31st May - 7th June 2005
The Great Astrolabe Reef (GAR) consists of two barrier reefs and a lagoon structure with several islands - including Ono Island - inside the lagoon. Kadavu is the biggest island in the area, with a fringing reef surrounding it.
There are fringing reefs around all the islands and patchy reef bommies close by. The eastern windward barrier reef breaks up at just a few passages to the open sea. The western leeward barrier reef is broken up more by passages, channels and stretches of bommie complexes. Only one nautical mile north of the GAR lies the smaller “North Astrolabe Reef” (NAR). It is an atoll of 4 nautical miles diameter.
Vitareef data was collected inside the lagoon on coral bommies next to the fringing reef on the north west side of Ono as well as on the outside of the western barrier reef at a bommie complex system (Alacrity Rocks). We also ran transects at this outside study site. Observational dives were made at different locations all around the Great Astrolabe Reef including the eastern (windward) side.
The water temperature inside the lagoon (measured from the RV Heraclitus anchored northwest of Ono) ranged between 25.7°C and 26.8°C.
Neither outside nor inside the lagoon were significant signs of bleaching found. But we heard first hand reports and read about previous bleaching events in 2000 and 2002 from which the Kadavu/Ono reef areas have not yet recovered. This concurs with our observations inside and outside the lagoon where we found low live coral coverage. Most live coral colonies are small in size, either the remains of formerly large colonies or newly recruited colonies.
There were no visible signs of disease.
Outside the lagoon, we observed visibility up to 25 metres. Inside the lagoon, the visibility was generally lower, averaging around 12 metres.
Several crown of thorns sea stars were observed inside the lagoon. Obvious feeding trails on Acropora colonies were found, along with small recently dead Acropora colonies in close proximity to the crown of thorn sightings.
One dive operator on Kadavu told us that approximately 2000 COTs were taken out of the reef by divers in 2004.
The Kadavu island group is 45 nautical miles south of Viti-Levu, Fiji’s main island. It is comprised of Kadavu, the fourth largest of Fiji’s islands, and 7 smaller islands: Ono, Dravuni, Vurolevu, Namara, Buliya, Yaukuvelevu, and Vanuakula. Ono is the largest of the seven, at 30 square kilometers. The reef surrounding these islands was named the Great Astrolabe Reef by French explorer Dumont d’Urville after he nearly lost his ship, the Astrolabe, to this reef in 1827. In the 1870s Kadavu was almost selected as the site for the capital of Fiji, due to the once thriving whaling business. Today the island group is home to 12,000 native Fijians living in 72 traditional village communities. There is one airstrip. The primary means of support for the population is subsistence agriculture and fishing. Local produce, non-native pine and seafood are also exported to mainland Viti-Levu and beyond to meet increasing demands for cash money. Naquara village, on the north side of Ono was the main contact for the crew’s interactions with the local community.
We studied mainly the western part of the Great Astrolabe Reef close to Ono Island. Ono’s fringing reef spreads from its rocky cliffs and starts in shallow water about 1 – 2 meters. The reef is patchy and surrounded by sand, the areas of hard substrate having low coral coverage. The patchy reef slopes down into the lagoon to 12 –15 meters deep where there are only a few live corals left. From this depth two coral bommies, situated approximately 100 m apart, rise up breaking the surface at low tide. Their diameter is about 20 m. On these bommies, there are large colonies of Diploastrea on the wall. The bommie mount is mostly covered with small Acropora colonies and leather soft corals. Several Acropora colonies are affected by coralline algae overgrowth and by tissue loss, maybe due to crown of thorns starfish grazing. From these two bommies the reef slopes further down into the lagoon to about 30 m. Vitareef data was collected on the two bommies.
The western barrier reef parallel in latitude with Ono Island is not a connected reef front but rather a complex of coral bommies rising from a depth of about 20–25 meters up to 1 m below the surface. Swells break on these bommies during low tide. The bommies have steep slopes with small overhangs and low scleractinian coral coverage. On some of the platform areas coral coverage was a little bit higher. The bottom between bommies was covered with rubble. Vitareef data was collected along the walls and transects were also laid along these bommie walls.
Coral diversity was higher outside the lagoon then inside, but coral coverage was higher inside. At both sites most colonies were fairly small. Some were obvious left overs from formerly large colonies, others were small newly recruited colonies.
On the inside bommies there were a lot of young healthy Acropora colonies, but also a lot of older Acropora with damaged tissue and coralline algae overgrowth on the skeleton.
