Mamanuka Group, Fiji S17°49.56' E17710.42' Reef Report
Health of the Mamanuka Group Reefs
Dates:

4th  - 7th August 2005

 

Reef type:

The Mamanuca Island Group lies about fifteen miles southwest of Nadi (mainland Viti Levu) and offers a range of different reef forms. There are fringing reefs around all the islands. Platform reefs are situated between islands and the Malolo barrier reef.  The barrier reef is broken up by several different passages.

We studied only one zone of this reef system, the crown-of-thorns starfish- invested Nuku platform reef. It lies one nautical mile south of Malolo Lailai Island. We collected Vitareef data and ran transects at the east side of Nuku Reef. Additionally we conducted observational dives at the fringing reef of Namotu Island, at Ro Ro Reef (part of the barrier reef), in Wilkes Passage (pass in the barrier reef) and inside the lagoon at our anchorage site.

 

 

Water Temperature:

The water temperature inside the lagoon (measured from the RV Heraclitus at anchor) ranged between 25C and 25.5C during the study period. 
 

Bleaching:

Significant signs of bleaching were found only inside the lagoon in our anchorage spot. There were a few small coral bommies  at a depth of approximately 27 meters. Several of the colonies on those bommies were completely or partially bleached.

In the study site (Nuku Reef) we did not encounter any obvious cases of bleaching. 
.

Disease:

We did not observe any coral diseases in  the area.
 

Visibility:

Inside the lagoon at Nuku Reef visibility ranged from 16 to 19 meters.

 

Crown of Thorns:

Crown-of-thorns starfish were only seen on Nuku Reef, not at any of the other visited sites. Their high number at Nuku Reef threatened the reef by leaving many dead or half-grazed coral colonies behind. On each observational dive at this reef we counted 10-20 individuals.

 
Geography:

In the lee of the main island, Viti Levu, lies the Mamanuca island chain. Sandy beaches, coral reefs, and clear water fringe these islands. Many of the islets have upscale resorts that attract travelers from around the world, although a few islets, like Mana and Malolo, still accommodate traditional Fijian villages.

The Malolo Barrier Reef surrounds and protects many of the islands of the Mamanucas from enormous waves. Within the calm waters of the reef are Malolo Island and Malololailai Island, which are the largest islands in the Mamanuca group.

In 1880, an American sailor named Louis Armstrong purchased Malololailai Island from the natives for a musket. The cove between the two islands has since then been named, Musket Cove, and a marina and resort have taken on this name as well. Now the island hosts an airstrip, two major resorts, timeshare condominiums, and a nine hole golf course.

Malolo, the larger of the two islands, is less developed than its neighbor island, Malololailai. Two native Fijian villages are located on the island. The island is also surrounded by mangrove swamp and blanketed in coastal forest. The highest point at 218 meters is Uluisolo. Villagers used this hill for fortification during past conflicts. During World War Two, US forces occupied Uluisolo as an observation point.

Beyond the two islands and within the Malolo Barrier reef is the Nuku Reef. The Nuku Reef is a platform reef that is separated from Malolo Island by a shallow pass. The reef is home to a plethora of corals and thousands of tiny vibrant fish.

 

Reef topography:

Nuku Reef is a very shallow platform reef of about one to two meters depth. At low tide parts of the reef are exposed. Its top is covered in branching and table Acropora coral colonies. The reef crest drops down to a sandy bottom of about three to four meters in places to about 12-15 meters. From there the bottom slopes downward slowly to a depth of about 40 meters inside the lagoon.

The fringing reef of Namotu Island consists of coral bommies in 10 to 20 meters of water. Some coral bommies are entirely formed by spreading Pavona clavus coral colonies.
        Wilkes Passage consists of a sandy/hard bottom in about 15-20 meters. It is covered with Tubinaria colonies and soft corals.
 

Coral Observations:

Nuku Reef was clearly dominated by Acropora colonies branching or forming large plates. On the top of the reef coral coverage was high. Approximately half of all colonies seemed in very good condition.  However, the other half had either died recently or lost most of their living tissue. The most obvious reason for the die-off can be attributed to crown-of-thorns starfish grazing. There were obvious fresh grazing marks on many of the plate corals, and crown-of-thorns starfish were often found in the proximity of recently dead colonies.

A few colonies, however, seemed to have been affected by a different threat. Tissue loss of some branching Acropora had started from the base of the colony, while crown-of-thorns starfish usually begin grazing from the tips of branching colonies. We do not know how this reef was affected by Fijis bleaching events in 2000 and 2002. Certainly there were several coral colonies that had died earlier than the most recent deaths attributed to crown-of-thorns starfish. We do not know if their death was caused by earlier crown-of-thorns starfish grazing or possible previous bleaching.

The fringing reef of Namotu Island was dominated by widely spreading colonies of Pavona clavus.
 

