Brunei Bay (Malaysia)

Year of compilation: 2003

Site description
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).

Key biodiversity
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).

Pressure/threats to key biodiversity
Limbang Estuary: The Limbang estuary faces a complex issue of conflicting land use. Due to its non-protected status, various forms of disturbances and threats currently plague the area. Non-sustainable logging. Clear-felling of mangoves for cordwood, charcoal and firewood is practised. Possibly when the timber from the Lawas mangroves is depleted, pressure on those of Limbang is bound to increase.Illegal logging through the clearance of up to 50 ha for coconut plantation and aquaculture schemes along Sungai Limbang.Erosion of the riverbanks by speedboats and large barges.Poaching of Proboscis Monkeys and Silvered Leaf-Monkeys.Illegal clearance for small settlements caused by illegal immigration.Planned clearance of area on the east bank of Sungai Limbang for establishment of prawn ponds by end of 1986. By the first quarter of that year, more than 100 ha has been totally cleared.Trusan-Sundar: Landuse within the Trusan-Sundar mangrove area includes;Large scale production forest (mangrove) for producing cord wood for export to Taiwan.Local fish smoking industry and small scale fishing for domestic consumption.Small scale hunting of Sambar Deer (Cervus unicolor) and Wild Pig (Sus scrofa).Fishing.Cultivation and settlements.The continuation of large scale clear-felling of mangrove areas without proper management and sustainability is one of the most prominent threat to the site. Illegal logging have also been reported. The occurrence of these two activities have impacted the soils here as they are highly susceptible to the acid sulphate condition after clearance. The damming of Sungai Trusan would likely to affect the silt load and continued accretion of intertidal zone.Menumbok Estuary: illegale cutting of "bakau" is wide-spread. Hunting and illegal fishing takes place. Recent proposals to build a bridge linking the Klias peninsula with the island of Labuan will have devastating effects on the area, and will also affect the entire Brunei bay.

Protected areas
Limbang Estuary: The Limbang estuary is currently not protected as most of it is untitled state land. However, its adjacent areas in Brunei Darussalam, Pulau Siarau, Pulau Berambang and Sungai Brunei are conservation areas.Howes and NPWO (1986) proposed the conservation of the mangroves and intertidal mudflats to link with adjacent areas in Brunei and to establish an International Coastal Resource Conservation Area within Brunei Bay. Forestry management proposals and restrictions on illegal logging were also recommended. Salter and MacKenzie (1981) also proposed that the Sungai Limbang mangroves be designated as a Wildlife Sanctuary for the protection of the Proboscis Monkey (Nasalis larvatus). Protection of the Limbang mangroves was also echoed by Bennett and Gombek (1986) based on the following reasons;Limbang is Sarawak's only major contribution to Brunei Bay fishery.Limbang links two protected sections of Brunei's mangroves. Destruction of Limbang will result in faunal degradation and reduced protection of Brunei areas.Brunei Bay's Proboscis Monkey (N. larvatus) population is the only significant one between Sandakan and Rajang Delta.Brunei Bay has a population of the Silvered Leaf-Monkey, an endangered primate in Sarawak.Trusan-Sundar: The majority of the Trusan-Sundar mangroves are contained within the Kenalian, Bumbun and Terentang Forest Reserves, where logging has been extremely intensive. Areas up to 1 ha or more have been virtually clear-felled. Only the area between Sungai Sepatai and Sungai Awat-Awat is relatively untouched with a more or less intact canopy. Howes and NPWO (1986) proposed the creation of a Wildlife Sanctuary and management of the mangrove production forest. Under the proposal, the site forms a link with areas in Brunei Darussalam as an International Coastal Resource Conservation Area. Salter and MacKenzie (1981) proposed the undisturbed mangroves between Kuala Bangau and Kuala Trusan to be made a Wildlife Sanctuary for the protection of Proboscis Monkeys (Nasalis larvatus).Menumbok: Menumbok Forest Reserve cover 5,710ha, under the category of Class I FR. The inter-tidal mudflats are not included within the forest reserve.

Habitat and land use
Limbang Estuary: The mangrove vegetation is extensive in the estuary, up to 10 km in width and extending inland along the main rivers for up to 20 km. The mangrove stands are mostly mature and untouched in most areas and those along the coast are accreting at the main rivermouths but eroding along some stretches in between. Degraded swamp forest and lowland mixed dipterocarp forest are found further inland. The mangrove consists of predominantly Nypa fruticans or Rhizophora apiculata. Others include Avicennia alba, Sonneratia alba, S. acida, Bruguiera cylindrica, Rhizophora apiculata and R. mucronata. Xylocarpus granatum, Kandellia reedii and Acanthus illicifolius occurs in isolated patches. Heritiera littoralis are found in the mangrove-lowland swamp forest transition zone (Howes and NPWO, 1986).Trusan-Sundar: Mature stands of Avicennia alba occurs along the coastal strip with a maximum height of 15 m. At most rivermouths, clusters of 20 m tall Sonneratia alba occurs. The inland area is dominated by Rhizophora apiculata, in most places formed a pure Rhizophora forest with very few other species present. Cleared areas of mangrove were colonised by Acrostichum speciosum and Bruguiera gymnorrhiza. Nipa fruticans are present along the upper reaches of the river system. In the coastal areas with sandy beaches, Casuarina equisetifolia occur (Howes and NPWO, 1986).Menumbok Estuary: large areas of mono-specific stands of Rhizophora apiculata along the lower reaches, a result of regular harvesting for poles and other uses. Further upstream, the mangroves are more diverse, with characteristic salinity-influenced communities (Bruguiera, Xylocarpus and Barringtonia, eventually becoming Nypa-lined. Between the Menumbok and Lawas estuaries is an area of sea grass (10 species recorded, James Bali, pers.comm.). This is the only significant area of sea grass in Sarawak, and supports a small and critically endangered population of Dugong Dugong dugon. Concentrations of sea turtles are regularly seen here (James Bali, pers.comm.).

Recommended citation
BirdLife International (2022) Important Bird Areas factsheet: Brunei Bay (Malaysia). Downloaded from on 15/08/2022.