Boundaries

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For Southeast Asia, delineations were determined using a bottom-up approach that employed both published and unpublished field data and expert assessment (Abell et al. 2008; Christensen 1992).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).