Hon Sup: 1.28 Hectares, and the Healthiest Reef on Son Tra

Son Tra lost 42 per cent of its coral reef area in a single decade. The small

submerged reef off Bai But came through it better than anywhere else on

the peninsula - yet almost everything known about it comes from surveys

in which it was one stretch of coast among many.

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In one morning in June 2026, volunteers working off the south shore of the Son

Tra Peninsula cut close to 100 kilograms of abandoned fishing net away from a

single reef [1]. The reef is called Hon Sup, and it covers 1.28 hectares - smaller than two football pitches [2]. It is also, on the only published evidence there is, the reef in the best condition on a peninsula that lost 42 per cent of its coral in ten years [2],[3].

• A reef in the mountain's wind shadow

Hon Sup is a submerged rock and reef platform off the southern shore of Son Tra, between Bai But and Bai Da, close enough to the surface to break water at low tide. The Institute of Oceanography's 2016 mapping, combining field survey with remote sensing, put the reef at 1.28 hectares - one of the smaller of the sixteen it charted around a peninsula then holding about 46.9 hectares in all, Bai But alone accounting for 6.22 [2].

Size is not substance. Son Tra's reefs are what the Institute calls non-fringing: coral grows thinly across boulders and bedrock rather than on a limestone framework it built itself, squeezed into a narrow band from the intertidal zone to roughly 10 metres [2]. A 2019 satellite and dive study from the University of Da Nang traced live coral to 14 metres, densest between 2 and 6 [3]. On most of the peninsula that band is 30 to 90 metres wide; along the Bai But to Hon Sup stretch it widens to 140-300, the broadest reef platform Son Tra has [3].

 Hon Sup lies off the southern shore of the Son Tra Peninsula, between Bai But and Bai Da, and is one of five areas Da Nang zoned for coral protection under Decision 54/2007/QD-UBND, in force on the water since June 2009. Named sites are those used in the Institute of Oceanography's 2016 peninsula survey. Schematic locator map; shore outline and positions are indicative, not surveyed.

The peninsula runs about 13 kilometres east from the city and rises to a forested massif [18], and that bulk matters. Da Nang's year splits sharply: a dry season from January to July delivering some 600 millimetres of rain, then a wet season from August to December carrying 80 per cent of the annual 2,000 [3]. The Han River empties into Da Nang Bay on the north-western side, and its plume is heavy enough that the 2019 study excluded the north-westernmost reefs, the

estuary leaving the water too turbid to classify [3]. Hon Sup sits on the far side of the mountain. That is worth something, because sediment kills coral slowly: it blocks the light the symbiotic algae need, shedding it costs energy the colony cannot spend on growth, and settled silt denies larvae the hard surface they need to attach.

Coral around Son Tra grows in a narrow depth band, thickest between 2 and 6 metres and petering out by 14 metres, on a base of boulder and bedrock rather than a reef framework of its own making. The Bai But-Hon Sup stretch carries the widest platform on the peninsula at 140 to 300 metres. Schematic cross-section; depths and widths from the Institute of Oceanography's 2016 survey and Nguyen et al. (2021), profile shape illustrative.

• Plenty of species, plenty of small fish, nothing much worth catching

The species counts for Son Tra disagree, instructively. The Institute's 2016 survey identified 177 coral species in 17 families, seven of them new records for these waters [2]; the Global Coral Reef Monitoring Network figure cited in the 2019 study puts Son Tra at 226 [3]. The two count different things - a targeted field campaign against a cumulative regional inventory - so neither is wrong, only differently scoped. Either places Son Tra mid-range for Vietnam, above Ha

Long-Cat Ba at 150 species and well below Nha Trang at 350 [2]. For Hon Sup itself the record thins to almost nothing: a dive survey reported by the Da Nang authorities counted roughly 54 coral species east of the rock and 58 west of it, two decades ago, unrepeated since [4].

