The Irrawaddy Dolphin in Mahakan River

Genetically the Irrawaddy Dolphin is closely related to the Orca. The species name brevirostris comes from the Latin meaning short-beaked. They have a large melon and a blunt, rounded Lead, and the beak is indistinct. The dorsal fin, located about two-thirds posterior along the back, is short, blunt and triangular. The flipper are long and broad. It is lightly coloured all over, but slightly more white on the underside than the back. Adult weight exceed 130 kg (287 lb) and length is 2.3 m (8 ft) at full maturity. These dolphins are thought to reach sexual maturity at 7 to 9 years. Its gestation period is 14 months, giving birth to a single calf every 2 to 3 years. Length is about 1 m (3 ft) at birth. Birth weight is about 10 kg (22 lb). Weaning is after two years. Lifespan is about 30 years.

The Irrawaddy Dolphin in Indonesia almost known as "Pesut" and founded in Mahakam river systems, including Semayang, Melintang, and Jempang lakes, in East Kalimantan. A survey reported approximately 100 dolphins in Semayang Lake and the Pela River and adjacent Mahakam River.

In Semayang Lake, Irrawaddy Dolphins perform daily migration from the lake to the Mahakam River, returning to the lake in the evening. They may be found at distance up to 1.300 km upstream in major rivers, and indication of movements of considerable extent (Reyes, 1991).

Kreb (in IWC, 2000) described her research on this species in the Mahakam River, and its associated lakes and in nearby coastal waters of East Kalimantan, Indonesia. The middle section of the Mahakam River and tributaries between Mum Kainan (180 km from mouth) and Melak (350 km from the mouth) was identified as primary dolphin habitat. The distribution changes seasonally an is influenced by water levels and perhaps variation in prey availability. Dolphins move into tributaries during high water and back into the main river when water levels recede. Most sightings were made at confluences and river bends.

Threat's

Incidental catch: Irrawaddy Dolphins are accidentally caught in fishing nets in Mahakam River.
Habitat degradation: Irrawaddy Dolphins from Semayang Lake were formerly observed in the Mahakam River up to Tenggarong and Samarinda. Since the 1980's, probably due to the intense activity related to the timber industry, they are no longer observed near these towns but only above Muara Kaman (Reyes, 1991). Habitat degradation may limit the distribution and abundance of Irrawaddy Dolphins, particularly in fresh water. Dams (Baird and Maunsouphom, 1997), gold mining using mercury abstraction techniques, increased sedimentation as a result of deforestation and other changes in river catchments, overfishing, harmful fishing techniques (poison and electrofishing), vessel traffic and noise pollution are all potential threats to this species. Coastal development with concomitant eutrophication is also cause for concern (IWC, 2000).
Overfishing: Reduction of fish populations in Indonesian rivers by illegal fishing methods is a serious threat.

Status
The Irrawaddy dolphins are classified by the IUCN as a critically endangered species.