South China Sea

The South China Sea (SCS) is a marginal sea of the Western Pacific Ocean, enclosed by southern China to the north, the Philippine archipelago to the east, the islands of Borneo and the Malay Peninsula to the south, and the Indochina Peninsula to the west. Covering approximately 3,500,000 km² (1,351,000 sq mi), it ranks among the most strategically vital bodies of water on Earth. Roughly one-third of global maritime trade by value — an estimated $3.4 to $5 trillion annually — passes through its waters, connecting the Indian Ocean economies and the Middle East to the manufacturing and consumer markets of East and Southeast Asia.

The sea sits at the confluence of some of the world's most critical maritime chokepoints. To the southwest, the Strait of Malacca — the world's busiest shipping lane — feeds tanker fleets, container ships, and bulk carriers from the Indian Ocean into the SCS's southern basin. To the northeast, the Luzon Strait provides the deepest direct channel between the South China Sea and the open Pacific Ocean, allowing passage for submarines and the largest deep-draft vessels without constraint. The Taiwan Strait to the north connects the SCS with the East China Sea and the ports of mainland China and Taiwan.

The South China Sea contains several island groups and reef features that have become flashpoints of geopolitical tension in the twenty-first century. The Paracel Islands (known in Vietnamese as Hoàng Sa and in Chinese as Xisha) lie roughly equidistant from the coasts of China and Vietnam and are currently administered by China following a 1974 military engagement. The Spratly Islands(Trường Sa / Nansha) are a scattered archipelago of more than 100 reefs, islets, and atolls spread across 425,000 km² of the southern SCS, claimed in whole or part by China, Vietnam, the Philippines, Malaysia, Brunei, and Taiwan. Scarborough Shoal (Huangyan Island / Bajo de Masinloc) and the Pratas Islands are additional contested features. These overlapping claims have shaped not only regional diplomacy but also the practical navigation environment mariners encounter when transiting the sea.

Beneath its surface, the South China Sea holds significant hydrocarbon resources. Proven oil reserves are estimated at approximately 11 billion barrels, with speculative assessments ranging as high as 125 billion barrels — figures that have added commercial urgency to the territorial disputes above. The sea also supports one of the world's most productive and heavily exploited fisheries, underpinning the food security of hundreds of millions of people across the region. For mariners, the South China Sea presents a complex operational environment defined by monsoon weather patterns, typhoon risk, shallow continental shelf hazards, dense vessel traffic, and a regulatory landscape shaped by both IMO instruments and bilateral agreements among the coastal states.

1. Geography & Physical Characteristics

The South China Sea is bounded by a broad arc of landmasses and islands that create a semi-enclosed basin open to adjacent water bodies through a series of straits and channels. To the north, the Taiwan Strait (approximately 180 km wide at its narrowest) separates the SCS from the East China Sea and provides a heavily trafficked corridor for vessels calling at Chinese east-coast ports. The Luzon Strait to the northeast, between the northernmost point of the Philippines and Taiwan, is the primary deep-water exchange channel between the South China Sea and the open Pacific. The Luzon Strait reaches depths exceeding 2,400 metres and is one of the few passages wide and deep enough for unrestricted movement of nuclear submarines and the largest surface combatants.

Along the southern margin, the Strait of Singapore and the Strait of Malaccaconnect the SCS to the Andaman Sea and the Indian Ocean. The Singapore Strait, approximately 105 km long and 16 km wide, carries the densest concentration of maritime traffic of any comparable waterway on Earth. Further east, the Karimata Strait between Borneo and Sumatra and the Sunda Strait between Sumatra and Java provide alternative southerly exits to the Java Sea and the Indian Ocean, used primarily when draft restrictions or Malacca Strait congestion necessitate an alternate routing.

The seabed topography of the South China Sea is strikingly varied. The Sunda Shelf — an exceptionally broad, shallow continental shelf extending from the Malay Peninsula, Sumatra, Borneo, and Java — underlies the southern and southwestern portions of the sea. Across much of this shelf, water depths do not exceed 60 metres, creating an extensive zone of navigational sensitivity for deep-draft vessels. The shelf was exposed as dry land during glacial periods of low sea level, and its flat, sediment-laden topography reflects that geological history.

