TheGulf of Thailand, historically known as theGulf of Siam, is a shallow inlet[1][2] adjacent to the southwesternSouth China Sea, bounded between the southwestern shores ofMainland Southeast Asia and the northern half of theMalay Peninsula. It is around 800 km (500 mi) in length and up to 560 km (350 mi) in width, and has a surface area of 320,000 km2 (120,000 sq mi).[3][4] The gulf is surrounded on the north, west and southwest by the coastlines of Thailand; on the northeast byCambodia; and theMekong Delta region ofVietnam. It opens to theSouth China Sea in the southeast.
The "Gulf of Thailand", historically known as the "Gulf of Siam", derives its name from the historical kingdom ofSiam, the former name of modern-day Thailand. The term "Gulf of Siam" was widely used in Westerncartography and geographical references for centuries until the country renamed itself to "Thailand" in 1939. This reflects thecolonial-era practice of naming regions based on the prevalent local political entities at the time.
"Siam" itself is anexonym, believed to have origins in theSanskrit word "Syama," which means "dark" or "brown," referring to the skin tone of the region's inhabitants. The term "Siam" was used internationally until 1939, when the country officially changed its name to Thailand. Following this renaming, "Gulf of Thailand" gradually became the more commonly used term, especially in official contexts, although "Gulf of Siam" is still occasionally used.
The Gulf of Thailand is bordered byCambodia,Thailand,Malaysia, andVietnam.[7][8] It occupies a seabed area of 304,000 km2 from 6° N to 13°30' N latitude and 99°E to 104° E longitude.[9]: 250 The northern tip of the gulf is theBay of Bangkok at the mouth of theChao Phraya River. The southern boundary of the gulf is defined by a line fromCape Bai Bung in southern Vietnam (just south of the mouth of theMekong River) to the town ofTumpat andPengkalan Chepa on the Malaysian coast.
The gulf is relatively shallow: its mean depth is 58 metres (190 ft) and the maximum depth is only 85 metres (279 ft).[9]: 250 This makes water exchange slow, and the strong water inflow from the rivers reduces the level ofsalinity in the gulf (3.05–3.25 percent) and enriches thesediments. Only at greater depths does water with a higher salinity (3.4 percent) flow into the gulf from theSouth China Sea. It fills the central depression below a depth of 50 metres (160 ft). The main rivers which empty into the gulf are the Chao Phraya, including its distributaryTha Chin River, theMae Klong, andBang Pakong rivers at theBay of Bangkok, and to a lesser degree theTapi River flowing intoBandon Bay in the southwest of the gulf.
The seabed morphology in the central depression of the gulf is characterised by the presence of elongated mounds and ridges arranged parallel to the axis of thebasin. This morphology, widespread within the gulf in water depths exceeding 50 m, covers an area of tens of thousands of square kilometres.
It reflects an interaction betweensediment dewatering and theerosional activity of the present-daybottom currents.[10] Thesediment dewatering and fluid seepage result in the formation of numerous small pits andpockmarks. The long-termerosion imposed bycurrents of stable orientation modifiespockmarks into long runnels and depressions, and ultimately leads to the formation of the large fields of elongated mounds and ridges, as well as the residual outliers of un-eroded mud and clay sheets.[10]
The map shows the probable extent of land and water at the time of theLast Glacial Maximum, 20,000 years ago and when thesea level was likely more than 110 meters lower
There are 75,590rai of coral reef in the gulf, of which five percent are considered to be in fertile condition. In 2010 severe coral bleaching occurred at most reef sites in the country. Bleaching of reefs in the Andaman Sea was more severe and extensive than that in the Gulf of Thailand.[12] In 2016,coral bleaching was detected at Ko Thalu and Ko Lueam inPrachuap Khiri Khan Province for the first time.[13] Scientists have determined that bleaching starts when seawater temperature rises beyond 30 °C for more than three weeks. Given the prolonged period of temperatures up to 32 °C at Ko Thalu in Prachuap Khiri Khan, five to ten percent of corals in the area are already bleached.[14]
