Gulf of Thailand

Geography

The Gulf of Thailand is bordered by Thailand, Cambodia, Malaysia, and Vietnam. It occupies a seabed area of 304,000 km² from 6° N to 13°30' N latitude and 99°E to 104° E longitude. The gulf is defined by Thailand since around 70% of the coastline is Thai. The northern tip of the gulf is the Bay of Bangkok at the mouth of the Chao Phraya River. The southern boundary of the gulf is defined by a line from Cape Bai Bung in southern Vietnam (just south of the mouth of the Mekong River) to the town of Tumpat and Pengkalan Chepa on the Malaysian coast. This is the largest gulf in Southeast Asia.

The gulf is relatively shallow: its mean depth is 58 metres (190 ft) and the maximum depth is only 85 metres (279 ft). This makes water exchange slow, and the strong water inflow from the rivers reduces the level of salinity in the gulf (3.05–3.25 percent) and enriches the sediments. Only at greater depths does water with a higher salinity (3.4 percent) flow into the gulf from the South 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 distributary Tha Chin River, the Mae Klong, and Bang Pakong rivers at the Bay of Bangkok, and to a lesser degree the Tapi River flowing into Bandon Bay in the southwest of the gulf. Other notable tributaries of the gulf are the Pattani River, Phetchaburi River, and the Pran Buri River.

Delimitation

The International Hydrographic Organization (IHO), the highest international authority on the delimitation of seas, considers the Gulf of Thailand to be a sea. In its world-renowned publication, "Limits of Oceans and Seas" (3rd edition, 1953), the IHO assigns it the identification number 47 and defines it as follows:

"47.—Gulf of Thailand .
On the south. A line running from the Western extreme of Cambodia or Camau Point (8°36'N) to the Northern extreme of the point on the East side of the estuary of the Kelantan River (6°14′N 102°15′E / 6.233°N 102.250°E)".

Cape Cà Mau is in Vietnam, the Kelantan River is in Malaysia.

Seabed morphology

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 the basin. 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 between sediment dewatering and the erosional activity of the present-day bottom currents. The sediment dewatering and fluid seepage result in the formation of numerous small pits and pockmarks. The long-term erosion imposed by currents of stable orientation modifies pockmarks 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.

Coastline

Around 70% of the gulf's coastline is Thai, 16.5% is Cambodian, 13% is Vietnamese, and 0.67% is Malaysian.

Bays

There are a couple of bays along the gulf's coasts of Thailand, Cambodia and Vietnam.

Islands

The larger islands in the gulf include:

• Ko Chang
• Ko Mak
• Ko Kut
• Ko Samui
• Ko Pha Ngan
• Ko Tao
• Ko Phaluai
• Ko Sichang
• Ko Lan
• Ko Phai
• Ko Khram
• Ko Samae San
• Ko Samet
• Ko Rang
• Ko Khangkhao
• Ko Man Nok
• Ko Wai
• Ko Phi
• Ko Kham
• Ko Sai
• Ko Kra
• Ko Losin
• Phú Quốc
• Thổ Chu Islands
• Hà Tiên Islands
• Bà Lụa Islands
• Nam Du Islands
• Koh Kong
• Koh Rong
• Koh Rong Sanloem
• Koh Sdach
• Koh Puos
• Koh Dek Koul
• Koh Russei
• Koh Ta Kiev
• Koh Preab
• Koh Thmei
• Koh Seh
• Koh Ach Seh
• Koh Tonsay
• Koh Tang
• Koh Pring
• Koh Poulo Wai

