Ombai Strait

Mudflats and mangroves at Kampungbaru, Ulmera, near Tibar Bay, East Timor, in 2018

In parts of the Ombai Strait, and especially close to the coast of Alor Island, elevated phytoplankton blooms or concentrations (and therefore availability of food for other species) have been observed, particularly during the dry south-east monsoon.[2][3] The physical structure of the strait makes it likely that such concentrations are driven by fronts and vertical flows caused by some combination of tidal currents or topographic effects.[3]

A study published in 2002 noted persistent flow-induced upwelling and frontal features in the strait, together with subjective evidence of extremely strong tides. The study concluded that the upwelling and frontal features coincided with large phytoplankton blooms and significantly affected the concentration and distribution of chlorophyll in the Savu Sea. According to the study report, it is likely that the mean flow associated with the ITF also plays a role in causing the blooms.[3]

Along the northern littoral of East Timor, including the strait, the shallow coastal waters are dominated by a seagrass bed about 2,200 ha (5,400 acres) in total area.[10]^{: 2-29–2-30} Seagrass beds protect coral reefs from sedimentation and are feeding grounds for dugong (Dugong dugon), an endangered species.[11]^: 82 Seven genera of seagrass are known to exist off East Timor's coastline as a whole.[10]^: 2-29

East Timor's coastal waters have also traditionally hosted mangrove forests. Together with seagrasses and coral reefs, they are the primary breeding grounds for many species of fish and shellfish. However, the country's mangrove cover has very substantially declined since 1940, and in the Ombai Strait is now confined to the coastline and hinterland between Tibar and the Bay of Dili.[11]^{: xvi, xvii, 3–5, 81, 82}[12]^: 3

A survey of Tibar Bay carried out in 2016 found three mangrove species, namely Sonneratia alba, which was the dominant species, Avicennia marina and Rhizophora stylosa.[13]^: 2, 8 The report of another survey, carried out in 2017, stated that the dominant species in the area was Sonneratia albia, and that there were some clusters of Rhizophora apiculata, Ceriops tagal, and Lumnitzera.[12]^: 14

The 2016 survey report estimated the total area of mangrove in the bay to be 14 ha (35 acres);[13]^: 2, 8 the 2017 survey report's estimate was 22 ha (54 acres). According to the latter report, tree density was relatively low, at around 100–400 trees per hectare, and the mangrove cluster was heavily degraded, due to direct and indirect human activities. The former activities included fish pond and salt pan development, mangrove cutting, and cattle grazing. The latter activities were sea level rise, and siltation due to erosion in the hills on the landward side of the mangroves.[12]^{: 7, 13–14, 22–23}

The Ombai-Wetar corridor is heavily used by whales, dolphins, whale sharks (Rhincodon typus) and manta rays. The two straits combine to form a major migratory route for them between the Pacific and Indian Oceans.[14] The Ombai Strait has been identified as a critical habitat for sperm whales (Physeter macrocephalus), blue whales (Balaenoptera musculus), giant oceanic manta rays (Manta birostris) and sea turtles (Cheloniidae), amongst other endangered, threatened and protected species.[15] East Timor, on the south side of the two straits, is a recognized ‘global hotspot’ for whales and dolphins.[16]

The report of a 2013 survey of Tibar Bay, on Ombai Strait's southern shore not far west of Dili, East Timor, noted that a local dive tourism operator had made frequent sightings of dugong along the coast immediately east of the bay, and that there were also anecdotal reports of saltwater crocodiles in the bay.[14]

A weedy scorpionfish (Rhinopias frondosa) at Tasitolu in 2007

A short distance further east, at Tasitolu, dugongs are often observed, although as of 2019 detailed information regarding their habitat location, size, and characteristics in the area was not available.[10]^: 2-29 The other sea life at Tasitolu includes cephalopods, crustaceans, frogfish, harlequin ghost pipefish, small octopuses, rhinopias, sea horses, and soft coral crabs.[17]

