Tonga

From 11 to 30 September 2013, the Khaled bin Sultan Living Oceans Foundation's Global Reef Expedition surveyed 59 stations in three island groups of Tonga: Ha'apai, Vava'u and, in the far north, Niuatoputapu. Tonga's islands run for more than 800 km along the convergent boundary of the Indo-Australian and Pacific plates, with the Tonga Trench to the east. Spalding et al. (2001) describe two parallel arcs: a western line of high volcanic islands, mostly too unstable for significant reef growth, and an eastern arc of islands built largely of reef material. Ha'apai is a complex scatter of reefs and low-lying islands in the middle of that eastern arc, and the Vava'u group anchors its northern end. Niuatoputapu lies far to the north, near Tafahi and the volcanic island of Niuafo'ou.

The expedition worked Ha'apai first (32 stations, 11-21 September), then Vava'u (21 stations, 22-28 September) and Niuatoputapu (6 stations, 29-30 September). Maximum depths dived ran from 13.1 m at TOHA07 in Ha'apai to 32.9 m at TONI54 on Niuatoputapu. Ha'apai's reefs are built around large shallow mounds and spurs separated by sand and rubble channels, along with lagoonal patch reefs and a barrier fore reef. Vava'u adds sheltered lagoon reefs, channel entrances, fore reefs below uplifted cliffs, caves, and a steep pinnacle wall. Niuatoputapu's fringing and barrier reefs have modest relief.

Hard coral cover averaged 32% across all 59 stations, from 8.6% at TOHA25, surge channels where algae covered 84% of the benthos, to 53% at TOHA18, both in Ha'apai. The three groups were close on average: Vava'u 33% (n = 21, from 9.6% at TOVA50 to 53% at TOVA44), Ha'apai 31% (n = 32) and Niuatoputapu 30% (n = 6, from 15% at TONI57 to 44% at TONI56). Algae separated them more clearly, averaging 44% in Ha'apai, 51% in Vava'u and 66% on Niuatoputapu. Table, staghorn, and bottlebrush Acropora dominate the shallow mound tops and terraces of Ha'apai and Vava'u, with massive Porites deeper and on the spurs. Niuatoputapu was the exception: its reefs were built mainly by mounding Astreopora, plating Montipora, Turbinaria, and Porites.

The Coral Health Index averaged 2.56 across the 19 stations where it was scored (scale of zero to five; higher is healthier): 2.28 in Ha'apai (n = 11) and 2.96 in Vava'u (n = 8), with none scored on Niuatoputapu. It ranged from 1.00 at TOHA18 to 4.24 at Pete's Paradise (TOVA34) in Vava'u. Crown-of-thorns starfish were recorded at seven Ha'apai stations, including a minor outbreak at TOHA26 and 12 starfish at TOHA23, and at four in Vava'u, with 15 on one bommie at the Japanese Coral Gardens (TOVA42). Culcita cushion stars and Drupella snails turned up widely, and the survey notes at TOVA39 record a high density of coral-eating snails. Anderson's photographs also record bleaching in the 2013 frames: pale gray-white staghorn and a small stark-white colony at TOHA01, a partly bleached poritid at TOHA04, bleached staghorn stands at TOHA18, a large bleached table at TOHA27, two small fully bleached pocilloporids at TOHA28 (recorded as dying), and paling branching colonies at TOHA29. The sampling sheets record bleaching among the Pocillopora sampled at 17 stations, nine in Ha'apai and eight in Vava'u (for example, all seven colonies 20-80% bleached at TOVA34). White syndrome was noted at five Ha'apai stations, and diseased Pocillopora at five of the six Niuatoputapu stations, where the TONI57 barrier reef also appeared to have suffered extensive tsunami damage.

What stands out in Tonga is that cover and condition did not track each other. TOHA18 recorded 53% cover with a health score of 1.00, and TOVA35, a high-cover stand of massive Porites known as the Aquarium, scored 1.50 because large dead patches had been taken over by damselfish algal lawns. Dead or partly dead staghorn stands under damselfish lawns recur from Ha'apai to Vava'u. Vava'u's scored stations averaged 2.96 on the health index and Ha'apai's 2.28, and sightings included banded sea snakes at TOVA36 and TOVA37, a banded sea snake at TOHA03, a large blotched stingray at TOHA06, and giant clams in all three groups.