History

Paleolake in Sundaland

During the Last Glacial Maximum (26,000-20,000 years ago) the 3-4 km thick ice sheets in the Northern hemisphere lowered the global sea level by about 120 meters. The climate was -6 Celsius colder in Thailand. The sea level was 125 meters lower so there was no seawater in the Gulf of Thailand which has a max depth of 85 meters. Sundaland was a subcontinent that connected the Mainland with the former islands of Western Indonesia in Maritime Southeast Asia. The Gulf of Thailand was a large paleolake with a major paleo-river system in Sundaland. The principal water source regions were the Eastern Malay Peninsula and the Chao Phraya Basin. It had a watershed of circa 1,020,000 km² (390,000 mi²). It was filled with fresh water from the rivers and rain. The Chao Phraya was the biggest tributary that supplied the paleolake with water. The modern river systems connected to the paleo-river system of the Gulf of Thailand are: Johor, Rompin, Endau, Kuantan, Kelantan, Tapi, Mae Klong, Chao Phraya, Bang Pakong. There was another major paleo-river that carried the water from the southeastern shore of the paleolake into the South China Sea. Sundaland existed until circa 15000 years ago. It disappeared when the sea levels began to rise.

Environment

Coral reefs

There are 75,590 rai 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. In 2016, coral bleaching was detected at Ko Thalu and Ko Lueam in Prachuap Khiri Khan Province for the first time. 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.

Water quality

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. Some gulf waters off Chachoengsao Province, Samut Sakhon Province, Samut Prakan Province, Bangkok, Rayong Province, Chonburi Province, Phetchaburi Province, Prachuap Khiri Khan Province, and Surat Thani Province were judged to have coastal waters in "poor" or "very poor" condition. Songkhla was the only province on the gulf with coastal water rated "good" quality.

Fisheries

Of Thailand's total marine catch, 41 percent is caught in the Gulf of Thailand and 19 percent in the Andaman Sea. Forty percent is caught in waters outside Thailand's EEZ.

Coastal erosion

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.

Plastic pollution

In February 2017, a 10 kilometer-long patch of plastic refuse was found floating off Chumphon Province. Thailand is among the world's worst plastic polluters. More than half of "land-based plastic waste leakage" into the sea originates from just five countries: China, Indonesia, the Philippines, Thailand, and Vietnam.

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. Filter feeding invertebrates tested off the coast of Chonburi Province showed high levels of microplastics, leading the authors to warn that, "Health risks are possible when people consume these contaminated marine organisms, particularly shellfish."

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 million tonnes per year.

Oil spills

In 2013, a pipeline leak resulted in an oil slick that went on to coat a beach on the nearby Ka Samet island.

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 of Rayong province 20 km away.

Tourism

The Gulf of Thailand's many coral reefs have made it attractive to divers. The tropical warmth of the water attracts many tourists. Some of the most important tourist destinations in the gulf are the islands of Ko Samui and Ko Pha Ngan in Surat Thani Province, Pattaya in Chonburi Province, Cha-am in Phetchaburi Province, Hua Hin in Prachuap Khiri Khan Province, and Ko Samet in Rayong Province. Technical diving is particularly popular around Ko Samae San due to it having the deepest dive sites in the Gulf of Thailand. The deepest technical dive site in the gulf is the Samaesan Hole. In media, the Samaesan Hole is commonly reported as a dangerous dive site due to a number of hazards including unexploded ordinance, strong unpredictable currents and its location in a commercial shipping lane. Red Bull recognised it as one of the most extreme dive sites in the world which has attracted experienced divers to the area.

In recent years, the bay has become known for its whale watching activities, targeting the endemic, critically endangered populations of cetaceans (Eden's whales, newly described Omura's whales, Chinese white dolphins, and Irrawaddy dolphins showing unique feeding behaviors), and dugongs. It was first classified by Müller in 1776 as Trichechus dugon. Five species of the sea turtles have been found in the Gulf of Thailand and on Andaman Sea coastlines: olive ridley turtles, green turtles, hawksbill turtles, loggerhead turtles, and leatherback turtles.

Territorial disputes

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 of Ko Kra and Ko Losin. A long-standing dispute between Cambodia and Vietnam in the Gulf of Thailand concerns mainly the island of Phú Quốc or Koh Tral in Khmer, which is off the Cambodian coast. Cambodia also claims 48,000 square kilometres (19,000 mi²) of shelf area.