What grows there shapes what survives there. Branching corals such as Acropora grow fast and build the three-dimensional structure reef fish shelter in, but their thin branches snap under an anchor chain and bleach first under heat. Massive Porites grow far slower as dense boulders, and they endure. Across the eight Son Tra sites surveyed in detail in 2016, live cover averaged about 23 per cent of the bottom - roughly 20 per cent hard coral, 3 per cent soft - and

soft coral appeared at only three of the eight, all on the southern side [2].

The fish tell a quieter story. Reef fish density barely moved between survey years, from 841.3 individuals per 400 square metres in 2005 to 947.9 in 2016 [2]. The composition had hollowed out: counts were dominated by small and ornamental species, while commercially valuable groups turned up at very low density and were absent at many stations [2]. A reef can keep its headline fish numbers while losing every fish anyone would want to eat, because small fast-breeding species fill the space that larger, slower, heavily fished ones vacate.

23% - Average live coral cover across the eight Son Tra sites surveyed in detail in 2016 - roughly 20 per cent hard coral and 3 per cent soft.

• Vietnam's reefs bleached in 2024. Nobody checked Son Tra.

Reef-building corals live in partnership with single-celled algae inside their tissue, which supply most of the animal's food and all of its colour. When water stays roughly a degree above the local summer maximum for long enough, the partnership turns toxic and the coral expels them, leaving white skeleton visible through transparent tissue. The coral is not dead. It is starving, and whether it recovers depends on how long the heat lasts.

The heat has been lasting. NOAA and the International Coral Reef Initiative confirmed a fourth global bleaching event on 15 April 2024, with bleaching documented in both hemispheres of every major ocean basin between February 2023 and April 2024 [6]. Vietnam was not spared: surveys at Con Dao that June found 97.5 per cent of coral cover bleached at 2 to 5 metres and 85.7 per cent at 10 to 12, and by October the acroporids there were dead, while massive Porites bleached at 94 per cent yet largely survived [5]. It also shows why some reefs get off lighter. In Nha Trang Bay, surveyed by the same method in the same months, bleaching reached only 33.6 per cent, and corals recovered extensively about six weeks after monsoonal upwelling began [5]. Heat stress is cumulative - it is the dose over weeks that kills - so anything that flushes warm water off a reef resets the accumulation before it turns lethal. Depth works the same way, which is why Con Dao's deeper corals fared better than its shallow ones [5].

All of which makes Son Tra's absence conspicuous. The Vietnamese assessment of the June-July 2019 bleaching covered Nha Trang, Ninh Thuan, Con Dao and Phu Quoc [7]. Da Nang was not among them, and no published survey of bleaching at Son Tra in 2019 or 2024 could be located for this article. Divers on the peninsula report bleaching as something they see [8]; an observation is not a measurement, and nobody has turned one into the other.

Son Tra's mapped coral reef shrank from 80.9 hectares in 2006 to 46.9 hectares in 2016 - a loss of 34 hectares, or 42 per cent, in about ten years. Peninsula-wide coastal waters of the Son Tra Peninsula. Source: Institute of Oceanography 2016 survey, reported by Nguyen Xuan Hoa (2017); 2006 baseline from Nguyen Van Long and Hoang Xuan Ben.

• Forty-two per cent of Son Tra's reef vanished in ten years

The headline number is stark and undisputed. Mapped reef area around Son Tra fell from 80.9 hectares in 2006 to about 46.9 in 2016 - a loss of 34 hectares, or 42 per cent, in roughly a decade [2],[17]. Confirmation came by another route: classifying 3-metre Planetscope imagery from June 2019 against 1,543 underwater photographs, the Da Nang team measured 0.47 square kilometres of live coral, about 47 hectares [3]. Two techniques three years apart landing on the same figure is as close to a settled fact as this coast offers. The loss is severe because reefs do not rebuild on that timescale: a colony needs 15 to 45 years to reach maturity, on the estimate used by the divers who restore these reefs [9]. Ten years of loss is several decades of replacement.

