Plataniste or Ganges river dolphin (Platanista gangética), Karnaphuli river, Bangladesh
| Photo Credit: The Hindu

The story so far: The first ever census of Gangetic dolphins, the only riverine dolphins in India, which began four years ago, has found 6,327 of the species in the river Ganga and its tributaries.

What did the census survey find?

The Environment Ministry on March 3, commemorated as World Wildlife Day, released a report by the Wildlife Institute of India (WII), a Ministry body, that said there are 6,324 Ganges river dolphins, and three Indus river dolphins (a related species). There are an average of 3,275 dolphins in the main stem of the Ganga; 2,414 in the Ganga tributaries; 584 in the Brahmaputra mainstream; 51 in the Brahmaputra tributaries; and three dolphins in the river Beas. The effort began in 2021 and spanned a cumulative distance of 8,507 km along the stretches of the river and its tributaries. The maximum number of dolphins were reported in Uttar Pradesh (2,397), followed by Bihar (2,220), West Bengal (815) Assam (635) Jharkhand (162) Rajasthan and Madhya Pradesh (95) and Punjab (3).

Why was such a survey necessary?

River dolphins grow slowly and are found in some of the most threatened habitats, vulnerable particularly to getting entangled in the nets of fishermen. Project Dolphin was launched on August 15, 2020 by Prime Minister Narendra Modi to evolve a programme to conserve both marine and riverine dolphins by engaging multiple stakeholders such as the forest department, fishermen, and the fisheries department. In all, 58 rivers were surveyed, of which 28 were traversed by boat and 30 by road survey where Gangetic dolphins have been sighted historically.

How was the survey conducted?

Nearly all wild animals that are surveyed from time to time are land animals. Observational techniques and statistical modelling methods have evolved over the years to estimate their numbers using camera traps, line transects, tracking their excreta, pug marks, and traces of their DNA. However, dolphins present a unique challenge. They are primarily underwater animals, and emerge for air only for seconds. This leaves very little time for an observer to characterise them. Being water-borne, they don’t leave traceable biological samples that can be collected for analysis. This doesn’t mean that wildlife research scientists haven’t attempted to count them.

The number of Ganges river dolphins has been estimated to be between 4,000 and 5,000 towards the end of the 20th century. This dropped to 1,800, according to a 2008 study. More recent studies estimate it to be between 3,500 and 4,500 though these are said to be “guesstimates,” and primarily based on counting the number of dolphins seen from a boat on pre-defined stretches.

For the Project Dolphin survey, the scientists used a combination of “visual and acoustic surveys.” Here, two teams of observers on a boat, coasting at a constant speed, independently recorded a dolphin sighting. Every time they sighted a dolphin, they noted the distance of the sighting from their boat, whether the animals were adults or ‘neonates’, and whether it was the same dolphin sighted before. Then, these observations were correlated with an acoustic survey.

Dolphins are largely blind and communicate via sounds, or echo-location like bats. Scientists used a hydrophone, or an underwater microphone, that can record these sounds. Using signal processing, it is possible to identify with a fairly high accuracy, unique individuals, the WII report said. For their final assessments, results from both these methods were combined with mathematical techniques.

What are the threats faced by the dolphins?

Accidental deaths from being entangled in fishing nets are a major threat. Unlike in poaching, where certain animals are actively hunted, dolphins wade into these nets and are unable to free themselves to break into the surface for air and are thus suffocated. Large-meshed gillnets made from nylon and polythene monofilament are particularly dangerous for them, says a report from the Wildlife Conservation Trust of India (WCT). According to the International Union for Conservation of Nature’s Red List, the Ganges river dolphin (Platanista gangetica) are classified as ‘Endangered.’ Studies estimate that 4%-5% of dolphins die this way. The WCT report, however, mentions that there have been instances where the dolphins, once caught in these nets, are sometimes “opportunistically killed.” The oil extracted from their fat, or blubber, is used as a bait in India and Bangladesh for catching catfish.