The Mahakam lakes and surrounding wetlands act as water storage as well as a trap of sediment contained in the water flowing into the lakes which are now known to become shallower, presumably as a result of an imbalance between sediment input and slow subsidence. Fishing is the primary source of livelihood in the Mahakam lakes area, most of the people around the lakes are fishermen. The middle Mahakam lake area is an area of intensive fishing activity with a productivity of 25,000 to 35,000 metric tons per year since 1970. Delta The Mahakam delta is a mixed fluvial-tidal dominated delta. The delta covers about , consisting of mangrove areas near the shore, Nypa swamps in the central areas, and lowland forest near the apex, corresponding to the first bifurcation. Fishery development in this area has converted a vast area of mangrove into shrimp ponds (tambak). However, recent mangrove restoration efforts have taken place in the delta by replanting mangroves in abandoned shrimp ponds and encouraging silvofishery. Many areas in the Mahakam delta are already naturally recolonized by mangrove vegetation contributing to ecosystem restoration. Mangroves also function as sedimentation enhancing strategies by capturing sediment causing accretion. The delta has three main distributaries system directed Northeast, Southeast and South. The area between distributaries consists of a series of tidal channels generally unconnected to the main distributaries. The distributary channels are narrow and rectilinear with the depth ranging from and distributary channel bifurcations appear every . This lower Mahakam area is the second most productive hydrocarbon basin of Indonesia which contains around 3 billion barrels of oil and 30 Tcf of gas reserves. Field geological investigations in this area was started in 1888 and in 1897 exploration drilling discovered oil at shallow depth of on the Louise structure. Production started in 1898 followed by expansion of exploration to the entire Mahakam. Ecology Mahakam and its floodplain is an ecologically important region. A total of 147 indigenous freshwater fish species had been identified from the Mahakam. The Mahakam hosts the freshwater dolphin Irrawaddy dolphin (Orcaella brevirostris; called Pesut by local people) a critically endangered species, which is included on CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora) Appendix I. The Mahakam river basin is also an important breeding and resting place for 298 bird species, among them 70 protected and five endemic species: Borneo dusky mannikin (Lonchura fuscans),