Dugong

Australia is home to the largest population, stretching from Shark Bay in Western Australia to Moreton Bay in Queensland.[16] The population of Shark Bay is thought to be stable with over 10,000 dugongs. Smaller populations exist up the coast, including one in Ashmore reef. Large numbers of dugongs live to the north of the Northern Territory, with a population of over 20,000 in the gulf of Carpentaria alone. A population of over 25,000 exists in the Torres Strait such as off Thursday Island, although there is significant migration between the strait and the waters of New Guinea.[11] The Great Barrier Reef provides important feeding areas for the species;[30] this reef area houses a stable population of around 10,000, although the population concentration has shifted over time. Large bays facing north on the Queensland coast provide significant habitats for dugong, with the southernmost of these being Hervey Bay and Moreton Bay.[16] Dugongs had been occasional visitors along the Gold Coast[31] where a re-establishment of a local population through range expansions has started recently.[32]

The Persian Gulf has the second-largest dugong population in the world, inhabiting most of the southern coast,[11] and the current population is believed to range from 5,800 to 7,300.[33] In the course of a study being carried out in 1986 and 1999 on the Persian Gulf, the largest reported group sighting was made of more than 600 individuals to the west of Qatar.[34] However, recent studies revealed severe declines both in population size and distributions among the region.[35] A 2017 study, for instance, found a nearly 25% drop in population since 1950.[33] Reasons for this drastic population loss include illegal poaching, oil spills and net entanglement.[34]

In the late 1960s, herds of up to 500 dugongs were observed off the coast of East Africa and nearby islands. Current populations in this area are extremely small, numbering 50 and below, and it is thought likely they will become extinct. The eastern side of the Red Sea is home to large populations numbering in the hundreds, and similar populations are thought to exist on the western side. In the 1980s, it was estimated there could be as many as 4,000 dugongs in the Red Sea. Dugong populations in Madagascar are poorly studied, but due to widespread exploitation it is thought they may have severely declined, with few surviving individuals.[36][37] In Mozambique, most of the remaining local populations are very small and the largest (about 120 individuals) occurs at Bazaruto Island,[38] but they have become rare in historical habitats such as in Maputo Bay and on Inhaca Island.[39][40] In Tanzania, observations have recently been increased around the Mafia Island Marine Park where a hunt was intended by fishermen but failed in 2009.[41] In the Seychelles, dugongs had been regarded as extinct in 18th century[42] until a small number was discovered around the Aldabra Atoll. This population may belong to a different group than that distributed among the inner isles.[43][44] Dugongs once thrived among the Chagos Archipelago and Sea Cow Island was named after the species, although the species no longer occurs in the region.[45][46]

A highly isolated breeding population exists in the Marine National Park, Gulf of Kutch,[47] the only remaining population in western India. It is 1,500 kilometres (932 mi) from the population in the Persian Gulf, and 1,700 kilometres (1,056 mi) from the nearest population in India. Former populations in this area, centered on the Maldives and the Laccadive Islands, are presumed to be extinct. A population exists in the Gulf of Mannar Marine National Park and the Palk Strait between India and Sri Lanka, but it is seriously depleted. Recoveries of seagrass beds along former ranges of dugongs, such as the Chilika Lake have been confirmed in recent years, raising hopes for re-colorizations of the species.[48] The population around the Andaman and Nicobar Islands are known only from a few records, and although the population was large during British rule, it is now believed to be small and scattered.[11] Once distributed throughout the coastal belt in Sri Lanka, the dugong numbers have declined in last two decades.

Dugong with attached remora off Lamen Island, Vanuatu

A small population exists today along the southern coast of China, where efforts are being made to protect it, including the establishment of a seagrass sanctuary for dugong and other endangered marine fauna ranging in Guangxi.[49][50] Despite these efforts, numbers continue to decrease, and in 2007 it was reported that no more dugong could be found on the west coast of the island of Hainan.[51] Historically, dugongs were also present in the southern parts of the Yellow Sea.[52]

In Vietnam, dugongs have been restricted mostly to the provinces of Kiên Giang and Bà Rịa–Vũng Tàu, including Phu Quoc Island and Con Dao Island,[53] which hosted large populations in the past.[54] Con Dao is now the only site in Vietnam where dugong are regularly seen,[55] protected within the Côn Đảo National Park.[56] Nonetheless, dangerously low levels of awarenesses for conservation of marine organisms in Vietnam and Cambodia may result in increased intentional or unintentional catches, and illegal trade is a potential danger for local dugongs.[54] On Phu Quoc, the first 'Dugong Festival' was held in 2014, aiming to raise awareness of these issues.[57]

