Sightings data of two freshwater-dependent cetaceans, the Ganges River dolphin Platanista gangetica gangetica and Irrawaddy dolphin Orcaella brevirostris, were recorded by captains of three nature tourism vessels operating in waterways of the eastern Sundarbans mangrove forest, Bangladesh. These data were used to investigate channel-type preferences and identify cetacean hotspots according to a scoring system based on group, individual and calf encounter rates, the co-occurrence of both species, and encounter rates in neighbouring segments recorded during monsoon, post-monsoon and dry seasons. Six 5-km segments were identified for priority conservation attention from a total of 38 that were surveyed on at least three occasions during each season. An investigation of habitat preferences evaluated 5-km segments that had been surveyed on five or more occasions (n = 69) and assigned them to one of 12 categories defined by channel width, sinuosity and the number of large and small confluences. Significant differences were found between observed and expected frequencies of occurrence in the different segment categories for Ganges River dolphin groups and individuals and for Irrawaddy dolphin individuals. Both Ganges River and Irrawaddy dolphins selected channels with more than two small confluences or at least one large confluence. Understanding the preferred habitat and identifying hotspots of freshwater-dependent cetaceans in the Sundarbans is the first step of a planning process for the potential establishment of a network of protected waterways for these threatened species.

Irrawaddy dolphins Orcaella brevirostiris are found in coastal waters from the Bay of Bengal east to Palawan, Philippines and south to northern Australia. They also occur in three large tropical river systems in South-east Asia: the Mekong, Mahakam and Ayeyarwady. In March and May 1997 approximately 350 km of riverine habitat in parts of north-east Cambodia were surveyed, discussions took place with local people, and reported dry season dolphin habitat was mapped. Our objectives were to investigate the status, habitat and distribution of dolphins in north-east Cambodia and identify threats to the continued survival of dolphins in the Mekong River Basin. Nine groups of dolphins were observed in the Mekong River. A ‘best’ estimate of 40 animals were seen. Irrawaddy dolphins were generally confined to sections of the river with water levels >8–10 m during the dry season. It appears that the Mekong River dolphin population is rapidly declining. In 1997 there were probably no more than 100¨C150 dolphins left in north-east Cambodia (including southern Laos) and no more than 200 within the entire Mekong River Basin, although these numbers remain tentative. Anthropogenic mortality is high, albeit largely unintentional, and there is considerable risk that the dolphin population will become locally extinct in the Mekong River in the near future. The establishment of community-managed deep water Fish Conservation Zones with government support may represent the best opportunity for reducing dry season dolphin mortality from large-meshed gillnet entanglement. Efforts to establish protected areas for dolphins are currently underway.