The main threat to this subpopulation is undoubtedly gillnet entanglement, which accounted for approximately 66% of the 46 deaths documented between 1995 and 2005. Five of these dolphins were eaten by local people, and the skins of two were used as medicine for skin allergy. Dolphins in the Mahakam often are observed feeding in close proximity to gillnets and fishermen use the dolphins’ feeding patterns to determine the location and time to set their gillnets. Deliberate kills accounted for 9% of the documented deaths, occurring mostly in isolated areas where the animals were rarely found. Vessel strikes caused 7% of the deaths. Seven percent of the deaths were judged to represent fetal or neonatal mortality, and electro-fishing and hook-fishing each caused 2% of the deaths (Kreb et al. 2007).
From 1974 until 1988, 28 dolphins were live-captured and taken to Jaya Ancol oceanarium in
The high density of gillnets in Semayang and Melintang lakes physically obstructs dolphin movements, thereby reducing available habitat. This problem, together with high sedimentation caused by de-vegetation of the surrounding shorelines, has probably eliminated these lakes as primary areas of occupancy, as reported by Tas’an and Leatherwood (1984). Leaks of chemical wastes, including mercury and cyanide, from retention dams at gold mines in the upper reaches occurred in 1997 and resulted in a massive fish kill (D. Kreb, pers. comm.). Cleaning waste from coal mines enters the Kedang Pahu tributary during floods, and on two occasions dolphins have been observed there with changes skin pigmentation (Kreb et al. 2007). An additional threat is heavy vessel traffic, particularly large coal barges that operate in narrow tributaries and which the dolphins actively avoid (Kreb and Rahadi 2004).
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