Summary of Reef Condition at selected sites at

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Summary of Reef Condition at selected sites at
Summary of Reef Condition at selected sites at Clerke Reef
and Mermaid Reef, Rowley Shoals, October 2008.
Prepared by:
Dr Andy Lewis
Marine Biology Consultant to North Star Cruises
Reef EcoTours
PO Box 137. Magnetic Island. Q. 4819.
Ph: 0419 712 579 email: [email protected]
Executive Summary
The Adventure Cruise vessel TRUE NORTH visits the Rowley Shoals on a regular basis in
early October each year. In 2008, three cruises ran back to back, each of 4 days duration,
with 2.5 days spent at Clerke Reef and 1.5 days spent at Mermaid Reef on each cruise.
We visit a number of different sites during these cruises, both with snorkel and dive
groups. The purpose of this report is to present a brief summary of the condition of the
reef observed during our visit, with information on reef structure, coral cover, fish
community attributes, and the presence of Crown of Thorns Starfish, Drupella gastropods,
and coral bleaching.
Data were collected via visual observation and digital photographs, with site locations
recorded on GPS. While the conclusions that can be drawn from such an approach are
necessarily qualitative, the author (Dr Andy Lewis) has extensive experience in
quantitative reef sampling methods including the GCRMN (Global Coral Reef Monitoring
Network) standard method for both fishes and corals, so the broad conclusions are likely
to be accurate.
Generally, the reef communities at both Clerke Reef and Mermaid reef appear to be in
very good condition, although there has been possibly a small decline in coral cover at
some sites due to the action of Crown of Thorns starfish and coral feeding gastropod
snails (Drupella sp). Coral cover is high through the majority of the lagoonal areas on both
reefs, particularly the habitats close to the eastern margins, with many sites having coral
cover in the range of 80-100% over extensive areas. Acroporid corals are the dominant
family in the lagoons. The Eastern reef flats on both reefs also have coral cover in the
range of 60-90% at the limited number of sites we visited. The upper reef slopes have
coral cover in the range of 30-60% with small robust massive and digitate colonies
predominating.
The fish and invertebrate communities also appear in good condition. Large predatory
fishes such as Serranids, Lutjanids, and reef sharks are common, and there is a good
diversity of species in the smaller reef associated families such as Chaetodontids (19 spp
recorded), Labrids, Pomacentrids, Scarids, and Acanthurids. Based on our brief visits, the
fish community at Mermaid Reef appears even more diverse and abundant than that of
Clerke reef, with large Maori Wrasse (Chelinus undulatus), Oceanic Coral Trout
(Plectropomus laevis), Potato cod (Epinephelus tuka) and reef sharks (Charcharhinus
amblyrhinchos, C. albimarginatus) noticeably more abundant. Giant Clams (T. gigas, T.
derasa, T. squamosa, T. maxima, T. crocea, and H. hippopus) are well represented, and
Trochus sp. were also present, although not in large numbers. Holothurians were common
in both lagoons.
Crown of thorns starfish (COTS - Acanthaster planci) were observed in Clerke Reef lagoon
in moderate numbers (ie. 2-4 starfish seen per 50min snorkel by Dr Lewis covering
perhaps 200-300m), and feeding scars could be seen quite frequently. The abundance of
COTS seemed a little higher than in 2007 and starfish were observed in the open and
feeding during the day, which indicates their population may be approaching outbreak
densities (as defined by AIMS). COTS were not as common at Mermaid Reef. Drupella sp.
Gastropods were observed at both reefs, and an area south of the main entrance channel
at Clerke reef was found to contain moderate to high numbers with evidence of recent
feeding and widespread but patchy coral mortality. No instances of coral bleaching were
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observed, although sea temperatures were not high enough in early October to produce
thermal bleaching. We note however, that the predominance of the bleaching sensitive
Acroporid corals in the lagoons makes both reefs susceptible to large declines in coral
cover in any future thermal bleaching events.
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Site Condition Reports
Clerke Reef
Site 1:
Position: Reef margin immediately south of Bedwell Island.
Date & Time: 28/9/08
10:46hrs
Location Description: A contiguous line of reef bordering the shallow water south of the
island, with deeper water on the south side.
Reef Structure: A contiguous line of reef with edges dropping into 4-5m
Coral Cover: 40-60% along edges with numerous Favid corals, patches of extensive
Staghorn Acropora growth forms in the deeper water along the reef edge.
Fish Community: Pomacentrids, Labrids, Scarids, Acanthurids, Siganids, Lutjanids,
Chaetodontids common, some small Serranids (Cephalopholis argus).
Images
Figure 1: Coral community on the reef margin south of Bedwell Island, Clerke Reef Site
1.
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Images (cont.)
Figure 2: Coral community on the shallow lagoon floor adjacent to the reef margin south
of Bedwell Island, Clerke Reef Site 1.
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Clerke Reef
Site 2:
Position: 17° 16.800S
119° 22.420E
Date & Time: 30/9/08
09:30hrs
Location Description: Shallow reef flat to the south of main entrance channel.
Reef Structure: Reef flat with occasional gullies of 1.5m depth below main flat level.
Coral Cover: 70-90% with numerous Acropora growth forms including digitate, plate,
and corymbose colonies.
Fish Community: Pomacentrids, Labrids, Scarids, Acanthurids, Siganids, Lutjanids,
Chaetodontids, Serranids (Plectropomus laevis, P. aureolatus, Cephalopholis argus).
Images
Figure 3: View of site 2, Clerke Reef.
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Figure 4: View of site 2, Clerke Reef.