By contrast, the South China Sea Basin in the central and northeastern portions of the sea reaches depths of up to 5,559 metres — the maximum depth of the entire body of water — in an abyssal plain underlain by oceanic crust. Volcanic seamounts rise from the basin floor, some reaching into shallow water and constituting submerged navigation hazards. The Nansha Trough marks the deeper trench-like depression running northeast of the Spratly Islands, reaching depths of around 4,000 to 4,500 metres.

Hainan Island, the largest island within the SCS proper (excluding the Philippine and Indonesian archipelagos), lies in the northern portion of the sea, separated from the Chinese mainland by the Qiongzhou Strait. With an area of approximately 33,900 km², Hainan hosts major naval and submarine facilities at Yulin, which have assumed growing strategic significance as China's naval posture has expanded. The Paracel Islands (Xisha/Hoàng Sa), a group of approximately 30 coral islands and reefs, are located about 350 km southeast of Hainan, while the Spratly Islands(Nansha/Trường Sa) lie in the southern-central SCS, the most contested and extensively occupied of the disputed island groups. Scarborough Shoal, approximately 220 km west of the Philippine island of Luzon, is a triangular chain of reefs and rocks enclosing a central lagoon.

2. Oceanography & Climate

The South China Sea is governed climatologically by the Asian monsoon system, which reverses seasonal wind direction twice annually and produces dramatic changes in sea state, current patterns, and weather conditions across the region. Mariners must understand these seasonal cycles thoroughly, as they dictate both safe passage timing and the nature of hazards encountered throughout the year.

The Southwest (SW) Monsoon prevails from approximately June through September. During this period, warm, moisture-laden air flows northeastward from the Indian Ocean across the Malay Peninsula and into the SCS. The resulting northeastward currents along the Vietnamese and Chinese coasts drive surface water transport in the same direction. This season is characterised by lower average wind speeds than the northeast monsoon, though the SW monsoon brings heavy rainfall, reduced visibility, and significant swell to the western SCS. The Northeast (NE) Monsoon dominates from November through March, reversing the wind pattern to produce strong, persistent northeasterly winds — typically Force 4 to 6, with occasional gale conditions — and a pronounced southward current along the coast of Vietnam and toward the Sunda Shelf. The NE monsoon brings the coldest and windiest conditions experienced in the SCS and generates the highest average wave heights of the year across the northern basin.

Sea surface temperatures (SSTs) in the South China Sea remain warm throughout the year, ranging from approximately 27°C in the cooler northern portions during winter months to 29°C or above across the southern and central basin in summer. The South China Sea Warm Pool — a persistent area of elevated SST exceeding 28.5°C in the southwestern basin — plays a role in fuelling convective rainfall and tropical cyclone development. The thermocline is well-defined, with the mixed layer typically extending to 50–100 metres depth before temperatures drop sharply.

The Kuroshio Current — the western Pacific's powerful northward-flowing boundary current — intrudes into the SCS through the Luzon Strait, particularly during winter, bringing warm, saline Pacific water into the northern basin. This intrusion creates a complex current structure and contributes to the SCS's salinity maintenance (32–34 ppt) despite substantial freshwater input from major river systems including the Mekong, Red River, Pearl River, and Chao Phraya.

The tidal regime of the South China Sea is unusually complex for a marginal sea of its size. In the northern portions — the Gulf of Tonkin and north of approximately 15°N — a semi-diurnal pattern (two high tides and two low tides per day of roughly equal height) dominates. In the central and southern SCS, the regime transitions to a diurnal pattern (one dominant high and one dominant low per day), particularly pronounced in the Gulf of Thailand and around the shallow Sunda Shelf. Tidal ranges are generally moderate (0.5–2.5 m) but can cause significant currents in constricted channels such as the Singapore Strait. Tropical cyclones (typhoons in the Western Pacific classification) are a critical seasonal hazard. The SCS lies within one of the world's most active typhoon development zones. Peak activity runs from August through October, with storms commonly tracking westward from the Philippine Sea through the Luzon Strait and across the northern SCS toward the Vietnamese and Chinese coasts. The Philippines, Vietnam, and Hainan are most frequently affected. Mariners must monitor NAVAREA XI warnings and tropical cyclone advisories from the Japan Meteorological Agency (JMA) and the Joint Typhoon Warning Center (JTWC) throughout the May-to-November typhoon season.

