Administratively, Brunei Bay comes under two countries (Malaysia and Brunei). The Malaysian parts of its shoreline are also divided between two States: Sarawak and Sabah. The situation is further complicated by the fact that Brunei constitutes two separate territories (Brunei and Temburong districts) which are separated, along this bay, by Sarawak (Limbang district). The eastern curve of the bay is formed by the estuaries of the Klias, Padas and Menumbok rivers, flowing out of the Klias peninsula in Beaufort district, Sabah (MYSB09).There are three separate areas within this bay which are significance for birds: Limbang estuary, Trusan-Sundar and Menumbok estuary. These are treated separately below.(I) Physical CharacteristicsLimbang Estuary: The Limbang estuary is orientated in a north-south direction, about 20 km in length, bordered to the north and south by the two states of Brunei Darussalam. The area is protected from the north-east monsoon by several Bruneian islands; Pulau Muara Besar (composed of successive sandridges with scrubby vegetation) and Pulau Baru-Baru and Pulau Berbunot (both predominantly rocky with a narrow mangrove strip). The estuary is the emergence point of three main rivers; the Sungai Manunggul, Sungai Limbang and Sungai Pandaruan, each having a high silt load. Intertidal mudflats, consisting of fine-grained pure muds, cover the rivermouth and are replaced by sand-mud ridges to the seaward. The mudflats can extend to over 1 km offshore at low tide in areas like Tanjung Api-Api and Tanjung Tubu-Tubu. The situation is reversed at the rivermouths of smaller rivers such as Sungai Sentabok and Sungai Sapukang (Howes and NPWO, 1986).Trusan-Sundar: The Trusan-Sundar mangroves are orientated in a west-east direction, about 20 km in length, and stretches from Kuala Trusan to Tanjung Perepat. Localised erosion is evident east of the mangrove headland i.e. north-west of Kampung Awat-Awat. Accreting mangrove fringe accompanied by extensive intertidal mudflats occur prominently at the mouth of Sungai Trusan (the area's main river), Kuala Bangau, Kuala Kenaljam and Tanjung Perepat. At Kuala Trusan, the flats extend up to 3 km offshore at low tide and consist of a complicated series of deep water channels with areas of exposed sediment forming 'islands'. The deposits at the rivermouth consist of pure, fine-grained muds and gradually replaced by course sand particles as it extend seaward. Some of the most seaward deposts consist primarily of pure course grained sands. The estuary is sheltered by Pulau Labuan and peninsular to the north-east (Howes and NPWO, 1986).Menumbok Estuary: a wide mangrove estuary with exposed inter-tidal mudflats, formed by the Menumbok and Padas rivers. The part of Brunei bay is sheltered by the exposed, northeast-southwest aligned, Klias ridge. The estuary is continuous with the smaller Sg. Klias estuary further south, also forming a mangrove delta. The Menumbok Forest Reserve is contiguous with the Padas Damit FR.(II) Climatic Conditions Average annual rainfall is 3,500 mm. Small variation in monthly rainfall. Rainfall during monsoon season (September-January) is about 400 mm/month (DWNP, 1987).
Brunei Bay is one of the most important wintering sites for migratory waterbirds along Borneo's northern coastline. Its large expanses of mudflats and numerous small islands and exposed sand/mud banks, located within a large, deep bay, provide a sheltered, food-rich environment with numerous high-tide roosts. The Sundar and Lawas areas increase habitat diversity with stretches of sandy beaches and rocky shores.Limbang Estuary: The large expanse mudflats and sandflats of the Limbang estuary is an important wintering and staging area for migratory shorebirds and egrets, attracted to its rich feeding ground. Two globally threatened species have been recorded, the resident Lesser Adjutant Leptoptilos javanicus and migratory Chinese Egret Egretta eulophotes. Other Near Threatened waterbirds recorded in the area include Oriental Darter Anhinga melanogaster, Grey-headed Fish Eagle Ichthyophaga ichthyaetus, Asian Dowitcher Limnodromus semipalmatus, Malaysian Plover Charadrius peronii and Far Eastern Curlew Numenius madagascariensis (DWNP, 1987; Sebastian, 1995; Smythies, 1999).Trusan-Sundar: The Trusan-Sundar mudflats and sandflats are important wintering and staging sites for globally threatened waterbirds especially shorebirds and egrets especially at Kuala Trusan and Kampung Awat-Tanjung Perepat stretch. The Malaysian Plover Charadrius peronii resides along some beach stretches in the estuary (Howes and NPWO, 1986). Four pairs in breeding plumage were seen in November 1995 on the sandy beaches at the mouth of Sg. Kabab (Sebastian, 1995). Grey-headed Fish Eagle Ichthyophaga ichthyaetus was recorded from Sg. Trusan-Sundar and Sg. Kabab (Sebastian, 1995).Three Great-billed Heron Ardea sumatrana were observed in Nov 1995 along Sg. Trusan-Sundar and Sg. Kabab (Sebastian, 1995), the only record of this species for Sarawak. Smythies (1999) attributes a record from Brunei Bay by Mohd. Jaya in 1987.Menumbok Estuary: One Chinese Egret seen on mudflats and five Storm's Stork Ciconia stormi seen upstream of Menumbok (lower part of Padas Damit FR) in December 1997 (Sebastian, 1998).
Non-bird biodiversity: (I) Globally threatened mammals (IUCN, 2002): ENDANGERED: Proboscis Monkey Nasalis larvatus; VULNERABLE: Dugong Dugong dugon; NEAR THREATENED: Silvered Leaf Monkey Trachypithecus cristatus; Long-tailed Macaque Macaca fascicularis; DATA DEFICIENT: Irrawaddy Dolphin Orcaella brevirostris(II) Globally threatened reptiles (IUCN, 2002): CRITICALLY ENDANGERED: Painted Terrapin Callagur borneoensis along Sg. Siang Siang (Sebastian, 1995). A small population of Estuarine Crocodile Crocodylus porosus survives in parts of the bay.(III)Plants: Nepenthes reinwardtiana recorded along Sg. Lawas, a very rare species in Sarawak (Howes and NPWO, 1986).
BirdLife International (2020) Important Bird Areas factsheet: Brunei Bay (Malaysia). Downloaded from http://www.birdlife.org on 27/02/2020.