Planetary Coral Reef Foundation Science Report
Planetary Coral Reef Foundation studied the Namena Barrier Reef system in collaboration with Wildlife Conservation Society. The data collected is being incorporated into WCS’s Seascsape project which aims for greater protection of Fiji’s reefs. PCRF would like to thank Linda Farley, David Olson, Michael Marnane, James Atherton, Alex, Didi, Matthew Dunlap and Matt Perkins for contributing to our study of this reef system.
June - July 2005
Fifteen miles south of Vanua Levu, Fiji’s second biggest island, the Namena Barrier reef extends into the Koro Sea. At the centre of the lagoon lies the dragon-shaped island, Namena. Surrounding this lush volcanic island is a vast expanse of coral reef known as the Namena Barrier Reef, which includes over 30 miles of pristine reef track.
About four nautical miles southeast of Namena Island the barrier reef comes to a southern point. From this southern point towards the northeast and northwest, run two strips of barrier reef about five nautical miles long on each side. On the seaward side, the reef drops off to a depth of between 600 and 1000 metres. The barrier reef is broken on each side by the North Save-A-Tack Passage (east side) and the South Save-A-Tack Passage (west side). From both passages northward the barrier reef continues as a more discontinuous reef front with many bommies and pinnacles running in a northwesterly direction. The prevailing winds come from the east and southeast most of the year, hence the eastern barrier reef is the windward side. Vitareef data collection and transects were conducted in three zones:
We spent one day tracking large sections of the reef contour, including the three study sites, using a handheld global positioning unit.
All study sites were located within the marine protected area of the Namena Barrier Reef system. Additionally we conducted observational dives within the marine protected area on the northeast and the southwest outside barrier reef, in the North Save-A-Tack Passage and on several bommies and pinnacles on the southwest outside barrier reef.
The water temperature inside the lagoon (measured from the RV Heraclitus anchored one nautical mile north of Namena Island) ranged between 24.9°C and 26.3°C. At the beginning of the study, the water temperature was 26.3°C, dropping down to 24.9°C during the ‘bugi-waloo’ – an eight-day wind cycle common in Fiji at this time of year. This weather system interrupted our study time.
No significant signs of bleaching were found either on the barrier reef or the fringing reef.
On the fringing reef of the island we observed corals which had died off relatively recently including large massive colonies and whole Acropora fields. The die-off was extensive off the southwest corner of the island, but to the east there were fewer dead colonies. It is possible that this die-off was the result of previous bleaching events; conversations with divers that have known this reef for many years revealed that bleaching was noticeable in the early 2000s.
At almost every dive site through out the study area we encountered some kind of coral disease. We suspected that most of the sightings were white band disease affecting mainly Acropora, but also a few Porites and several Pocillopora colonies showed what appeared to be some kind of disease evident by a similar white band. We also noticed a condition resembling yellow blotch/band disease on a couple of Porites colonies. One small Porites colony was sighted with an orange-colored band separating the live tissue from the white skeleton.
Outside the barrier reef we observed visibility between 35 and 40 meters, inside the barrier reef around 30 meters, and on the fringing reef between 14 and 23 meters.
During the entire time of the study we encountered just a single crown of thorns starfish, which was sighted on Namena Island’s fringing reef.
Namena earns its uniqueness from its environmental status. In 2003, the lagoon surrounding Namena Island and the barrier reef were declared a marine reserve, known as the Namena Marine Park. Fishing restrictions have allowed marine life to flourish around the remote island. In addition to the marine reserve, a sanctuary on the island of Namena Island supports red-footed boobies, nesting sea birds. Green and hawksbill sea turtles lay their eggs on the beaches surrounding the island during the months of January, February, and March.
The barrier reef has an interesting topography including reef walls, numerous passes and channels, and coral pinnacles. The variety of seascapes supports a diverse assemblage of fish and corals.
Our study site on the outside of the eastern barrier reef consists of drop-offs down to almost a thousand meters in places. Vitareef data were collected on the wall and on reef crest. Transects were laid to cover both the crest and the upper wall. In both transects the crests were about seven to eight meters deep.