3. Marine Ecology & Biodiversity

The South China Sea sits adjacent to and partially overlaps with the Coral Triangle — the region encompassing the Philippines, Indonesia, Malaysia, Papua New Guinea, Solomon Islands, and Timor-Leste that is recognised as the global centre of marine biodiversity. The SCS itself supports extraordinary ecological richness, particularly across the reef systems of the Spratly and Paracel Islands, where coral biodiversity rivals that of the Great Barrier Reef. More than 500 species of reef-building coral have been recorded in SCS waters, alongside approximately 3,000 species of fish and a vast array of invertebrate life.

The reefs of the Paracel and Spratly Islands serve as critical habitat and feeding grounds for several globally threatened species. All six species of marine turtle found in the Indo-Pacific — including the green turtle (Chelonia mydas), hawksbill turtle (Eretmochelys imbricata), and leatherback (Dermochelys coriacea) — use SCS reefs and beaches for nesting and foraging. The waters support the world's largest known aggregations of whale sharks (Rhincodon typus) in certain seasonal feeding zones, as well as small and declining populations of dugong (Dugong dugon) in seagrass meadows of the Sunda Shelf and Gulf of Thailand. Both species are listed as Vulnerable on the IUCN Red List.

Commercially, the South China Sea has historically been one of the world's most productive fishing grounds, supporting a regional fishing industry employing an estimated 3.7 million people and providing protein for hundreds of millions across Southeast Asia. Key commercial species include yellowfin and skipjack tuna, Indian mackerel, squid, anchovy, and various demersal species. However, decades of intense fishing pressure — including widespread use of destructive methods such as blast fishing and cyanide fishing — have severely depleted stocks. The FAO and independent assessments consistently classify the South China Sea's fish stocks as heavily overexploited, with some estimates suggesting catches have declined by as much as 70–95% from historical peaks in certain areas.

Mangrove ecosystems and seagrass beds fringe the coastlines and shelf areas of the SCS, providing essential nursery habitat for juvenile fish and invertebrates as well as critical carbon sequestration function. Both ecosystem types have suffered significant losses from coastal development, aquaculture pond construction, and land reclamation. The large-scale artificial island construction projects undertaken by China in the Spratly Islands since 2014 have been identified as among the most catastrophically damaging single events in the history of tropical reef ecology, with an estimated 160 km² of coral reef destroyed or buried under dredged material — losses that will require centuries to reverse even under the most optimistic ecological recovery scenarios.

4. Maritime Trade Routes & Shipping

No single body of water on Earth carries a greater concentration of global trade by value than the South China Sea. Estimates vary between $3.4 trillion and $5 trillion worth of cargo annually, representing approximately one-third of total global maritime commerce. The sea serves as the central artery of intra-Asia trade and the essential bridge between the manufacturing centres of Northeast Asia and the energy suppliers, consumer markets, and raw material sources of Southeast Asia, South Asia, the Middle East, and Europe.

The scale of energy dependence on South China Sea transit routes is particularly striking. Approximately 40% of the world's liquefied natural gas (LNG) trade passes through the sea, carried primarily by LNG tankers from Australian export terminals (Darwin, Gladstone, Karratha) and Qatari terminals in the Persian Gulf bound for Japanese, South Korean, Taiwanese, and Chinese import terminals. Oil tanker traffic is equally dominant: an estimated 60% of South Korean and Taiwanese oil imports and 80% of Chinese crude oil imports — the latter representing one of the largest single commodity flows by volume in global trade — transit the SCS, most entering via the Malacca Strait. The VLCC (Very Large Crude Carrier) trade routes from the Persian Gulf and West Africa to Chinese and Japanese refinery terminals pass in their entirety through either the Malacca-Singapore Strait corridor or the Lombok-Makassar Strait alternative.

