bases of the colonies were weakened by the tissue loss.
Corals were surrounded by sand and sediment. The sediment layers leave few chances for new coral recruiting since recruits need clear hard substrate to settle upon. Those sediment layers can also cause very turbid waters when stirred up by wind and wave movements. During our study we experienced very turbid waters with wind blowing strongly for days. In turbid waters the light reaching corals is reduced. Good light conditions are essential for the photosynthesis of the corals’ symbiotic zooxanthellae algae, which support coral growth. Therefore excessive sedimentation can reduce coral growth and coral recruiting, probably occurring at the Uri inside reef.
According to the village the inside reef began to die off after the tectonic uplifting of the land. After the uplifting mangroves had better conditions to grow in the shallow reef areas south of Uri Island. Hence the mangroves almost closed the entire area between Malakula mainland and Uri Island resulting in Port Stanley Bay becoming a closed bay with effectively only one opening to the outer sea. The water circulation with open ocean water has been highly reduced.
Our impression of the inside reef was that many coral colonies were decreased in size due to tougher conditions dealing with sedimentation and reduced water circulation. Coral abundance has probably been reduced, too, since the changes in Port Stanley Bay have occurred. We believe that the reef is still in the process of adapting to its new environmental conditions and transforms into a different type of reef.
In the MPA at the outer fringing reef of Uri Island fishing has been banned for the past 15 years. The effects of those fishing regulations are clearly visible. The abundance of commercial fish is high. A large number of different snappers, big-sized groupers and parrotfish inhabit this area.
|School of fusiliers on a slope at the outer fringing reef|
Schools of different fusiliers, bar jacks (Carangoides orthogrammus), filament-fin parrotfish (Scarus altipinnis) and bumphead parrotfish (Bolbometopon muricatum) patrol the slopes of the reef. Together with many different smaller reef fish and the occasional white tip reef shark (Triaenodon obesus) the fish life is very diverse.
Extraordinary is the high abundance of crown of thorns starfish predators. Yellowmargin triggerfish (Pseudobalistes flavimarginatus) and Moustache triggerfish (Balistapus viridescens) are spread throughout the entire outer fringing reef protecting their territories against other fish. Further several very big Napoleon wrasses (Cheilinus undulates) were spotted on the outer fringing reef.
In the MPA inside Port Stanley Bay a clam farm garden has been established, though fishing is allowed. In the MPA the fish population resembles the typical picture of an inside reef. Many very small reef fish, in particular damselfish, dominate the scene. But also schools of fusliers and paddletail snappers (Lutjanus gibbus) were sighted inside the MPA. Even a small dogtooth tuna (Gymnosarda unicolor) passed through the transect area.
According to the village, fish stocks and fishing success have decreased gradually since the tectonic uplifting initiated the changes in the coral reef.
|Giant clam in the clam garden of the MPA|
inside Port Stanley Bay
According to local reports, the following turban snails which are fished are present on the outside reef: Turbo lajonkairii, Turbo marmoratus, and Turbo militarius, with the green snail, Turbo marmoratus, being the most commercially valuable (2,400 Vatu per kilo/ about 20$US per kilo). The corallivorous snail, the purple coral snail (Coralliophilia violacea) was present both inside and outside the fringing reef and was noted on all transect dives taken. The trochus, Tectus niloticus, was present on the outside fringing reef only.
Both reefs show good presence of giant clams, with Tridacna squamosa observed on every transect dive taken. Tridacna maxima was also present on one of the transect dives taken outside the fringing reef. The scallop, Pedum spondyloideum, had a strong presence inside the fringing reef inhabiting big Porites coral heads while none were observed on the outside transects.
Bioeroding invertebratess were most prolific inside the fringing reef with a very strong presence of the bioeroding mussel Lithophaga zittelliana, dwelling mainly on encrusting Acropora, while bioeroding snails were only present on the outside fringing reef transects. Christmas tree worms (Spirobranchus giganteus), and the coral hermit crab (Paguritta gracilipes) were observed both on the inside and outside fringing reef. Of these, the bioeroding mussels are the most harmful to the coral colonies as they burrow into the coral head producing tunnels, which greatly weaken the coral and combined with other burrowers can cause the eventual death of the coral. The only presence of sea urchins on transects occurred on the outside fringing reef, Diadema being the only genus observed.
The presence of sea cucumbers (beche-de-mer) was limited to the sandy rubble areas of the inside fringing reef, though none of those noted were commercially high value species, for example flowerfish (Bohadschia graeffei) and pinkfish (Holothuria edulis), both low value species commercially.
Port Stanley Bay turned out to be an extremely good location to observe dugongs. Several times we spotted dugongs surfacing around the anchored ship as well as from the small boat driving along Uri Island. Their breathing pattern suggested that they were mainly feeding on seagrass in the area.
On the first transect dive inside the bay, a diver was approached by a dugong which then swam through the studied reef area. Twice crewmembers spotted a dugong underwater while snorkeling. It seemed that there were several individuals in the bay remaining in the same areas for longer times. One dugong always seemed to be within the area of our study site inside the bay. On the outside we did not spot any dugongs or other marine mammals.
Turtles also seemed to be very common around Uri Island. During our dives at the outside fringing reef we spotted on average one turtle per dive. Hawksbill turtles and green turtles were encountered underwater while diving and on the surface while driving along in the small boat.
Inside the bay we regularly spotted green and hawksbill turtles surfacing for a breath. One fisherman approached our ship to sell a young green turtle that he had just caught in his canoe. According to the fisherman turtles are caught for a meal whenever the chance is given.
Since the tectonic uplifting the coral reefs around Uri Island have undergone severe changes when comparing their present state and condition with the reports of the villagers of how the reef used to look. The corals on the outside reef flat have died off. Parts of the reef inside the bay have died off and colonies appear to have decreased in size over time. Currently the reef seems to have adjusted to the new environmental conditions, leaving the outside reef a diverse habitat for many corals and fish, while the reef inside the bay has to struggle with less water circulation and higher sedimentation.
The desire of the village to cut out some of the mangroves between Uri Island and Malakula mainland is unlikely to succeed in reversing the process. Mangroves will probably grow back faster again than they can be cleared out. Additionally the uplifting of the land cannot be reversed. Hence, water circulation cannot be promoted enough to have an impact on the reef. Apart from the naturally-caused changes occurring on the inside reef, the only other major concern seems to be the unidentified coral condition on the outer reef, which apparently causes some coral colonies to lose their tissue quickly. Since most of the other corals seem to be in a very good state and very abundant, the reef can probably cope with the loss of a few corals. Nevertheless this condition should be monitored over time.
The no-take zone on the outside reef is inhabited by a high number of commercial fish, which should ensure the survival of a healthy fish stock in the surrounding areas. The villagers’ complaints about the reduced fish stock inside of the bay is possibly due to a reduction of the reef. The reduced reef area might not keep the potential to supply a growing human population with the same amount of fish as in the past. Therefore the area should be carefully managed with fishing regulations concerning catching rates and aggregation and spawning times for different fish families.