Eastern Borneo




J. Hales



Major Habitat Type

Tropical and subtropical coastal rivers

Drainages flowing into

Celebes Sea, Makassar Strait

Main rivers to other water bodies

The largest river in the ecoregion is the Mahakam, which is the second largest river in Borneo after the Kapuas. Tributaries include the Semayang, Belayan, Kedang Kepala, and Kedang Rantau. Other rivers in the ecoregion include the Kajan and Tuwau. The largest lakes in the ecoregion are lakes Melintang, Jempang and Semayang, which form part of the Mahakam system.



This ecoregion lies in eastern Borneo from the Mahakam basin in the south to the Kajan basin in the north. These boundaries lie within East Kalimantan (Indonesia).

Freshwater habitats

The largest basin in the ecoregion is the Mahakan, which has a drainage area of roughly 78,000 km2. Rising in the Müller Mountain Range, the Mahakan’s upper course is characterized by steep gradients and rapids. The middle course, which extends between Long Bagun and Long Iram, flows in a fast, strait course through long valleys. The lower Mahakan meanders through a broad valley and vast floodplain with many oxbow lakes and three large buffer lakes Melintang, Jempang and Semayang. The Mahakan empties into the Makassar Strait via the Mahakan Delta at Samarinda (Kottelat 1995).

Terrestrial habitats

Terrestrial ecoregions include Borneo montane rain forests [IM0103], Borneo lowland rain forests [IM0102], Southwest Borneo freshwater swamp forests [IM0153], Borneo peat swamp forests [IM0104], Sundaland heath forests [IM0161], and Sunda Shelf mangroves [IM1405].

Description of endemic fishes

The ecoregion contains around 30 endemics, a third of which are in the family Cyprinidae. Some of those endemic to the Mahakan basin include Barbonymus mahakkamensis, Crossocheilus nigriloba, Osteochilus bellus, Osteochilus kelabau, Osteochilus repang, and Thynnichthys vaillanti (Kottelat 1995). Pseudohomaloptera is the only endemic genus.

Justification for delineation

For Southeast Asia, delineations were determined using a bottom-up approach that employed both published and unpublished field data and expert assessment (Abell et al. 2008; Christensen 1992). Traditional species lists for the Mahakam basin, which has long been considered to be inhabited by a fauna significantly different from those of the rest of Borneo. A critical reevaluation has shown this to be largely (but not completely) a collecting artifact. The Mahakam is noteworthy (when compared to other large Borneo rivers) by the number of freshwater species with marine ancestors or affinities. It also has a number of buffer lakes (M. Kottelat pers. comm. 2006).


  • Abell, Robin,M.L. Thieme,C. Revenga,M. Bryer,M. Kottelat,N. Bogutskaya,B. Coad,N. Mandrak,S.C. Balderas,W. Bussing,M.L.J. Stiassny,P. Skelton,G.R. Allen,P. Unmack,A. Naseka,R. Ng,N. Sindorf,J. Robertson,E. Armijo,J.V. Higgins,T.J. Heibel,E. Wikramanayake, (2008). "Freshwater Ecoregions of the World: A New Map of Biogeographic Units for Freshwater Biodiversity Conservation" BioScience 58 (5) pp. 403-414.
  • Kottelat, M. (1995). "The fishes of the Mahakam River, East Borneo: an example of the limitations of biogeographic analyses and the need for extensive fish surveys in Indonesia" Tropical Biodiversity 2 (3) pp. 401-426.
  • World Wildlife Fund (WWF) (2001) \Terrestrial Ecoregions of the World\ "<"http://www.worldwildlife.org/wildworld/profiles/terrestrial_nt.html">"
  • Christensen, M.S. (1992). "Investigations on the ecology and fish fauna of the Mahakam River in East Kalimantan (Borneo), Indonesia" Int. Rev. Gesamt. Hydrobiol 77 (4) pp. 593-608.
  • IUCN (2012) \IUCN Red List of Threatened Species. Version 2012.2\ "<"www.iucnredlist.org">" (21 May 2013)