The Strait of Malacca is the world's most strategically critical chokepoint. At its narrowest point — the Phillips Channel south of Singapore — the navigable channel is approximately 2.8 km wide, constraining traffic to a formally managed Traffic Separation Scheme (TSS). More than 90,000 vessels per year transit the Malacca and Singapore Straits, including container ships, tankers, bulk carriers, LNG vessels, and naval vessels. The three riparian states — Indonesia, Malaysia, and Singapore — cooperate under the Cooperative Mechanism on Safety of Navigation and Environmental Protection in the Straits of Malacca and Singapore (established 2007) to fund dredging, hydrographic surveys, aids-to-navigation maintenance, and vessel traffic management. The STRAITREP Mandatory Ship Reporting System operates throughout the TSS, requiring all vessels of 300 GT and above — and all vessels carrying hazardous cargo regardless of size — to report to designated authorities upon entering each reporting sector.

For vessels where Malacca Strait draft restrictions apply — the effective maximum draft for safe transit is approximately 20.5 metres, limiting the largest VLCCs — or where congestion or security concerns motivate an alternative routing, the primary bypass routes are the Lombok Strait (between the Indonesian islands of Bali and Lombok, minimum depth approximately 250 m, suitable for all vessel sizes including fully laden VLCCs) and the Sunda Strait (between Sumatra and Java, with navigable depths of 20–25 m, suitable for most vessels but not the largest tankers). Both alternatives add two to four days to the voyage between the Indian Ocean and the South China Sea, with corresponding increases in fuel cost and voyage duration.

Container shipping in the SCS is enormous in scale. The five busiest container ports in the world — Shanghai, Singapore, Shenzhen, Ningbo-Zhoushan, and Guangzhou — are all connected to or located on the South China Sea. The major liner trade lanes run northward from Singapore through the SCS along the Vietnamese coast and through the Taiwan Strait to Japanese and South Korean ports (the Northeast Asia corridor), or eastward through the Luzon Strait into the Pacific for transpacific services to North American west coast ports. China's Belt and Road Initiative (BRI), the maritime component of which is frequently termed the “Maritime Silk Road,” has driven substantial investment in port infrastructure around the SCS, including Hambantota (Sri Lanka), Gwadar (Pakistan), and Kyaukphyu (Myanmar), with the explicit goal of creating alternative access routes and reducing strategic dependence on the Malacca chokepoint.

Piracy and armed robbery against ships remains a concern in portions of the SCS, particularly in the approaches to the Malacca Strait and in the Sulu and Celebes Seas east of Borneo. The IMB Piracy Reporting Centre (PRC), based in Kuala Lumpur, publishes quarterly reports categorising incidents by type and location. While the number of incidents in the Malacca Strait proper has declined dramatically since coordinated naval patrols were introduced in 2004 under the Malacca Strait Patrols initiative, the Sulu Sea corridor between Mindanao, Sabah, and the southern Philippines continues to experience kidnapping-for-ransom incidents by criminal and militant groups.

5. Key Ports & Harbours

The South China Sea is rimmed by some of the world's busiest and most strategically important ports. Each plays a distinct role in the regional and global maritime trade system.

Singapore (SGSIN)

Consistently ranked as the world's second-busiest port by gross tonnage and frequently the busiest by container volume, Singapore occupies a commanding position at the junction of the Malacca Strait and the South China Sea. The Port of Singapore handled approximately 37–39 million TEU annually in recent years and bunkered over 50 million tonnes of marine fuel — making it the world's largest bunkering port. The Maritime and Port Authority (MPA) of Singapore operates the Singapore Vessel Traffic Information System (VTIS), which monitors all vessel movements in the Singapore Strait and port approaches across multiple VHF working channels (Ch 14, Ch 10, Ch 9, Ch 7). Singapore offers the full range of port services including ship repair at Sembcorp Marine and Keppel yards, extensive container transhipment at Tuas Mega Port (currently under phased construction to consolidate all terminal operations), and comprehensive LSFO (low-sulphur fuel oil) and LNG bunkering under the IMO 2020 sulphur cap regime.

Hong Kong (HKHKG)

Hong Kong's Kwai Tsing container terminal complex — comprising eight terminals operated by multiple operators including Hutchison Ports and Modern Terminals — remains one of the busiest container port clusters in the world, handling approximately 14–18 million TEU annually. The port benefits from exceptional natural deep-water anchorages in the Victoria Harbour and adjacent waters, with vessel drafts to approximately 15 metres accommodated alongside at Kwai Tsing. Hong Kong also functions as a major transhipment hub for cargo to and from Pearl River Delta manufacturing centres (Guangzhou, Shenzhen, Dongguan), serviced by an extensive river barge and Pearl River Delta ferry network.

