Gangetic river dolphins in the Indian Sundarbans struggle with swelling salinity

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.

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.

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.

Through sightings on boat and surveys from land, coupled with interviews of local boatmen and fishers, from 2013 to 2016, the authors underscored that glacial melting and sea level rise due to climate-induced changes affect the salinity level.

But water diversion and commissioning of large barrages upstream also has had a “great impact” on the salinity profile of the rivers downstream in central Sundarbans, which lose their freshwater supply for much of the year.

“Significant increase in salinity levels were documented in the river Ganges in India after the commissioning of the Farakka Barrage,” the study said, referring to the project in West Bengal that began operations in 1975.

Declining flow of Ganga biggest threat to Gangetic dolphin population

Looking at the pan-India scenario, Sinha explained the biggest threat to Gangetic dolphins is the declining flow in the river Ganga as erection of dams and barrages and water-intensive agriculture in the basin contribute to the base flow petering out and fragmenting their habitats.

This was evidenced by the fact that dolphins in the main channel of the Ganga were split into two subpopulations in 1975 when the Farakka Barrage was commissioned.

In Uttar Pradesh, the Lower Ganga Barrage at Narora (1966) and the Middle Ganga Barrage at Bijnor (1984) further fragmented the Ganga main stem population into four subpopulations. Dolphins have now been extirpated above the Middle Ganga Barrage at Bijnor.

Today they occur in three subpopulations bounded by the Bijnor, Narora and Farakka barrages, said Sinha.

“Other than the barrages on the main stem of the Ganga, their counterparts on different tributaries like Triveni barrage on river Gandak along the Indo-Nepal border at Valmiki Nagar, Bihar, Kosi barrage on river Kosi, Girijapuri Barrage on river Ghaghara in Bahraich district in Uttar Pradesh and others are responsible for the fragmentation,” he said.

Saving habitat to save the species

In India, which forms 80 percent of the territory of the existing Gangetic dolphin population (spreading into neighbours Nepal and Bangladesh), the state of Bihar harbours 50 percent of the mammals in the country.

“Through actual sighting in India, Bangladesh and Nepal, we can say there are at least 3750 dolphins in total with a majority in the Ganga river system. Over the last 200 years, their habitat has shrunk by 20 percent. Given the fact that they are slow breeders, for now we can say the numbers are stable,” Sinha said.

A 2018 National Mission for Clean Ganga (NMCG) report says the dolphin population, which was about 10,000 in late 19th century, reduced to 3,526 in 2014.

Even in the country’s only protected area for the Gangetic dolphin at Vikramshila Gangetic Dolphin Sanctuary (VGDS) in Bihar, experts have documented a decline in numbers from 207 in 2015 to 154 in 2017.

“It was notified in 1990 but still it lacks an effective management plan,” Sinha said.

They are not starving in Indian waters, Sinha believes, but entanglement in fishing nets and hunting for meat and oil need to be checked further. It is not too late to save them, stressed Sinha.

“But we need to save its habitat to save the subspecies. The extinction of the baiji or Chinese river dolphin was a wake up call for us. We need communities to participate,” said Sinha. Baiji, a freshwater dolphin species found in the Yangtze River in China, is reported to have been driven to extinction by human activities  “In India, with increasing pollution load and river traffic, we need to get our acts in order.

The study authors note: “Motorised boats in good number (10–25 per hour) travel at an average speed of 6–10km per hour across the dolphin movement route in different segments of the major rivers in south Bengal and near the inhabited islands of the Sundarbans. It is likely that underwater noise does affect the behaviour of dolphins, which rely on sound for sensing of the environment.”

“The decline of Ganges rive dolphin (GRD) population in the Indian Sundarban needs to be substantiated with further study, however the reduced freshwater flow in the channels and gradual rise of salinity clearly suggest a habitat unsuitability for GRD in this estuary,” the study states.

 

CITATION:

Mitra, S., & Chowdhury, M. R. (2018). Possible range decline of Ganges River Dolphin Platanista gangetica (Mammalia: Cetartiodactyla: Platanistidae) in Indian Sundarban. Journal of Threatened Taxa, 10(13), 12738-12748.

Banner image: Gangetic dolphin. Photo by Zahangir Alom/Marine Mammal Commission/National Oceanic and Atmospheric Administration/Wikimedia Commons.