In Thailand, the present distribution of dugongs is restricted to six provinces along the Andaman Sea,[58] and very few dugongs are present in the Gulf of Thailand.[59] The Gulf of Thailand was historically home to large number of the animals, but none have been sighted in the west of the gulf in recent years,[11] and the remaining population in the east is thought to be very small and possibly declining.[60] Dugongs are believed to exist in the Straits of Johor in very small numbers. The waters around Borneo support a small population, with more scattered throughout the Malay archipelago.[11]

All the islands of the Philippines once provided habitats for sizeable herds of dugongs. They were common until the 1970s, when their numbers declined sharply due to accidental drownings in fishing gear and habitat destruction of seagrass meadows. Today, only isolated populations survive, most notably in the waters off the Calamian Islands in Palawan, Isabela in Luzon, Guimaras, and Mindanao. The dugong became the first marine animal protected by Philippine law, with harsh penalties for harming them.[61][62][63] Recently, local marine trash problem in the archipelago remained unabated and became the biggest threat to the already dwindling population of Dugongs in the country. Litters of plastic waste (single-use sachets, plastic bottles, fast food to-go containers etc.) and other non-biodegradable materials abound in the coastal areas. As these materials may be mistaken as food by dugongs, these may lead to death due to plastic ingestion. Overpopulation and lack of education of all coastal fisherfolk in the Philippines regarding marine trash are clearly harming the coastal environment not only in Palawan but also across the islands of the Philippines.[64]

Populations also exist around the Solomon Islands archipelago and New Caledonia, stretching to an easternmost population in Vanuatu. A highly isolated population lives around the islands of Palau.[11]

A single dugong lives at Cocos (Keeling) Islands although the animal is thought to be a vagrant.[65][66]

Today, possibly the smallest and northernmost population of dugongs exists around the Ryukyu islands, and a population formerly existed off Taiwan.[11] An endangered population of 50 or fewer dugongs, possibly as few as three individuals, survives[67] around Okinawa.[68] New sightings of a cow-calf pair have been reported in 2017, indicating a possible breeding had occurred in these waters.[69] A single individual was recorded at Amami Ōshima, at the northernmost edge of the dugong's historic range, more than 40 years after the last previous recorded sighting.[70] A vagrant strayed into port near Ushibuka, Kumamoto, and died due to poor health.[71] Historically, the Yaeyama Islands held a large concentration of dugongs, with more than 300 individuals. On Aragusuku Island, large quantities of skulls are preserved at an utaki that outsiders are strictly forbidden to enter.[72][73] Dugong populations in these areas were reduced by historical hunts as payments to the Ryukyu Kingdom, before being wiped out because of large-scale illegal hunting and fishing using destructive methods such as dynamite fishing after the Second World War.

Populations around Taiwan appear to be almost extinct, although remnant individuals may visit areas with rich seagrass beds such as Dongsha Atoll.[74] Some of the last reported sightings were made in Kenting National Park in the 1950s and 60s.[75] There had been occasional records of vagrants at the Northern Mariana Islands prior to 1985.[76] It is unknown how much mixing there was between these populations historically. Some theorize that populations existed independently, for example, that the Okinawan population were isolated members derived from the migration of a Philippine subspecies.[77] Others postulate that the populations formed part of a super-population where migration between Ryukyu, Taiwan, and the Philippines was common.[78]

It has been confirmed that dugongs once inhabited the water of the Mediterranean[79][80] possibly until after the rise of civilizations along the inland sea. This population possibly shared ancestry with the Red Sea population, and the Mediterranean population had never been large due to geographical factors and climate changes.[81] The Mediterranean is the region where the Dugongidae originated in the mid-late Eocene, along with Caribbean Sea.[82][83]

Typical dugong feeding area in Moreton Bay

Dugongs, along with other sirenians, are referred to as "sea cows" because their diet consists mainly of sea-grass. When eating they ingest the whole plant, including the roots,[16] although when this is impossible they will feed on just the leaves.[11] A wide variety of seagrass has been found in dugong stomach contents, and evidence exists they will eat algae when seagrass is scarce.[17] Although almost completely herbivorous,[18] they will occasionally eat invertebrates such as jellyfish, sea squirts, and shellfish.[16] Dugongs in Moreton Bay, Australia, are omnivorous, feeding on invertebrates such as polychaetes[85] or marine algae when the supply of their choice grasses decreases. In other southern areas of both western and eastern Australia, there is evidence that dugongs actively seek out large invertebrates. This does not apply to dugongs in tropical areas, in which fecal evidence indicates that invertebrates are not eaten.[11]