Port Klang (MYPKG)

Port Klang, located 38 km west of Kuala Lumpur on the Strait of Malacca, is Malaysia's primary gateway and ranked consistently among the world's top 15 container ports. It comprises two principal container terminals: Westport, operated by Westports Malaysia and handling the majority of container throughput (approximately 10 million TEU annually), and Northport (Northport (Malaysia) Bhd), which handles conventional cargo, ro-ro, and bulk in addition to containers. Port Klang also handles palm oil, timber, and other Malaysian agricultural and resource exports, and serves as a transhipment hub for the Malacca Strait corridor.

Manila (PHMNL)

The Port of Manila is the Philippines' principal international gateway, located on Manila Bay on the western coast of Luzon. The Manila International Container Terminal (MICT), operated by International Container Terminal Services Inc. (ICTSI), is the country's largest container facility. Manila Bay provides a well-protected anchorage for vessels awaiting berth allocation, though the approaches are affected by seasonal typhoon activity and require careful attention to weather routing during the May–November typhoon season. The Philippine Ports Authority (PPA) manages port operations and vessel traffic in Manila Bay.

Ho Chi Minh City / Cai Mep (VNCMT)

Vietnam's primary southern gateway is anchored by the Cai Mep International Terminal (CMIT) in the Cai Mep–Thi Vai port cluster, approximately 80 km southeast of Ho Chi Minh City on the Thi Vai River. Cai Mep is capable of accommodating ultra-large container vessels (ULCV) of up to 18,000 TEU capacity with drafts to 14 metres, making it the deepest-draft capable container terminal in Vietnam. The port cluster has grown rapidly as Vietnam's export-oriented manufacturing sector — particularly electronics, textiles, footwear, and furniture — has expanded, drawing direct calls from major Asia–Europe and transpacific liner services that previously required transhipment via Singapore.

Kaohsiung (TWKHH)

Kaohsiung is Taiwan's largest port and a major transhipment hub in the northeastern SCS, consistently ranked among the world's top 15 container ports (approximately 9–10 million TEU annually). Its natural deep-water harbour accommodates the largest container vessels, and its geographic position on the southwestern coast of Taiwan places it at the intersection of the Japan/Korea–Southeast Asia corridor and the transpacific trade lanes. Kaohsiung handles Taiwan's semiconductor, electronics, and petrochemical exports, and serves as a hub for transhipment to smaller regional ports.

6. Historical & Strategic Significance

The South China Sea has been a conduit of human movement, trade, and cultural exchange for at least 4,000 years. Long before European contact, Malay, Cham, Javanese, and Chinese mariners crossed the sea using the monsoon winds to trade spices, porcelain, silk, and other commodities across a network of ports from Guangzhou and Quanzhou in the north to Malacca, Srivijaya, and the Javanese kingdoms in the south. This pre-colonial maritime network — often described as the Maritime Silk Road— made Southeast Asia one of the most economically integrated regions on Earth centuries before the arrival of European traders.

Portuguese navigators arrived in the SCS in the early sixteenth century following Vasco da Gama's rounding of the Cape of Good Hope in 1498, capturing Malacca in 1511 and establishing a trading presence that gave Lisbon effective control of the sea's western gateway for over a century. The Dutch East India Company (VOC) wrested control of Malacca from the Portuguese in 1641 and dominated the SCS trade until the rise of British power in the late eighteenth and nineteenth centuries. British colonisation of Singapore (founded by Stamford Raffles in 1819), Penang, and Hong Kong (ceded in 1842 following the First Opium War) established the port infrastructure that continues to dominate the sea's trade architecture today.

The Second World War transformed the strategic calculus of the South China Sea. Japanese forces swept through the region from 1941–1942, and the sea became a theatre of some of the largest naval engagements in history. The Battle of Leyte Gulf in October 1944 — the largest naval battle by total vessels engaged in history — involved Japanese fleet movements through the South China Sea via the Sibuyan Sea and Surigao Strait in a final attempt to destroy the Allied amphibious forces at Leyte. The Allied victory sealed Japanese control of the sea and opened the route to the liberation of the Philippines.