1.28 ha - The mapped area of the Hon Sup reef in the Institute of Oceanography's 2016 survey, out of about 46.9 hectares left around the whole peninsula.

The loss was not evenly spread, and the pattern is the point. On the northern shore live cover fell below 10 per cent, the bottom silted and much of it under macroalgae: hard coral at Bai Bac collapsed from 31.9 per cent in 2006 to 0.6 in 2016, and at Mui Lo from 21.3 to 7.5 [2]. Several southern sites moved the other way over the same decade, gaining cover rather than losing it [2]. The 2019 classification put Hon Sup on the better side of that split: 19.73 per cent of

the Bai But-Hon Sup segment was in good condition, against 7.87 per cent across the peninsula as a whole [3].

Of the five Son Tra stretches classified from 2019 satellite imagery and dive photography, Bai But-Hon Sup had the smallest share of reef in poor condition and the largest share in good condition - though no Son Tra reef reached the excellent class, and Cu Lao Cham outperformed the whole peninsula. Reef segments of the Son Tra Peninsula and Cu Lao Cham Island. Source: Nguyen et al. (2021), PeerJ 9:e12413.

The causes named by the Institute of Oceanography are land reclamation, coastal construction, sediment deposition and overfishing, with waste and pollution from towns, fishing, restaurants and tourism as continuing threats [2]. Storms compound them: after Typhoon Noru struck in October 2022 the restoration group SaSa reported more than half the natural reef at Bai But destroyed [10]. But the damage volunteers carry ashore is mostly gear. Lost nets settle over branching colonies, shade them and saw at them with every swell until the branches part. Anchors do in seconds what took decades to build. One SaSa member described a metal crab trap nearly 20 metres long lying across the reef, which took two hours to remove [11].

• Protected on paper since 2007, repaired in practice by volunteers

Hon Sup has had legal protection for nearly twenty years. Decision 54/2007/QD-UBND of 13 September 2007 set management rules for the waters from Hon Chao to south Hai Van and around Son Tra, naming an 82-hectare strict protection area covering Hon Sup, Bai But, Bai Bac, Huc Lo, Vung Da and East Bai Bac [12]. When the fishing and coral-harvesting ban took effect in June 2009 it was reported as covering nearly 100 hectares, extending 300 metres

from shore to an average depth of about 12 metres [13] - one figure is the area named in the decision, the other what the drawn boundary enclosed. The city zoned five areas in all: Hon Sup, Bai But, Huc Lo-Vung Da and Bai Bac under strict protection, and Bai Nom as an ecological restoration zone, each buoyed [14].

Enforcement has been the weak link from the start. Reporting in 2008 found no buoys or markings of any kind on the supposedly protected water, and responsibility for the entire coast from south Hai Van to Son Tra handed to a Fisheries Sub-Department too thinly staffed to cover it; the same account recorded anchor scars, trampling and fishing at Bai Nom, Hon Sup and Huc Lo [12].

What has been done underwater has largely been done by volunteers. SaSa Team Marine Rescue, founded in Da Nang in 2016 and led by Le Chien, built a coral nursery at Bai But that year, growing 36 species across more than 400 square metres, among them the uncommon Acropora loripes [9]. The method is unglamorous: divers collect broken and diseased fragments, mount them on stainless frames and nursery tables, and return them to the bottom once stabilised [11],[15]. Members must hold international dive certification, sessions run six to eight hours in visibility of 5 to 10 metres, and about 90 per cent of the funding comes from members' own pockets [9].

The group's reported results should be read as what they are. SaSa states it has restored 400,000 square metres of reef at Bai But and more than 20,000 at Hon Sup, lifting cover at Bai But from 16 per cent to around 60 [9]. Those areas far exceed the 6.22 and 1.28 hectares mapped at the two sites in 2016 [2], so they must describe a working footprint rather than mapped reef, and no independent survey has verified the cover figures. The gear is easier to credit: by 2019 the group reported cutting close to 10 tonnes of net free [15], and the June 2026 Clean Up Son Tra day took nearly 100 kilograms off Hon Sup in one morning [1],[16]. SaSa has since dismantled the Bai But nursery and released the young colonies into open water because, in Le Chien's account, there was nobody to guard it [9].