Most dugongs do not feed on lush areas, but where the seagrass is more sparse. Additional factors such as protein concentration and regenerative ability also affect the value of a seagrass bed.[16] The chemical structure and composition of the seagrass is important, and the grass species most often eaten are low in fiber, high in nitrogen, and easily digestible.[11] In the Great Barrier Reef, dugongs feed on low-fiber high-nitrogen seagrass such as Halophila and Halodule,[16] so as to maximize nutrient intake instead of bulk eating. Seagrasses of a lower seral are preferred, where the area has not fully vegetated. Only certain seagrass meadows are suitable for dugong consumption, due to the dugong's highly specialized diet. There is evidence that dugongs actively alter seagrass species compositions at local levels. Dugongs may search out deeper seagrass. Feeding trails have been observed as deep as 33 metres (108 ft), and dugongs have been seen feeding as deep as 37 metres (121 ft).[11] Dugongs are relatively slow-moving, swimming at around 10 kilometres per hour (6.2 mph).[82] When moving along the seabed to feed they walk on their pectoral fins.[21]

Dugong feeding may favor the subsequent growth low-fibre, high-nitrogen seagrasses such as Halophilia and Halodule.[86] Species such as Zosteria capricorni are more dominant in established seagrass beds,[87] but grow slowly, while Halophilia and Halodule grow quickly in the open space left by dugong feeding.[86] This behavior is known as cultivation grazing, and favors the rapidly growing, higher nutrient seagrasses that dugongs prefer.[86] Dugongs may also prefer to feed on younger, less fibrous strands of seagrasses,[87] and cycles of cultivation feeding at different seagrass meadows may provide them with a greater number of younger plants.

Due to their poor eyesight, dugongs often use smell to locate edible plants. They also have a strong tactile sense, and feel their surroundings with their long sensitive bristles.[18] They will dig up an entire plant and then shake it to remove the sand before eating it. They have been known to collect a pile of plants in one area before eating them.[21] The flexible and muscular upper lip is used to dig out the plants. This leaves furrows in the sand in their path.[18]

Dugong mother and offspring from East Timor

A dugong reaches sexual maturity between the ages of eight and eighteen, older than in most other mammals.[88] The way that females know how a male has reached sexual maturity is by the eruption of tusks in the male since tusks erupt in males when testosterone levels reach a high enough level.[89] The age when a female first gives birth is disputed, with some studies placing the age between ten and seventeen years, while others place it as early as six years.[11] There is evidence that male dugongs lose fertility at older ages.[17] Despite the longevity of the dugong, which may live for 50 years or more, females give birth only a few times during their life, and invest considerable parental care in their young.[88] The time between births is unclear, with estimates ranging from 2.4 to 7 years.[11]

Mating behaviour varies between populations located in different areas.[18] In some populations, males will establish a territory which females in estrus will visit.[11] In these areas a male will try to impress the females while defending the area from other males, a practice known as lekking.[18] In other areas many males will attempt to mate with the same female,[11] sometimes inflicting injuries to the female or each other.[16] During this the female will have copulated with multiple males, who will have fought to mount her from below. This greatly increases the chances of conception.[18]

Females give birth after a 13–15 month gestation, usually to just one calf.[88] Birth occurs in very shallow water, with occasions known where the mothers were almost on the shore.[17] As soon as the young is born the mother pushes it to the surface to take a breath.[20] Newborns are already 1.2 metres (4 ft) long and weigh around 30 kilograms (66 lb).[16] Once born, they stay close to their mothers, possibly to make swimming easier.[17] The calf nurses for 14–18 months, although it begins to eat seagrasses soon after birth.[11] A calf will only leave its mother once it has matured.[18] When need to be nursed, calves would suck their flippers in a 'thumb sucking' fashion as observed in calves under human care.