The post-war era brought the emergence of competing territorial claims codified in the 1982 United Nations Convention on the Law of the Sea (UNCLOS). UNCLOS established the framework of 12-nautical-mile territorial seas, 24-nautical-mile contiguous zones, and 200-nautical-mile Exclusive Economic Zones (EEZs), which the SCS coastal states have invoked — often incompatibly — to assert rights over overlapping sea areas. China's nine-dash line, a U-shaped demarcation enclosing approximately 90% of the South China Sea and covering the EEZs of Vietnam, the Philippines, Malaysia, Brunei, and Indonesia, has no recognised basis in UNCLOS.

In 2016, an Arbitral Tribunal constituted under Annex VII of UNCLOS — established at the Philippines' initiative — ruled that China's nine-dash line claims had no legal basis under international law and that China had violated the Philippines' sovereign rights within its EEZ. China refused to participate in the proceedings and rejected the award. The United States has conducted regular Freedom of Navigation Operations (FONOPS) — naval transits through contested waters — to challenge what it characterises as excessive maritime claims, a practice that has been a persistent source of friction with China. ASEAN member states and China have been engaged in long-running negotiations over a Code of Conduct for the South China Sea, though agreement on a legally binding text has remained elusive. Estimated hydrocarbon reserves of up to 11 billion barrels proven oil and potentially 125 billion barrels on speculative assessments, along with substantial natural gas deposits, continue to add economic stakes to the geopolitical contest.

7. Navigation Safety & Hazards for Mariners

The South China Sea falls within NAVAREA XI (Western Pacific), coordinated by Japan. Mariners must subscribe to NAVAREA XI navigational warnings and maintain a continuous NAVTEX watch on appropriate frequencies (518 kHz for English-language broadcasts) when transiting the sea. NAVAREA XI warnings cover new dangers, uncharted wrecks, changes to lights and buoys, military exercise notifications, and environmental hazards. Additional national NAVTEX broadcasts are issued by coastal state authorities including the Philippines, Vietnam, and Singapore for local area warnings.

Tropical cyclone risk is the single most significant meteorological hazard in the SCS. The typhoon season runs from May through November, with peak activity from August through October. Storms typically develop in the Philippine Sea east of Luzon and track westward or northwestward through the Luzon Strait into the northern SCS before curving northward toward Hainan or the Vietnamese coast. Vessels should avoid approaching within 100 nautical miles of a tropical cyclone centre and must maintain a continuous weather watch on VHF Ch 16 and receive regular WX NAVTEX bulletins. Mariners should consult JMA typhoon forecasts, JTWC bulletins, and company weather routing services when scheduling transits during the typhoon season.

Shallow-water navigation across the Sunda Shelf requires close attention to charted depths, tide-corrected UKC calculations, and echo sounder monitoring. The shelf extends widely from the Malay Peninsula, Sumatra, and Borneo, with depths typically in the 40–60 metre range over vast areas. While such depths are navigable by all but the deepest-draft bulk carriers and tankers, squat at high speed in shallow water can be significant — UKC reductions due to squat must be calculated for all passages at transit speed in water depth-to-draft ratios below approximately 1.5.

The Spratly Islands area presents particular navigational dangers. Many reef features within the Spratlys are inadequately charted, with CATZOC D or U ratings common across much of the area. Military installations on Chinese-occupied artificial islands have introduced exclusion zones and operating restrictions that may not be reflected in current ENCs. Mariners should consult the latest Admiralty Sailing Directions (NP31, China Sea Pilot Volume I and II) and current NAVTEX/NAVAREA warnings before transiting the area, and should maintain a generous safety margin from all charted and reported reef features.

In the Singapore and Malacca Straits, the Traffic Separation Scheme is mandatory for vessels of 300 GT and above under COLREG Rule 10. The STRAITREP reporting system requires all qualifying vessels to report on VHF to the appropriate sector authority (VTS Singapore: Ch 14, 10, 9, and 7 depending on position; VTS Klang: Ch 12; Indonesian Maritime Safety Communication Centre: Ch 16) when entering each STRAITREP sector. ECDIS passages must incorporate the full TSS geometry and the current edition of Chart 5500 (Mariners' Routeing Guide, Singapore Strait) or its equivalent. Vessels should be aware that non-compliance with STRAITREP is a deficiency under Port State Control and may result in detention.