Coastal water monitoring results in 2015 from 202 sampling locations, collected twice annually, indicate that no Thai coastal waters were found to be in excellent condition. Sixteen percent of coastal water was of good quality, 72 percent was of fair quality, 9 percent was of poor quality and 3 percent was of very poor quality. The quality of all coastal waters exhibited similar percentages — most were of fair quality — except for the Inner Gulf of Thailand, where the coastal water was poor to very poor. In comparison to coastal water quality as measured in 2014, water quality has deteriorated.[15]: 52 Some gulf waters offChachoengsao Province,Samut Sakhon Province,Samut Prakan Province, Bangkok,Rayong Province,Chonburi Province,Phetchaburi Province,Prachuap Khiri Khan Province, andSurat Thani Province were judged to have coastal waters in "poor" or "very poor" condition.[15]: 54 Songkhla was the only province on the gulf with coastal water rated "good" quality.[15]: 56
Of Thailand's total marine catch, 41 percent is caught in the Gulf of Thailand and 19 percent in theAndaman Sea. Forty percent is caught in waters outside Thailand'sEEZ.[12]
Thailand has 1,660 kilometres of coastline bordering the gulf. "Severe erosion", more than five metres of coastline loss per year, afflicts 670 kilometres of that total. At least some of the erosion is attributable to the clearing of mangrove forests to make way for shrimp farms.[16]
In February 2017, a 10 kilometer-long patch of plastic refuse was found floating offChumphon Province.[17] Thailand is among the world's worst plastic polluters. More than half of "land-basedplastic waste leakage" into the sea originates from just five countries: China, Indonesia, the Philippines, Thailand, and Vietnam.[18]
The Thai Marine and Coastal Resources Department has noted that at least 300 sea animals on average—60 per cent of which are whales and dolphins—die from eating plastic fishing gear and trash each year.[17]Filter feeding invertebrates tested off the coast ofChonburi Province showed high levels ofmicroplastics, leading the authors to warn that, "Health risks are possible when people consume these contaminated marine organisms, particularly shellfish."[19]
Thailand's Pollution Control Department (PCD) estimates that plastic waste in the country is increasing at an annual rate of 12 percent, or around two milliontonnes per year.[17]
In 2013, a pipeline leak resulted in an oil slick that went on to coat a beach on the nearby Ka Samet island.[20]
In late January 2022, a leak in the pipeline operated by the Star Petroleum Refining Public Company Ltd caused a spill of 20 to 50 tonnes across 47 km2 of water, with some oil reaching the coast ofRayong province 20 km away.[20][21]
In recent years, the bay has become known for itswhale watching activities, targeting the endemic, critically endangered populations ofcetaceans (Eden's whales, newly describedOmura's whales,[22]Chinese white dolphins, andIrrawaddy dolphins showing unique feeding behaviors), anddugongs.[23][24][25] It was first classified byMüller in 1776 asTrichechus dugon.[26] Five species of the sea turtles have been found in the Gulf of Thailand and the Andaman sea coast, including olive ridley turtles, green turtles, hawksbill turtles, loggerhead turtles, and leatherback turtles.[27]
The area between Malaysia, Thailand, Cambodia, and Vietnam is subject to several territorial disputes. Malaysia and Thailand have chosen to jointly develop the disputed areas, which include the islands ofKo Kra andKo Losin.[28] A long-standing dispute between Cambodia and Vietnam in the Gulf of Thailand concerns mainly the island ofPhú Quốc or Koh Tral in Khmer, which is off the Cambodian coast.[29] Cambodia also claims 48,000 square kilometres (19,000 mi2) ofshelf area.[30][31]
^ab"Limits of Oceans and Seas"(PDF) (3 ed.). International Hydrographic Organization. 1953. p. 23. Archived fromthe original(PDF) on 8 October 2011. Retrieved28 December 2020.
^Dzurek, Daniel J. (2005). "Maritime Agreements and Oil Exploration in the Gulf of Thailand". In Ganster, Paul; Lorey, David E. (eds.).Borders and Border Politics in a Globalizing World. Lanham: S.R. Books. pp. 301–313.ISBN0-8420-5104-X. [Reprint ofDzurek, Daniel J. (1998). "Maritime Agreements and Oil Exploration in the Gulf of Thailand". In Blake, Gerald; Pratt, Martin; Schofield, Clife; Brown, Janet Allison (eds.).Boundaries and Energy: Problems and Prospects. London: Kluwer Law. pp. 117–135.ISBN90-411-0690-1.]
^Schofield, Clive (2008). "Maritime Claims, Conflicts and Cooperation in the Gulf of Thailand".Ocean Yearbook Online.22:75–116.doi:10.1163/221160008x00064.
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