Red Sea

The Red Sea is geologically a young ocean, a basaltic spreading rift that has been pulling Africa and Arabia apart for some 70 million years. Its shallow link to the Indian Ocean has closed many times, most recently during the Pleistocene, and many of today's reefs are thin modern veneers over older Pleistocene reefs (Spalding et al. 2001). Together with the Gulf of Aden it may be the most biologically diverse reef area outside Southeast Asia, with many endemic species. South of the Gulfs of Suez and Aqaba both shores are lined with fringing reefs that run unbroken for tens of kilometers, usually with narrow reef flats. On the Arabian side the continental shelf is narrow or absent in the north and broadens fairly rapidly south of about 20°N; offshore, the Al Wadj Bank holds a complex of fringing, patch, and barrier reefs with small islands, and from Al Wadj to Jeddah runs a broken barrier-type structure that has been called the Little Barrier Reef. Surveys between 1997 and 1999 recorded about 260 hard coral species on the Saudi coast. Spalding et al. (2001) estimate about 6,660 km² of coral reef for Saudi Arabia, a national figure that includes its Arabian Gulf coast, and about 3,800 km² for Egypt; Souter et al. (2021) give 13,605 km² for the Red Sea and Gulf of Aden together and 7,800 km² for the northern and central Red Sea subregion, which is the subregion all but two sites on this page belong to; the two Al Qunfudah reefs sit south of it.

This page gathers 35 sites in two national branches. Saudi Arabia holds 24 in three units: Jeddah / KAUST (Thuwal) / Rabigh with 15 — eight off Thuwal worked with Coastline (KAUST), six off Jeddah, and one shore-entry reef north of Rabigh — Yanbu with seven offshore reefs in the Yanbu and Umluj area dived with Umluj Dive Center, and South of Jeddah with two unnamed reefs off Al Qunfudah, dived together on 30 August 2024 with Al Qunfudah Diver. Egypt holds 11 in one unit, four at Marsa Nakari and seven at Wadi Lahami, dived with Red Sea Diving Safari. Thirty-three fall in one NOAA Coral Reef Watch ecoregion, Northern and Central Red Sea; the two Al Qunfudah reefs fall in Southern Red Sea, so the page spans two heat-stress provinces, with 33 of 35 sites in the northern one. The settings run from a wave-lit reef flat a meter or two under the surface at Alrass Alakhdar and low-relief house-reef pavement, rubble, and sand at Marsa Nakari, through high-relief spur-and-groove honeycombed with swim-throughs at Cement Reef, submerged pinnacles at Habili El Ghadeer, Habili Daisy, and Habili Gazella, and Porites-carpeted slopes at Um Mankout, Om Halhala, and Salah, to near-vertical walls at Rose Reef and Thetis and eroded, heavily bioeroded caverns rising from sand at Cave Reef. Three members are artificial or barely coralline — the Cement Wreck and Chicken Wreck off Thuwal and Jeddah, and Drogon Corals, mostly a sand plain with isolated bommies — while a fourth, Abu Galawa-small, is a steel shipwreck with scattered coral colonies, most on its open ribs. Recorded maximum depths run from 13.4 m at Habili Gazella to 36.0 m on Rose Reef's wall at the 24 sites that have one. Two sites have no photographs and no cover value at all: Middle Reef off Thuwal and Al Shbareer reef off Yanbu.

Published point-count cover exists for 33 of the 35 sites and averaged 20%, a narrow band from 13% on the Cement Wreck to 30% at Alrass Alakhdar. The two branches are close together: 19% across the 22 Saudi sites with a value and 22% across all 11 Egyptian sites. Algal cover stayed low everywhere, from 3.4% at Om Halhala to 12% at Long Reef, with the Egyptian sites clustered at the low end. The community is consistent along both coasts — pocilloporids in lavender, magenta, rose, and hot-pink color forms, corymbose, digitate, staghorn, and table Acropora, knobby and columnar Porites, Seriatopora hystrix, faviids, and Millepora-type fire coral, with cobalt Acropora on the upper slope at Cement Reef. Individual sites stand out for structure rather than for cover. Coral Gardens off Jeddah is built on giant Porites mounds several meters across; Um Mankout carries a near-continuous Porites framework with almost no bare pavement; Anderson's Reef-II is set apart by a bommie the size of a small building with its crown covered in lilac and gold arborescent soft corals; and Shabrour-north is a broad sand plain studded with bommies and low patch reefs.