Fish Observations:

The fish observed on transects were predominantly small or juveniles with few predatory species. The transects were performed in shallow water on the reef top so smaller fish would be expected here. Even on the edge of the reef top where the depth dropped to about 5 meters there were very few predatory species or larger fish. Few fish moved through the area. One white tip reef shark was seen in the area of the transects.

At the fringing reef of Namotu Island we encountered a high abundance of Sergent fish. Not only these, but all other fish, including some passing Trevalleys, showed a very curious behavior towards divers, suggesting that fish feedings occur in the area.  This may be related to nearby tourist operations.

 

Invertebrate Observations:

All dive sites in the Mamanuca Group showed a low Giant Clam population. Not a single individual was seen on the Nuku reef or on the outside of the barrier reef and the Giant Clams that were seen on other dive sites usually were of small sizes.

Sea cucumbers are more abundand and quite diverse. We saw different species of the genera Holothuria, Thelenota, Stichopus and Bohadschia. Organism size was often increasing with depth, reaching impressive sizes up to 50 cm (Holothuria sp.)

Bioeroding mussles and worms occur at an average number affecting mainly massive Porites heads. Corallivoreous snails were not noticed.

Inside of the lagoon close to the pass there are huge areas of Soft Coral gardens, with beautiful Tree Corals (Dendronephthya sp.) spread all over the seafloor. In these areas the soft corals are the dominant sessile organisms, with the abundance of hard coral being very low.

The beautiful Nuku Reef, our Vitareef study site, is currently affected by a major outbreak of Crown of Thorn starfish (Acanthaster planci). A high percentage of the Acropora table colonies on the reef-flat showed white patches of bare skeletton, the feeding trail of these carnivores. On the first transect-dive we counted 24 individuals ranging from 15 cm to 40 cm in size on an area of 80 m.

In low numbers the Crown of thorn starfish help to keep balance between hard corals and other space-competing organisms on the reef. Grows their population to a plague level though, they can be a serious threat to the entire reef system.

The Crown of Thorns feed on the coral polyps, leaving behind dead coral skeleton which is fast overgrown by algae. Soon there is no more live coral left and the starfish move along the reef, attacking new coral colonies and reproducing with explosive speed. With all these corals being killed and algae taking over their space, many organisms on the reef loose their habitat and disappear. As coral-feeding fish become rare, the population of herbivorous fish and invertebrates may increase, leading to a shift in population dynamics and in general- to a reduction of diversity. A reef that has suffered a severe Crown of Thorn-Plague may need 10 to 15 years to regenerate.

The reasons for these occasional outbreaks are still a mystery. Theories suggest that an increased phytoplancton production due to sewage and other human waste result in a higher survival-chance of the starfishs larvae. Also the harvesting of the natural predators (e.g. trumpet triton shell, humphead wrasse) may lead to a population growth.

The Nuku Reef is mainly colonized by big Acropora colonies, leaving little space for other sessile invertebrates like sponges and soft corals. We identified Leather Corals (Lobophytum sp., Sinularia sp.) and Tree corals (Dendronephthya sp.) but their abundance is very low.

Only one sea cucumber (Bohadschia graeffei) was seen on both transect dives.

Hardly any sea urchins were counted on this reef, which might be due to the dense Acropora population, making observations underneath the big colonies quite difficult.

 

Marine Mammals:

Around Namotu Island and the Wilkes Passage we encountered a large pod of about 150 Dwarf Spinner Dolphins on several occasions. Within this pod there were several different groups consisting of either males or females with many young calves with pink bellies. They spent much time on the surface snagging or foraging. When we approached the dolphins with the small boat, they started to ride the bow, at times jumping high out of the water and performing a couple of twists. They remain with the small boat for about 20 minutes. The dolphins moved off when two tourist-operated small boats arrived in the area, driving at high speed straight towards them.

A Dwarf Minke whale repeatedly surfaced in close proximity to the ship during an onboard party.  The first surfacing occurred soon after a song containing humpback whale recordings began, and surfacing continued until the song ended.    
 

Turtles:

On the outside of Ro Ro Reef close to Wilkes Passage a surfacing Hawksbill turtle was spotted several times. It was unclear if it was the same individual or if there were several individuals in that area. Twice during dives Green turtles were spotted underwater. 
 

Threats to the Reef:

The biggest threat to the reef appears to be the crown-of-thorns starfish invasion at Nuku Reef. Nuku Reef is under high grazing pressure by these organisms, and one can observe the rapid decimation of the presently high live coral coverage. The spread of these organisms should be carefully monitored, as we did not encounter any of the natural predators of the crown-of-thorns starfish.

 

 





Fringing reef of Namotu Island

Reef Report
Vitareef Data
Coral& Fish ID
Transect
GPS Data