Last three days of diving/surveys at Va'vau, the northern island group of the Kingdom of Tonga. Most islands are fringed by reefs (about 18.7°S, 174.0°W).
Max. dive depth=30.2 m
Mean survey temperature=26.2°C
Coral cover (2013)=33±4% (mean±standard error)
Coral Health Index=3.50/5 (moderate)
From 26 to 28 September 2013, during the Khaled bin Sultan Living Oceans Foundation's Global Reef Expedition, we surveyed eight stations on the fringing and barrier reefs of southern and western Vava'u. Transect coral cover ranged from 18% to 47%. The best-developed stations carried shallow terraces of branching and table corals, staghorn thickets, and large massive colonies, whereas the poorest were rubble pavements covered in turf and crustose coralline algae.
Interesting observations included crown-of-thorns starfish, a sheer wall, giant clams, and pincushion sea stars. I also sampled 26 Pocillopora colonies at four stations; of the 22 genotyped, 19 were P. acuta and 3 were P. damicornis. Five sampled colonies, four of them at TOVA47, were 60–95% bleached, although regional heat stress that year peaked at only 1.1°C-weeks.
TOVA45 (Sept. 26, 2013)-no P. damicornis present
TOVA46 (Sept. 26, 2013)-no P. damicornis present
TOVA47 (Sept. 26, 2013)
TOVA47 sampled colonies (n=8)
TOVA48 (Sept. 27, 2013)-no P. damicornis present
TOVA49 (Sept. 27, 2013). Did not dive at TOVA50.
TOVA51 (Sept. 27, 2013)
TOVA51 sampled colonies (n=8)
TOVA52 (Sept. 28, 2013) from the surface
TOVA52 underwater
TOVA52 sampled colonies (n=6)
TOVA53 (Sept. 28, 2013)
TOVA53 sampled colonies (n=4)
Our biomarker analysis of Tongan pocilloporids flagged 12 of 88 colonies as statistical outliers, with higher RNA/DNA ratios and elevated expression of three stress-related genes (Mayfield et al. 2017), and we later developed a coral health index for Tonga's reefs (Mayfield et al. 2021). More recently, a team documented one of the largest coral colonies ever recorded, a giant Pavona clavus, that supports fish communities comparable to those of nearby reefs (Crisp et al. 2026).
Since our surveys, NOAA Coral Reef Watch's Southern Tonga virtual station has recorded severe heat stress (≥8°C-weeks) in 2014, 2015, 2022, 2023, and 2026, peaking at a record 20.6°C-weeks in March 2026.
References
Crisp S, van Hulten D, Southworth L, Havea T, Fatongiatau V, Finau M, Bridge TCL, Smallhorn-West P (2026) Size and ecology of a giant Pavona clavus coral colony in the Kingdom of Tonga. Ecology and Evolution 16(7):e73940. https://doi.org/10.1002/ece3.73940
Mayfield AB, Chen CS, Dempsey AC (2017) Biomarker profiling in reef corals of Tonga's Ha'apai and Vava'u archipelagos. PLOS ONE 12(11):e0185857. https://doi.org/10.1371/journal.pone.0185857
Mayfield AB, Dempsey AC, Chen CS (2021) Assessing coral health in the Kingdom of Tonga with a coral health index. Platax 18:53–78. https://doi.org/10.29926/platax.202112_18.0007