Ganges river dolphins , Indus valley dolphins : rarest of their kind

GANGES DOLPHINS :


India’s national aquatic animal, the endangered Gangetic river dolphin (Platanista gangetica gangetica), is rarely visible in the waterways of the Indian Sundarbans. A study sheds light on the rise in salinity in the water and reduced freshwater flow for the mammal’s disappearing act in the iconic estuarine habitat. Records dating back to 1879 reveal the freshwater-loving mammals swam along the entire length of the Ganga and Brahmaputra rivers, and all their tributaries from the delta at the Bay of Bengal till the Himalayan foothills. Even in the month of May, when the Ganga was very low, dolphins were seen as far up the Yamuna in Delhi.

At the gathering of the Ganga, Brahmaputra and Meghna rivers that form the Sundarbans mangrove region and its numerous channels, these dolphins now struggle to survive, restricted only to certain pockets. A survey of a nearly 100-km stretch of the Sundarbans delta in India adjoining Bangladesh, has confirmed the presence of the dolphin populations only in the westernmost segment, in the lower reaches of the river Hooghly, where the salinity is lower than that of natural seawater.


Gangetic river dolphins are disappearing from the Indian Sundarbans due to a combination of factors such as increasing salinity, sedimentation and reduced freshwater flow. Photo by Kartik Chandramouli/Mongabay-India.

The mammals stayed away from the central Sundarbans, found the survey, where siltation in the waterways has disrupted freshwater flow leading to high salinity levels.


The easternmost part of the Indian Sundarbans having freshwater connectivity with river Padma of Bangladesh is moderately saline but the salinity level increases downstream and the southwest part of Bangladesh Sundarbans is hyper-saline. Published in the Journal of Threatened Taxa, the survey indicates a “possible decline in the range of Platanista gangetica in the Indian Sundarbans” attributing the extirpation to a triple whammy of elevated sedimentation, reduced freshwater discharge and swelling salinity.

On the other hand, there is continued occurrence of Irrawaddy dolphin (Orcaella brevirostris) in the Indian part of the estuary.

India’s “Dolphin Man”, ecologist Ravindra Sinha agreed with the findings and observed that earlier, in the entire Sundarbans including different water channels and tributaries/distributaries one could spot the Ganges dolphins.

“Gangetic dolphins are obligatory freshwater animals and they never enter the sea. They are found in brackish water zones such those in the Sundarbans estuary. But freshwater flow has declined over the decades and sea water has ingressed, increasing the salinity. They are rarely visible now, whereas once they were plenty,” Sinha, Vice Chancellor Nalanda Open University, told Mongabay-India.

A surfacing Gangetic river dolphin. Photo by Ravindra Sinha.

Inhabiting one of the most densely populated regions of the world, Gangetic river dolphin is one of the only four surviving river dolphins globally, as Yangtze River dolphin is virtually extinct, noted Sinha, one of the authors of the Conservation Action Plan for the Gangetic Dolphin 2010-2020.


The study highlighted a drop in numbers and the species occupying a lesser area than its historic range in the Sundarbans, illustrated by the fact that dolphins were consistently encountered in all seasons mostly in hyposaline (low salinity) waters and in moderate salinity which occurs close to the estuarine mouth of Ganges.

The research team included Sangita Mitra (presently with National Biodiversity Authority, Chennai) and Mahua Roy Chowdhury, a marine biologist from the University of Calcutta, West Bengal.


INDUS DOLPHINS :


The Indus River dolphin, locally known as bhulan is a freshwater dwelling cetacean species found in the Indus River, Pakistan. It is a flagship species and is an indicator of the biological health of aquatic and terrestrial environment adjoining the Indus River. The current distribution range of the Indus River dolphin is about 1000 km stretch of the Indus River which includes main Indus channel and active channels connected to it between Jinnah and Kotri barrages.

The existing population of Indus River dolphin is about 1300 dolphins found between Chashma and Kotri barrages in the Indus River.

Indus River dolphins are roughly the same color as the river, gray or brown, though they sometimes are lighter on their undersides. Their “beaks” are distinctively swollen at the tip and very long, reaching 20% of the length of their bodies, with large, visible teeth. In contrast to their “beaks”, their dorsal fins are rather small and reduced compared to other river dolphins. Large flippers and flukes, combined with long and remarkably flexible necks, probably help the dolphins navigate effectively. Platanista minor has external ears located below their eyes, but their eyes are very small and probably can only see shadowy, unclear images. Though Platanista minor and Platanista gangetica barely differ physically except for slight differences in tail lengths, the two species are distinguishable by their ranges. Platanista minor lives only in the Indus River system, while Platanista gangetica only inhabits the Ganges River system. Females are larger than males.




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