Irrawaddy dolphins were recently split into two species. The 'true' Irrawaddy, O. brevirostris is known to occur in five freshwater systems and the nearshore marine waters of Southeast Asia extending as far west as the east coast of India, while the newly described snubfin dolphin, O. heinsohni occurs in the coastal waters of northern Australia and southern Papua New Guinea. Over the next couple of weeks, we'll hear from researchers working on both species so technically this should be called the "Genus of the month" but it makes sense to do it this way, so stick with me!
But before we get started with the "notes from the field", here's a quick overview of the Irrawaddy dolphin - with the extracts taken/borrowed mostly from the "Irrawaddy Action Plan - Status and Conservation of Freshwater populations of Irrawaddy dolphins" by Brian Smith et al, produced as a result of a workshop that WDCS co-funded in 2005 - no point in reinventing the wheel and Brian tells us how it is so perfectly!
(And if anyone would like a copy of the report then please just ask!)
Freshwater populations occur in three river systems - the Ayeyarwady (formerly Irrawaddy, where the dolphin gets its name from of course) in Myanmar (formerly Burma), the Mahakam in Indonesian Borneo, and the Mekong in Laos, Cambodia and Vietnam; and two partially isolated brackish or freshwater lakes – Chilika Lagoon in India and Songkhla Lake in Thailand. There is (or was) also a sub-population in the Malampaya Sound in the Philippines however little is known about their current status. However they, and the Ayeyarwady, Mahakam, Mekong and Songkhla populations are classified as “critically endangered” on the IUCN Red List. According to IUCN criteria, there's not enough information available to assess the Chilika Lake population, however, it is considered likely to be endangered due to it's low population size and high mortality rate. (NH says - More dolphins are dying than are being born into the population and if things continue at this rate perhaps by the time sufficient information is available for a listing, it'll be joining the other freshwater populations as being designated "critically endangered").
All five freshwater populations are believed to be demographically isolated from members of the species occurring in marine waters. The downstream range extents of the riverine populations are about 180, 500 and 1000 km from the sea in the Mahakam, Mekong and Ayeyarwady rivers, respectively, and, with a single exception of a carcass recovered near the mouth of Chilika Lake (which may have drifted through its opening during low tide), no strandings or sightings of Irrawaddy dolphins have been documented along adjacent coastlines within 80 km of both Chilika and Songkhla lakes.
The freshwater populations also represent a fairly unique adaptation within the Order of Cetacea as members of a species that is found in both nearshore marine and riverine/marine appended lake environments. They share this attribute with only two other cetaceans: the finless porpoise, Neophocaena phocaenoides in the Yangtze River of China and the tucuxi, Sotalia fluviatilis in the Amazon and Orinoco rivers of South America.
Less is known about the marine populations of Irrawaddy dolphins however a significantly larger population of at least a few thousand animals occurs in the inner and outer Sundarbans Delta of Bangladesh. Interestingly, Irrawaddy dolphins do not occur in fluvial waters of the Ganges-Brahmaputra-Meghna river system upstream of the Sundarbans Delta, possibly due to inter-specific competition from Ganges River dolphins Platanista gangetica, which are true freshwater specialists.
Other marine populations are known from the coastal waters of Thailand, Sumatra and Malaysian Borneo and we'll be hearing more about them in the coming weeks too!
May is going to be Irrawaddy overload - with hopefully some exciting news towards the end of the month ... but for the moment i'll leave you to consider these rather special and relatively small dolphins!