The outside bommie walls were highly covered with coralline algae and some filamentous algae. Due to the steepness of the slopes and therefore bad light conditions for corals, coral coverage would have been anyway low at this particular site. But a lot of the existing corals were severely decreased in size or had died off reducing the total coral coverage for this site.
On observational dives at the eastern barrier reef a higher coverage of hard corals was found.
There was a general lack of both diversity and populations of larger predatory fish at nearly all the sites dived in the GAR, but especially at the study site west of Ono.
While anchored in the area, we learnt about the destructive fishing practice which uses a local weed, commonly known as ‘duva’. The plant is crushed and then squirted in an area underwater to stun fish. The fish are then easily collected. However, the effect is indiscriminate and all fish within that area will be affected, also the corals. Duva has been widely used in Fiji although there have been great efforts to eliminate the practice. It seems that parts of the GAR have not yet complied with the ban, or that we were witnesses to the aftermath of previous fish collections using duva.
The results of our transects on the outer reef west of Ono showed a lack of predatory fish. On the northeast outer North Astrolabe Reef, there was a much more predictable array of fish. The low state of health of the coral reef on the west of Ono plus overfishing/destructive fishing have probably contributed to the low diversity in fish at our study sites.
World Wildlife Fund have played a key role in establishing a no-take zone, managed by the village of Waisomo on Ono. Enforcement of this protected area should help to return fish stocks to their previous level of abundance.
On each of the bommies inside the lagoon we found about 4 or 5 Crown of Thorns sea stars, feeding mainly on Acropora table-colonies. There were few giant clams inside the lagoon and only a small number outside. However, most of our diving took place at depths between 7 and 15 meters and there might be a bigger amount of clams at a deeper level.
On the transect dives at the Alacrity-Rocks we found only 2 sea cucumbers, whereas the coral bommies inside the reef showed a healthy population of these animals.
Quite a lot of massive and encrusting hard corals, such as Porites and Montipora, were colonised by the scallop Pedum spondyloideum which lives inside crevices of these corals. Also the marks of bioeroding mussels and vermetid snails were found on hard coral colonies and coral rock.
Sea urchins were mainly represented by Echinostrephus sp. which excavate depressions into rock for protection.
No corallivorous snails were seen on any of our dives.
During our time at the Great Astrolabe Reef at the end of May into June we had no encounters with marine mammals. There is a pod of pilot whales known to pass around the GAR but we unfortunately did not meet them. However, another yacht which arrived a few days after us had encountered a whale in the channel between Kadavu and Ono. He suspected it was a minke whale.
On the voyage from Suva to the Great Astrolabe Reef and back to Suva there were also no marine mammal sightings.
Close to the RV Heraclitus’ anchorage inside the lagoon we had one sighting of a turtle surfacing, but we did not encounter any turtles during our dives throughout the study on the western side of the GAR. During observational dives on the eastern barrier reef six turtles (Green and Hawksbill) were encountered.
The GAR is threatened by many factors. The two bleaching events and the slow recovery from them are one concern. The second concern is found in over fishing. Due to its proximity to Suva, the area has been heavily fished. This pressure is not that dominant anymore as commercially important stocks have already been exploited. But there is still fishing pressure from the local communities. The villages on Kadavu Island are not dependent on fishing and harvesting of the reefs. They have good soil on their island making agriculture a sustainable source. However, the slash and burn technique is rampant on the slopes of most of the islands in the group and this has caused a great deal of sedimentation and runoff onto the reefs.
Ono’s soil is not as fertile as Kadavu’s and it has also been destroyed by the imported pine tree, which are being grown everywhere. We met with resort owners on Ono who are planting native hardwoods in an effort to reverse the pine-tree trend.
Ono’s inhabitants can grow their own vegetables in their gardens, but we were told by one of the villagers of Nangera that the soil would not sustain an economical use. Therefore Ono’s inhabitants are dependent on fishing and harvesting of the reef to meet increasing financial demands. They catch fish and collect sea cucumbers for sale in Suva’s markets.
Another concern is the possibility that ‘duva’ is still being used for fishing or that the reef is taking a long time to recover from its previous use (see fish observation section).
For the outer leeward western barrier reef we found a low scleractinian coral coverage of about 14%. One third of this total live coral coverage resulted from a couple of huge Diploastrea colonies on the transects. This implies a pretty low abundance for all the other scleractinian genera.
Out of all the scleractinian coral colonies 44 % were in an overall healthy state while 38 % were threatened by decreasing in size or having recently died off.
From an observational point of view the health of the reef would have been estimated much lower than 44 %. The relatively high number of healthy corals is probably represented by a high number of young newly recruited Acropora coral colonies. For most other genera we found hardly any healthy or unblemished colonies.