The area on the lagoon side of the barrier reef surrounding our study sites consists of a shallow slope covered in sand and rubbe. There are only a few small areas of hard bottom, and the most common coral genus growing in these patches is Acropora. These patches are found in about three to four meters of water.
Moving farther into the lagoon, the sandy bottom slopes down to about 35 meters. Within this zone, we chose two different patches to lay transects and collect Vitareef data. The first consisted of a flat shallow area of about 1.5 to two meters depth which developed into a small crest, dropping down to a depth of four meters. The transect was laid to cover both the area of the crest and the extending flat area. The second transect was laid on an entirely flat patch in about two to three meters of water.
A fringing reef extends along the entire south side of Namena Island. In places it slopes down gradually. In others, it drops from a very shallow crest of about one meter depth down to about seven to 12 meters. The fringing reef also extends to the north side of the island.
Observational dives were conducted in the North Save-A-Tack Passage (eastern barrier reef). Here visibility depended very much on the tides, with the lagoon emptying out as high tide ran out to low.. The passage consists mainly of a hard bottom covered with sand and is about 20 meters deep. There are coral bommies all along the edge of the pass, at their shallowest point about ten to 16 meters deep.
Observational dives were also conducted at the bommies and pinnacles of the southwest barrier reef. They originate at about 30 meters depth and rise up to about five meters below the surface.
Coral diversity was relatively high in all three study zones, but on the flats of the inside barrier reef small Acropora colonies were most abundant. Coral colonies of the outside and inside barrier reef were mostly small in size. Some coral colonies are visibly suffering from disease - we sighted on average at least three affected colonies during each dive.
At one time, the fringing reef of the south side of the island must have been a beautiful reef with numerous large coral colonies. Unfortunately many of the corals have died, either partially or entirely. There were entire Acropora fields that had died and subsequently been broken up into large pieces of rubble. Macroalgae and filamentous algae were quite abundant.
Just in front of the resort jetty spreads a small Acropora field. We observed that the red algae Galaxaura rugosa was smothering every single coral, overgrowing the branching Acropora beginning at the base of the colony.
Fishing has been banned from inside the lagoon and the passes of Namena for many years. It is enforced as much as possible by the marine reserve scheme established at this reef. While we were working in the area, Didi (one of the staff of Wildlife Conservation Society) chased off a fishing boat. Trawling is allowed on the outer reef. The effects of this no-take zone are very apparent. The fish life at Namena is plentiful and extremely diverse, including large predators.
Reasonable numbers of pelagic fish were observed at the outer barrier reef study site – white tip reef sharks, jacks and double-lined mackerels. From observational dives at the entrance to the North Save-a-Tack pass (just to the north of this study site) we know that there are high and very diverse pelagic populations in this area. At the pass, we observed schools of scalloped and great hammerhead sharks, plenty grey reef sharks and white-tip reef sharks, many dogtooth tuna and Spanish mackerel, extremely large schools of big eye trevally, blue jacks and schools of barracudas.
Namena also provides a healthy grouper population. On a dive in the pass of the outer barrier reef we would on average see at least ten groupers. We observed different size classes of groupers, even the bigger individuals that aren’t normally observed when spear fishing is practiced in the area.
The conditions of the different types of reef studied – the outside barrier reef, the inside barrier reef and the fringing reef of Namena Island – were reflected well in the fish population studies at each site. The shallow inside barrier reef has a high diversity of small reef fish. The fringing reef of the island is also high in diversity in smaller reef fish. During observational dives on the bommies and pinnacles of the western barrier reef we were covered in beautiful clouds of Anthias.
The efforts to preserve intact fish populations at Namena are paying off, providing spectacular underwater scenes of large and multiple schools of fish. Even without having explored all of Fiji’s reefs, it is clear that Namena and its reef system are worth protecting.
