Tonga - Ha'apai (TOHA30)

Setting

Intermediate-exposure fore reef. Fringing reef off northeast Ha'apai (LOF Tonga field report). Shallow mounds extend out from 3-4 m to 10 m, then drop vertically to 30-40 m, ending in a sand/rubble/coral field that continues to slope gently seaward. A number of deep channels separate each mound. There is 80-100 cm relief on the vertical surfaces.

General benthic features

The frames are of a coral-topped mound and its drop-off, shot from the mound top looking out and over the edge. Image 1, Image 8, Image 2, Image 7, Image 3, Image 4, and Image 10 all show the same setting: a densely colonized reef top running out to an edge, with deep open water immediately beyond it and nothing but blue below.

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The community in these frames is pale-toned and highly mixed — corymbose and digitate acroporids, thin tabular plates, gray foliose and plating sheets, green corymbose colonies, brown branching clumps, rounded massives, and pavement crusted in pink and mauve coralline algae. Image 3 holds the set's cleanest single structural subject, a broad pale-gray tabular plate sitting proud on the crest; Image 1 has a large cream-yellow domal massive at its left edge; Image 10 has a bright yellow tabular colony on the flank. Image 6 is the best structural frame in the set. It is shot at the lip of the drop-off looking along and over it, with the reef edge crowded in the foreground — a bright yellow-cream massive with large polygonal corallites, tan branching and digitate colonies, and a pale plating colony — and the face falling away into open blue at the right of the frame.

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It shows that the edge is abrupt and that living cover runs right up to and over it, but the frame does not follow the wall down and settles nothing about the deeper community (e.g., Image 6). Two close frames record a giant clam (Anderson's identification). Image 5 is a tight view of a clam deeply embedded in the framework, its brown mantle edged in iridescent white along a strongly zigzag shell margin, beside a tan massive with large polygonal corallites and red-pink coralline crust. Image 9 is a wider version of the same close-range framework — staghorn branches at the upper left, a second clam with a mottled olive and cream mantle, a free-living disc coral, and gray foliose plates — and is the best frame in the set for reading the fine-scale packing of the reef top.

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The deep part of the wall had many 1-2 m gorgonian sea fans. Soft coral increases in cover as you go shallow (5.0% at 30 m; 30% at 8 m). Parts of the wall had 10-20% cover of Millepora; at the edge were large groups of corallimorphs.

Coral cover

• Reference: Khaled bin Sultan Living Oceans Foundation, Global Reef Expedition (in situ transect)
• 40%, from the Khaled bin Sultan Living Oceans Foundation's in situ transect survey (Global Reef Expedition site data); the photographs illustrate the site.
• Transect mean 40 ± 6.5% SE (n = 8 transects; Living Oceans Foundation per-transect benthic data); the headline above is this all-transect mean.

Stressors

No bleaching, disease lesion, or corallivore feeding scar resolves with confidence. The frames cannot establish depth zonation, do not reach the base of the wall, and cannot confirm the survey's deeper records — the large foliaceous colonies at 35 m, the 1–2 m gorgonian sea fans, the Millepora cover, or the corallimorph patches at the edge. Typical recent thermal stress: mean annual maximum of 8.1°C-weeks (Degree Heating Weeks, 2021-2025, NOAA CRW; Southern Tonga virtual station). The 2013 survey notes for this site record snails on a number of the smaller Acropora, including staghorn coral (Khaled bin Sultan Living Oceans Foundation expedition site notes, 2013; regional summary in Bruckner 2014). The Living Oceans Foundation transects here recorded 6.8% cyanobacteria and no recently dead coral.

Fish and other fauna

Fish are conspicuous in the water column. Clouds of dark planktivores and damselfishes hang over the reef edge in Image 2, Image 7, Image 3, and Image 10, and Image 10 also carries a bannerfish and a butterflyfish among them. Small yellow fish appear among the colonies in Image 7.

Comparisons with other data

Apart from the Living Oceans Foundation survey shown here, we found no other surveys at this site as of 2026. If you would like to share yours, please email us.

Other notes

Eleven habitat frames survive from the dive (Ha'apai, 21 Sep 2013); there are no sampled-colony frames, so this is a habitat account throughout. None of the target Pocillopora ('no Pdam'), Anderson's sampling target on this expedition, was found here, which is why the site carries no sampled-colony series. Two frames are poor. There is no scale object, quadrat, or transect line anywhere in the set, so no colony dimension here should be treated as measured.

Survey team's site notes: The top of the mound is a scoured hardground near the reef crest, with moderately high cover (30-40%) of staghorn, digitate, thick plating, and tabular acroporids (some over 2 m), Pocillopora, Favia stelligera, Coscinaraea, Leptoria, Goniastrea, Lobophyllia, large Favites crusts, and many other species, and 25-30% soft coral near the edge of the drop-off. The mounds drop near vertically, with small ledges, overhangs, and crevices. At 10 m it is mostly Montipora, digitate acroporids, staghorn, medium-sized Porites, Pocillopora, Favia stelligera columns, and Favites crusts. At 15 m plates of Isopora, staghorn, digitate acroporids, Porites, and F. stelligera dominate with 30-50% live cover. Coral cover ranges from 5.0-50%, consisting of plating Montipora, submassive Porites, small acroporids and pocilloporids, Goniastrea, and other species. Deeper are numerous sheets of Mycedium, Merulina, Pachyseris, Leptoseris, Echinophyllia, and other species. At the base of the wall in 35 m are very large foliaceous Mycedium, Pachyseris, Merulina, Leptoseris, and Montipora. Further south is a steep slope (45-60 degree) with 80-100% cover of plating and encrusting corals extending from 32-20 m. This included 3-5 m encrusting Porites lobata, P. rus, Montipora, and other species. Further along was a large stand of thick-branched staghorn coral, with mounds of P. rus, Montipora, Goniastrea, and many other species.