19.7% - Share of the Bai But-Hon Sup segment classified in good condition in 2019, against 7.9 per cent peninsula-wide.

• The survey that has never been done

No published study treats Hon Sup on its own. Its area comes from a peninsula-wide mapping exercise, its condition from a segment it shares with Bai But, its species list from a dive count two decades old. Nothing measures bleaching at Son Tra in 2019 or 2024, the years Vietnam's reefs were hit hardest, and no independent survey has tested the restoration results reported here. The question worth putting to a research vessel is specific: does the southern shore's

position behind the massif actually buffer it against heat, and if so, for how much longer?

What the record does show is consistent, and it points one way. On a peninsula that lost nearly half its reef in a decade, the stretch that held on best is the one furthest from the river mouth, widest across the seabed, and on the sheltered side of a mountain. Hon Sup's advantage was not conferred by the protection order it has carried since 2007; it was conferred by geography, and it has been maintained since largely by people cutting net off coral by hand. A reef of 1.28

hectares is a small thing to be the best of anything. That it is the best Son Tra has left is the measurement that matters.

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References

[1]. Tien Phong. "Giai cuu san ho khoi 'sat thu' luoi ma o bien Da Nang" [Rescuing coral from the ghost-net killer in Da Nang waters]. Tien Phong, 14 June 2026. https://tienphong.vn/giai-cuu-san-ho-khoi-sat-thu-luoi-ma-o-bien-da-nang-post1851248.tpo (accessed 20 August 2026).

[2]. Nguyen Xuan Hoa. "Cac ran san ho o vung bien ven bo ban dao Son Tra" [Coral reefs in the coastal waters of the Son Tra Peninsula]. Institute of Oceanography, Nha Trang; published in Van Nghe Da Nang, 2 October 2017. https://vannghedanang.org.vn/cac-ran-san-ho-o-vung-bien-ven-bo-ban-dao-son-tra-nguyen-xuan-hoa-6643.html (accessed 20 August 2026).

[3]. Nguyen, K.V., Duong, V.C., Kieu, K.T., Tran, T.V., Huang, C., Reef, R. and Hoang, T.M. "Characterizing the spatial distribution of coral reefs in the South-Central Coast region of Viet Nam using Planetscope imagery." PeerJ 9:e12413, 2021. https://peerj.com/articles/12413/ (accessed 20 August 2026).

[4]. Da Nang People's Committee portal. "Ra Son Tra lan bien" [Diving off Son Tra]. danang.gov.vn. https://www.danang.gov.vn/web/guest/gioi-thieu/chi-tiet?id=687&_c=169 (accessed 20 August 2026).

[5]. Tkachenko, K.S., Dung, V.V., Ha, V.T. and Phong, H.H. "Unprecedented coral bleaching in Southern Vietnam in 2024: the only hope for thermal refuges?" Coral Reefs 44, 881-894, 2025. https://link.springer.com/article/10.1007/s00338-025-02651-8 (accessed 20 August 2026).

[6]. National Oceanic and Atmospheric Administration. "NOAA confirms 4th global coral bleaching event." NOAA, 15 April 2024. https://www.noaa.gov/news-release/noaa-confirms-4th-global-coral-bleaching-event (accessed 20 August 2026).

[7]. Hoang, P.K., Tuan, V.S., Quang, T.M., Hoc, D.T. and Tuyen, H.T. "Bleaching of coral in Nha Trang, Ninh Thuan, Con Dao and Phu Quoc islands in June-July 2019." Vietnam Journal of Marine Science and Technology 20(4A), 55-60, 2020. doi:10.15625/1859-3097/15649. Cited here as listed in the reference list of Tkachenko et al. (2025), reference 5; not consulted directly.