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Clerke Reef
Site 3:
Position: 17° 18.829S
119° 21.893E
Date & Time: 4/10/08
09:00hrs
Location Description: Coral wall at the Southern end of the most eastern lagoon.
Reef Structure: Convoluted coral wall with nearby patch reefs, edges dropping into 67m.
Coral Cover: 40-80% along edges, 50-60% on top. Porites (massive) and staghorn
Acropora growth forms along the edges and digitate and corymbose colonies on the top of
the patch reef. Extensive beds of staghorn Acropora over the deeper sand bottoms
adjacent to the reef.
Fish Community: Pomacentrids, Labrids, Scarids, Acanthurids, Siganids, Lutjanids,
Chaetodontids, Serranids (Plectropomus laevis, P. aureolatus, Cephalopholis argus)
Images
Figure 5: View of reef wall, site 3, Clerke Reef.
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Images (cont.)
Figure 6: View of staghorn Acropora thickets, site 3, Clerke Reef.
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Clerke Reef
Site 4:
Position: Western end of main entrance channel
Date & Time: 4/10/08
11:20hrs
Location Description: Large coral bommies at the western end of the main entrance
channel.
Reef Structure: Large coral bommies coming up to tide datum, with sand bottoms in
between at depths 6-12m.
Coral Cover: 80-100% staghorn Acropora growth forms along the bommie edges and
digitate and corymbose colonies on the top of the patch reef.
Fish Community: Pomacentrids, Labrids, Scarids, Acanthurids, Siganids, Lutjanids,
Chaetodontids, Serranids (Plectropomus laevis, P. aureolatus, Cephalopholis argus)
Images
Figure 7: View of coral cover on bommie wall in main entrance channel, site 4, Clerke
Reef.
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Figure 8: View of coral cover on bommie top in main entrance channel, site 4, Clerke
Reef.
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Clerke Reef
Site 5:
Position: 17° 17.481S
119° 22.631E
Date & Time: 4/10/08
15:20hrs
Location Description: Outer reef crest on eastern side of Clerke Reef.
Reef Structure: Reef crest and upper slope with frequent gutters and surge channels in
the top 10m.
Coral Cover: 40-70% with digitate Acropora and small compact massive corals (Poritidae
and Favidae) common, occasional larger massive corals on the upper slope.
Fish Community: Pomacentrids, Labrids, Scarids (including large school of
Bolbometapon muricatum) , Acanthurids, Siganids, Lutjanids, Chaetodontids, Balistids,
Serranids (Plectropomus laevis, P. aureolatus, Cephalopholis argus)
Images
Figure 9: View of coral cover on reef crest, site 5, Clerke Reef.
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Figure 10: View of coral cover on the upper reef slope, site 5, Clerke Reef.
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Mermaid Reef
Site 1:
Position: 17° 05.431S
119° 39.088E
Date & Time: 30/9/08
16:20hrs
Location Description: Eastern margin of the lagoon at the western edge of the reef flat.
Reef Structure: Undulating reef margin in 3-4m
Coral Cover: 60-90% with abundant Acropora growth forms including digitate, plate, and
corymbose colonies. Poritid, Favid and Agaricid corals also present.
Fish Community: Pomacentrids, Labrids, Lutjanids, Carangids, Scarids, Acanthurids,
Siganids, Chaetodontids moderately common, some small Serranids (Plectropomus
areolatus, Cephalopholis argus).
Images
Figure 11: View of patch reef edge, site 1, Mermaid Reef.
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Mermaid Reef
Site 2:
Position: 17° 05.803S
119° 39.137E
Date & Time: 5/10/08
09:00hrs
Location Description: Reef flat on the eastern edge of the lagoon.
Reef Structure: Shallow reef flat with gullies and intermittent sand areas.
Coral Cover: 70-100%. Plentiful staghorn, digitate and corymbose Acropora, with some
massive Porites and Favids.
Fish Community: Pomacentrids, Labrids, Scarids, Acanthurids, Siganids, Chaetodontids
all common, Serranids (Plectropomus areolatus, Cephalopholis argus) common, and
several Maori Wrasse (C. undulatus) sighted. White tip reef sharks (Triaenodon obesus)
and gray reef sharks (Carcharhinus amblyrhynchos) sighted close to the deeper lagoon
edge.
Images
Figure 12: Reef flat adjacent to site 2, Mermaid Reef.
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Images (cont.)
Figure 13: Reef flat adjacent to site 2, Mermaid Reef.
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Mermaid Reef
Site 3:
Position: 17° 03.265S
119° 38.722E
Date & Time: 6/10/08
08:30hrs
Location Description: Outer reef crest on the eastern side of Mermaid Reef, north of
the entrance channel.
Reef Structure: Shallow reef crest with steep outer slope bisected by numerous gullies
and surge channels.
Coral Cover: 20-40%. Compact digitate Acropora and small massive colonies common on
the crest and upper slope, with some larger massive corals at depths greater than 5m.
Fish Community: Pomacentrids, Labrids, Scarids, Acanthurids, Chaetodontids, Balistids,
Anthiids, Caesionids, Lutjanids, Serranids (P. laevis), and several Maori Wrasse (C.
undulatus) sighted. White tip reef sharks (Triaenodon obesus) observed over the flat and
gray reef sharks (Carcharhinus amblyrhynchos) sighted along the deeper reef front. A
school of 10-15 Bump head parrotfish (Bolbometapon muricatum) seen.
Images
Figure 14: Coral cover on the reef slope, site 3 Mermaid Reef.
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Images (cont.)
Figure 15: Coral cover on the upper crest, site 3, Mermaid Reef.
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