What this set records is the 2023-24 Red Sea marine heatwaves: 33 of the 35 sites carry a bleaching percentage. Across them, bleaching averaged 42% of coral tissue and ranged from 14% at the first Al Qunfudah reef to 74% at Shabrour-north in Wadi Lahami. The two branches read very differently. The Egyptian mean is 59% and ten of the eleven Egyptian sites are above 50%; the Saudi mean is 33% and only four of the twenty-two Saudi sites with a value are. Shabrour-north reached 74% and Shark Reef off Thuwal 72%, where whole slopes were white and three crown-of-thorns starfish were photographed amid the bleaching. Branching, massive, and fire corals were all affected on the Saudi coast, and branching and massive corals on the Egyptian coast, where the fire coral was dead and overgrown with algae: at Om Halhala the Porites framework bleached en masse while nearby Acropora thickets held color, at Bhadur Reef soft corals, fungiids, and anemones bleached along with the hard corals, and the corals on the Abu Galawa-small wreck were almost entirely white. Some colonies showed fluorescent (colorful) bleaching, among them a pale-blue thicket at Shabrour-south and vivid pink-lavender, blue, and yellow colonies at Shark Reef and north Rabigh. Other pressures were photographed too: a derelict fish trap overgrown by corals and a diver towing a trap full of fish at Um Mankout, where Anderson logged "clear evidence of overfishing"; an old line across Cement Reef and a fouled pipe at Al Fasmah; and red or rust-orange cyanobacterial mats over coral bommies, dead rock, or dead framework at Gazella Reef, Habili Daisy, and Shark Reef.

That country gap should not be read as a difference between Egyptian and Saudi reefs, because it is confounded with when each coast was dived. All eleven Egyptian sites were surveyed in a single four-day window, 20 to 23 September 2024, at about 25.5 to 26.0°C-weeks of accumulated heat, more than at any Saudi visit, with a published dive temperature of 31.5°C at Marsa Nakari-North (Mayfield 2025, Environments 12:248, Appendix A) — a snapshot taken at the peak of a severe event. The 24 Saudi sites were surveyed over fifteen months, September 2023 to November 2024, and several were dived repeatedly in winter and in summer, at per-visit heat loads from 1.4 to 22.3°C-weeks and, on seven visits, none recorded at all; their site pages list each visit. The Saudi visits made at the heaviest heat loads bleached in the Egyptian band: Bhadur Reef 62% at about 22.3°C-weeks, Shark Reef 72% at 20.6 and 21.8, Thetis 62% at 19.9 in October 2023. Accumulated heat does not explain the scatter either, though. The clearest case is Al Qunfudah: its two reefs sat at about 20.4°C-weeks and bleached 14% and 18%. The north Rabigh shore site read 65% at about 16.9°C-weeks while Long Reef read 20% at 16.5; Anderson's Reef-I and Reef-II read 17% and 19% at 11.9; and seven visits bleached with no accumulated heat stress recorded at all: Rose Reef at 23%, Cave Reef at 27%, Abu Ejeiah at 13%, and Cement Reef at 20% in February 2024, and Abu Galawa at 25%, Mr. Umluj at 28%, and the Cement Wreck at 25% in April 2024. Where the heat did bite, cover followed: Thetis went from 33% in October 2023 to 13% in August 2024, when the wall was darker and dominated by soft corals and sponges.

Two independent comparisons sit beside these figures rather than being averaged into them. At Marsa Nakari, Reef Check stations 0.10 and 0.25 km from the two house-reef sites recorded 29% and 27% hard coral in 2025 (40% and 47% in 2024, against CRD's 25% and 23% that year — a different method on what may be the same reef). At Wadi Lahami, Reef Check's House Reef 2 station, 1.55 to 1.85 km from the two Shabrour sites, recorded 56% in 2009 and 44% in 2023, against 24% and 25% in September 2024. These 33 sites are also the reference region for CRD's multi-model computer-vision comparison, in which several segmentation models are being scored against a set of frames hand-annotated by Anderson. Every cover figure on this page is the published CoralNet point count of Mayfield (2025, Environments 12:248), in which most frames were annotated by CoralNet's AI, trained and checked by Anderson. For the longer view, Souter et al. (2021), writing before the 2023-24 heatwaves, had coral cover in the northern and central Red Sea falling from 35% in 1997 to 30% in 2002 and then climbing slowly to 39% by 2019, while algal cover rose from 5.2% in 2010 to 11.5%. They name tourism and coastal development as the main local drivers of loss in the northern Red Sea and land runoff, eutrophication, and overfishing in the central Red Sea, with Saudi Arabia's Vision 2030 coastal tourism plans an added pressure; and they note that the region's only sampling units with more than fifteen years of data lie on the Egyptian coast, where more frequent disturbance has produced a stepwise decline, with half of those that lost at least a fifth of their coral never regaining 90% of it.