[8]. Thanh Nien. "'Coi troi' san ho duoi day bien Hon Sup o Da Nang" [Freeing the coral on the seabed at Hon Sup, Da Nang]. Thanh Nien, 3 June 2026. https://thanhnien.vn/coi-troi-san-ho-duoi-day-bien-hon-sup-o-da-nang-185260603230302647.htm (accessed 20 August 2026).

[9]. Lan Anh. "Gieo mam su song giua long bien Da Nang" [Sowing the seeds of life in the sea at Da Nang]. Bao Nong nghiep va Moi truong, 22 August 2025. https://nongnghiepmoitruong.vn/gieo-mam-su-song-giua-long-bien-da-nang-d768273.html (accessed 20 August 2026).

[10]. FairPlanet. "Saving Vietnam's coral reefs." FairPlanet. https://www.fairplanet.org/story/saving-vietnams-coral-reefs/ (accessed 20 August 2026).

[11]. Nguoi Lao Dong. "Tinh nguyen lan bien trong san ho" [Volunteer divers planting coral]. Nguoi Lao Dong, 13 September 2020. https://nld.com.vn/thoi-su/tinh-nguyen-lan-bien-trong-san-ho-20200913203846238.htm (accessed 20 August 2026).

[12]. Sai Gon Giai Phong. "Da Nang: San ho 'hap hoi'!" [Da Nang: coral in its death throes]. Sai Gon Giai Phong, 25 January 2008, quoting Decision 54/2007/QD-UBND of the Da Nang People's Committee, 13 September 2007. https://www.sggp.org.vn/da-nang-san-ho-hap-hoi-post234422.html (accessed 20 August 2026).

[13]. VOV. "Cam khai thac hai san vung bien quanh ban dao Son Tra" [Fishing banned in the waters around the Son Tra Peninsula]. Voice of Vietnam, 30 June 2009. https://vov.vn/xa-hoi/cam-khai-thac-hai-san-vung-bien-quanh-ban-dao-son-tra-115031.vov (accessed 20 August 2026).

[14]. Wikipedia (Vietnamese). "Bai But." https://vi.wikipedia.org/wiki/B%C3%A3i_B%E1%BB%A5t (accessed 20 August 2026). Used only for the zoning summary and the 2015 measures, both corroborated by references 12 and 13.

[15]. Bao Tai nguyen va Moi truong. "Nhung nguoi tre 'cuu' san ho" [The young people rescuing coral]. 13 September 2019. https://baotainguyenmoitruong.vn/nhung-nguoi-tre-cuu-san-ho-293302.html (accessed 20 August 2026).

[16]. Cong nghiep Moi truong. "Lan bien nhat rac, giai cuu san ho vi mot Son Tra xanh" [Diving for waste and rescuing coral for a green Son Tra]. 14 June 2026. https://congnghiepmoitruong.vn/lan-bien-nhat-rac-giai-cuu-san-ho-vi-mot-son-tra-xanh-16923.html (accessed 20 August 2026).

[17]. Thuy san Viet Nam. "He sinh thai vung bien ven bo suy giam: Da Nang co bo lo Son Tra?" [Coastal ecosystems in decline: is Da Nang neglecting Son Tra?]. 6 July 2020. https://thuysanvietnam.com.vn/he-sinh-thai-vung-bien-ven-bo-suy-giam-da-nang-co-bo-lo-son-tra/ (accessed 20 August 2026).

[18]. Wikipedia (Vietnamese). "Khu bao ton thien nhien Son Tra" [Son Tra Nature Reserve]. https://vi.wikipedia.org/wiki/Khu_b%E1%BA%A3o_t%E1%BB%93n_thi%C3%AAn_nhi%C3%AAn_S%C6%A1n_Tr%C3%A0 (accessed 20 August 2026). Used only for peninsula dimensions.