Ombai Strait is also part of the Coral Triangle, which is recognised as a global centre of marine biodiversity.[18] The coastal zones of East Timor have important areas of coral reef ecosystems, such as the ones at Tasitolu and in the Bay of Dili. At Tasitolu, the reefs west of Dili Rock are live, healthy and diverse, but those to the east of the rock have lower coverage and more reef rubble. At the Port of Dili inside the Bay of Dili, there are at least four different species of coral.[10]^{: 2-29–2-30}

The Dili area, on the southern edge of the strait, is a recognised nesting ground for sea turtles, of three different species: hawksbill sea turtle (Eretmochelys imbricata; Critically Endangered (CR), according to the International Union for Conservation of Nature (IUCN) Red List), olive ridley sea turtle (Lepidochelys olivacea; Vulnerable (VU)), and green sea turtle (Chelonia mydas; Endangered (EN)). On both the Tasi Tolu Beach and the Beto Tasi Beach near Dili's international airport, there is regular hatching of sea turtles.[10]^: 2-29 However, in most parts of East Timor sea turtles are overharvested for their eggs, skin, meat and carapace for handicraft making.[11]^: xviii

The northern edge of the strait abuts two Important Bird Areas (IBAs), Gunung Muna (on Alor Island, Indonesia),[19] and Atauro Island (East Timor).[20] The southern edge of the strait is also the northern shore of four East Timorese IBAs. From west to east, they are Be Malae (just to the west of the Loes River), Maubara (a short distance east of the village of Maubara), Tasitolu (just to the west of Dili), and Areia Branca no Dolok Oan (on the eastern side of the Bay of Dili).[21]

A container ship, Selatan Damai, anchored off Dili, East Timor, in 2018, with Atauro in the background

Under the United Nations Convention on the Law of the Sea, a commercial vessel or aircraft has freedom of navigation or overflight solely for the purpose of continuous and expeditious transit of a strait between one part of the high seas or exclusive economic zone and another.[22] That transit passage principle applies to the Ombai Strait and Wetar Strait, as they combine to link two oceans.[23][24]

The Ombai-Wetar route is longer in distance than its Strait of Malacca-Singapore Strait counterpart. The former route is therefore not really a preferred alternative path for west to east commercial traffic.[23] However, it is sometimes used, and is considered the safest route, for the largest oil tankers transiting between the Persian Gulf and Japan, and is also used by vessels transiting between Australia and the Java Sea or East Asia.[24]

The two straits are always critical for East Timor, both in relation to its own international trade and as routes for internal transport.[24] In the Ombai Strait, internal East Timorese ferry services operate regularly between Dili and both Atauro and Oecusse.[25] If there were any disruption in the flow of commercial shipping on the Malacca-Singapore route, the Ombai-Wetar route would also have a crucial role to play in global trade, especially for the Asia-Pacific region.[24]

USS Blue Ridge in the Bay of Dili in 2000, with Dili in the background

The ships and aircraft of all nations, including warships, auxiliaries, and military aircraft, enjoy the right of unimpeded transit passage through straits such as the Ombai-Wetar route and their approaches. Submarines are free to transit the route submerged, as that is their normal mode of operation.[26]

The channels of the Ombai-Wetar route are extremely deep.[23] East Timor has refused advances by China seeking off-shore oil rights and permission to set up radar arrays ostensibly to counter illegal fishing.[24]

The two straits therefore provide a route for undetected access by nuclear-powered submarines between the two oceans they connect.[23][24] In particular, they are an alternative deep water route for submerged U.S. submarines moving between Guam and stations in the Indian Ocean via the Molucca Passage, Indonesia, which separates Sulawesi from Halmahera.[8] Although the Ombai-Wetar route is a less direct link between Guam and the Indian Ocean than the Lombok-Makassar route, it is useful because it has limited traffic and very little surveillance, with the consequence that passing submarines are less likely to be tracked.[8]

By the mid-1970s, the Ombai-Wetar route had been identified as one of five international straits essential for passage by U.S. ballistic missile submarines, the others being Gibraltar, Malacca-Singapore, Lombok and Sunda.[24][27] In 1977, a paper published by the International Institute for Strategic Studies, based in London, ranked the Ombai-Wetar route as the second-most-important strait in the world to U.S. defense interests, after the Strait of Gibraltar.[28] The two straits are now also considered by China to be strategically significant.[24]