Despite being legally protected in many countries, the main causes of population decline remain anthropogenic and include hunting, habitat degradation, and fishing-related fatalities.[9] Entanglement in fishing nets has caused many deaths, although there are no precise statistics. Most issues with industrial fishing occur in deeper waters where dugong populations are low, with local fishing being the main risk in shallower waters.[11] As dugongs cannot stay underwater for a very long period, they are highly prone to deaths due to entanglement.[22] The use of shark nets has historically caused large numbers of deaths, and they have been eliminated in most areas and replaced with baited hooks.[16] Hunting has historically been a problem too, although in most areas they are no longer hunted, with the exception of certain indigenous communities. In areas such as northern Australia, hunting remains the greatest impact on the dugong population.[11]

Vessel strikes have proved a problem for manatees, but the relevance of this to dugongs is unknown.[11] Increasing boat traffic has increased danger,[16] especially in shallow waters. Ecotourism has increased in some countries, although the effects remain undocumented. It has been seen to cause issues in areas such as Hainan due to environmental degradation.[11] Modern farming practices and increased land clearing have also had an impact, and much of the coastline of dugong habitats are undergoing industrialization, with increasing human populations.[16] Dugongs accumulate heavy metal ions in their tissues throughout their lives, more so than other marine mammals. The effects are unknown. While international cooperation to form a conservative unit has been undertaken,[93] socio-political needs are an impediment to dugong conservation in many developing countries. The shallow waters are often used as a source of food and income, problems exacerbated by aid used to improve fishing. In many countries, the legislation does not exist to protect dugongs, and if it does it is not enforced.[11]

Oil spills are a danger to dugongs in some areas, as is land reclamation. In Okinawa the small dugong population is threatened by United States military activity. Plans exist to build a military base close to the Henoko reef, and military activity also adds the threats of noise pollution, chemical pollution, soil erosion, and exposure to depleted uranium.[11] The military base plans have been fought in US courts by some Okinawans, whose concerns include the impact on the local environment and dugong habitats.[68][94] It was later revealed that the government of Japan was hiding evidence of the negative effects of ship lanes and human activities on dugongs observed during surveys carried out off Henoko reef.[95] One of the three individuals has not been observed since June 2015, corresponding to the start of the excavation operations.[96]

If dugongs do not get enough to eat they may calve later and produce fewer young.[16] Food shortages can be caused by many factors, such as a loss of habitat, death and decline in quality of seagrass, and a disturbance of feeding caused by human activity. Sewage, detergents, heavy metal, hypersaline water, herbicides, and other waste products all negatively affect seagrass meadows. Human activity such as mining, trawling, dredging, land reclamation, and boat propeller scarring also cause an increase in sedimentation which smothers seagrass and prevents light from reaching it. This is the most significant negative factor affecting seagrass.[11]

Halophila ovalis—one of the dugong's preferred species of seagrass—declines rapidly due to lack of light, dying completely after 30 days. Extreme weather such as cyclones and floods can destroy hundreds of square kilometers of seagrass meadows, as well as washing dugongs ashore. The recovery of seagrass meadows and the spread of seagrass into new areas, or areas where it has been destroyed, can take over a decade. Most measures for protection involve restricting activities such as trawling in areas containing seagrass meadows, with little to no action on pollutants originating from land. In some areas, water salinity is increased due to wastewater, and it is unknown how much salinity seagrass can withstand.[11]

Dugong habitat in the Oura Bay area of Henoko, Okinawa, Japan, is currently under threat from land reclamation conducted by Japanese Government in order to build a US Marine base in the area.[97] In August 2014, preliminary drilling surveys were conducted around the seagrass beds there.[98] The construction is expected to seriously damage the dugong population's habitat, possibly leading to local extinction.[99]

The Australian state of Queensland has sixteen dugong protection parks, and some preservation zones have been established where even Aboriginal Peoples are not allowed to hunt.[16] Capturing animals for research has caused only one or two deaths;[11] dugongs are expensive to keep in captivity due to the long time mothers and calves spend together, and the inability to grow the seagrass that dugongs eat in an aquarium.[16] Only one orphaned calf has ever been successfully kept in captivity.[18]

Worldwide, only three dugongs are held in captivity. A female from the Philippines lives at Toba Aquarium in Toba, Mie, Japan.[11] A male also lived there until he died on 10 February 2011.[100] The second resides in Sea World Indonesia,[101] after having been rescued from a fisherman's net and treated.[102] The last one, a male, is kept at Sydney Aquarium, where he has resided since he was a juvenile.[103] Sydney Aquarium had a second dugong for many years, until she died in 2018.[104]

Gracie, a captive dugong at Underwater World, Singapore, was reported to have died in 2014 at the age of 19, from complications arising from an acute digestive disorder.[105]