Armed robbery against ships remains a real risk in certain parts of the SCS. The IMB Piracy Reporting Centre (PRC) in Kuala Lumpur tracks incidents and issues quarterly reports. Mariners should register voyages with the PRC where appropriate, maintain anti-piracy watches, implement physical security measures in accordance with BMP5 (Best Management Practices for Protection against Somalia-based Piracy) principles adapted to the SCS context, and report all incidents to the IMB PRC (Tel: +60 3 2031 0014) as well as to the relevant coastal state authority. The Sulu Sea and Celebes Sea east of Borneo require heightened vigilance for kidnapping-for-ransom operations, and some flag states and maritime security organisations issue specific voyage guidance for this corridor.

Dense fishing vessel traffic is an ever-present hazard throughout the SCS, particularly during the pre-monsoon and post-monsoon transition seasons. Hundreds of thousands of small fishing vessels — many without AIS, adequate lighting, or radar reflectors — operate across the sea. A proper lookout (COLREG Rule 5) incorporating visual, radar, and AIS monitoring is essential, with radar gain set appropriately to detect small wooden or fibreglass vessels. Mariners should reduce speed in areas of known high fishing vessel density, particularly at night and in reduced visibility.

8. Environmental Issues

The South China Sea faces a confluence of environmental stresses that threaten its long-term ecological function and the food security and livelihoods of the 270 million people living in its coastal communities. These pressures are compounded by the political complexity of transboundary conservation governance across eight sovereign states with competing territorial claims.

Overfishing and illegal, unreported, and unregulated (IUU) fishing are the most pervasive and damaging long-term threats to the SCS ecosystem. The sea's fish stocks have been under sustained and increasing pressure for decades, and catches are declining despite greater fishing effort — a classic indicator of stock collapse. IUU fishing is widespread and enforcement is hampered by limited patrol capacity, contested jurisdiction, and the sheer size of the sea. China imposes a seasonal fishing moratorium in the northern SCS each year (typically May–August) to allow some stock recovery, but the effectiveness of this measure is disputed by other coastal states that do not recognise Chinese jurisdiction over the relevant waters.

Plastic pollution is a rapidly growing crisis in the SCS. Five of the ten countries contributing the highest volume of plastic to the world's oceans — China, Indonesia, the Philippines, Vietnam, and Malaysia — border the South China Sea, and a significant share of their plastic waste enters the marine environment through river systems, coastal communities, and inadequate waste management. Plastic debris concentrates in surface gyres and accumulates on reef systems and shorelines throughout the sea, with severe consequences for marine life including entanglement and ingestion by turtles, cetaceans, and seabirds.

The risk of oil spills from the extraordinary volume of tanker and LNG traffic transiting the SCS is a persistent environmental concern. A major tanker accident in the shallow and ecologically sensitive Malacca Strait or on the Sunda Shelf would be catastrophic for the coral reefs, mangrove forests, and seagrass meadows of the surrounding region. MARPOL Annexes I (oil) and V (garbage) are in force throughout the SCS, and the Malacca and Singapore Straits have been designated a Particularly Sensitive Sea Area (PSSA) and a Special Area under MARPOL Annex I — meaning that any discharge of oil or oily mixtures is prohibited. Enforcement of MARPOL compliance by coastal state authorities varies across the SCS, with Singapore maintaining among the most rigorous inspection and enforcement regimes in the region.

Climate change is imposing accelerating pressures on the SCS environment. Increasing sea surface temperatures are driving more frequent and severe coral bleaching events across the Spratly and Paracel reef systems, with back-to-back bleaching years threatening the long-term viability of reef ecosystems already weakened by construction damage and sedimentation. Sea level rise threatens the low-lying islands and atolls of the SCS — several of which are barely above high-tide level under natural conditions — as well as the densely populated delta coastlines of Vietnam (Mekong Delta), China (Pearl River Delta), and Thailand (Central Plain). Intensifying tropical cyclones, driven by warmer sea surface temperatures, amplify storm surge and wave damage risk to coastal communities and infrastructure throughout the region. The IPCC has projected continued warming and sea level rise across Southeast Asian coastal zones through 2100, with scenarios ranging from manageable to severe depending on global emissions trajectories.