The majority of Namena´s reefs showed a healthy population of most invertebrate families. Different species of giant clams (Tridacna maxima, Tridacna squamosa, Tridacna derasa) were seen even in areas that would ordinarily be easily accessible to free-divers.
High- and low-value beche-de-mer species, for example greenfish (Stichopus choloronotus), prickly redfish (Thelenota ananas) and flowerfish (Bohadschia graeffei) inhabit the inside and outside barrier reef, as well as – in lower numbers – the fringing reef of the island.
The dominant sea urchin species throughout the lagoon are Echinostrephus spp. and Echinometra mathaei. Both are known to excavate small holes into the rock and even live coral for protection. The highest abundance of these bioeroding organisms was found on the inside barrier reef.
In spite of the high number of observational dives we saw only one single crown of thorns starfish, indicating that the effect of these animals on the living reef is probably negligible.
Especially on the fringing reef we observed different kinds of what were probably corallivorous snails feeding on live coral colonies, mainly Acropora. However, we were unable to identify these snails by observation. The highest abundance and diversity of invertebrates were found on shallow patches on the inside barrier reef. The high value greenfish and other beche-de-mer species as well as giant clams occur in this area in a high density.
At the northeast section of the outer barrier reef we encountered a group of about 30 to 40 pan tropical spotted dolphins while driving the small boat. There seemed to be four different groups within the pod structure. They were very friendly and stayed with the small boat for about 45 minutes. They rode the bow while the boat was moving and jumped high out of the water. Twice an observer entered the water, and even then the dolphins remained close by, turning towards the person and echo locating.
When the expedition departed Namena for Nadi, we also had two groups of pan tropical spotted dolphins riding the bow of the Heraclitus. There was one newborn dolphin within the first group.
Namena Island has nesting beaches for hawksbill and green turtles. During January, February and March the resort closes, so turtles do not get disturbed when laying their eggs. According to information by a Wildlife Conservation Society staff member, there has been a decrease over the last several years in the number of turtles nesting on Namena’s beaches.
We performed manta towing of one observer along the eastern outside and inside barrier reef and along the entire south fringing reef. On the outside barrier reef we counted one green turtle, eight hawksbill turtles and four unidentified turtles (either hawksbill or green). There were no turtle sightings on the inside barrier reef. On the fringing reef we counted one green and two unidentified turtles (either hawksbill or green) while manta towing. All turtles encountered were hovering over the reef or sitting in between the corals in about five to ten meters of water and moved off quickly as our small boat passed.
During dives we observed only one hawksbill turtle on the fringing reef and one in the North Save-A-Tack Passage of the eastern barrier reef.
Threats to the Reef:
As the southern tip of the Namena Barrier Reef is a marine protected area (designated since 2003) with no-fishing and no-taking rules in certain areas, fishing currently presents little threat to the reef.
There are several dive companies, based mainly in the Savu Savu area, which bring divers to the Namena Barrier Reef. Each diver pays a $20 fee to dive within the marine reserve. An increase in the number of divers, however, could threaten some of the more popular dive sites around the reef. The currents around the passes and on the outer barrier reef can be strong, and evidence such as broken coral tips at the so-called “Chimneys” and other pinnacles on the southwest side of the barrier reef suggests divers are causing perceptible damage to the reef.
The large number of dead corals on the fringing reef could be due to a former bleaching event. Many dead corals were overgrown by algae, probably encouraged by nutrients derived from the feces of the large bird population that nests on the island.
We were surprised to find the extreme cover of the red algae Galaxaura rugosa on the corals just in front of the resort’s jetty. The branching Acropora colonies affected were still alive. We have no information on what is causing the algae to spread.
The biggest threats to the reef seem to be conditions unmanageable on a local scale, such as the spreading of coral diseases and future bleaching events. Therefore it is even more important to continue protecting these reefs, minimizing the impact of local stresses so that the system is more able to fight off globally caused stresses.
The protected reefs of Namena provide an incredible source of fish that almost certainly repopulates other less populated areas through ocean currents and gyres. These reefs are an immensely important resource for the recovery and provision of recruits to other threatened reefs in the area.
