Counting our vulnerable dolphins – The Hindu

The Environment Ministry study could locate only three Indus river dolphins. FIle

Earlier this week, the Ministry of Environment, Forests and Climate Change released the findings of a population study of dolphins found in Indian rivers, putting their numbers at 6,327. These playful creatures with their torpedo-like bodies generate excitement whenever they are spotted. People flock to see them. Urban teenagers describe them as ‘cute’.

There are two kinds of river dolphins. Facultative river dolphins occur in both marine and fresh waters. In India, Irrawaddy dolphins are seen around the Chilka lake, where an estimated 155 of these are a major tourist attraction, and off the Sunderbans. The smaller tucuxi is found in the Amazon and Orinoco rivers, and the Yangtze finless porpoise is about 1 metre long.

Obligate river dolphins are found only in fresh water bodies. The Yangtze river dolphin of China is presumed to be extinct, having been last spotted in 2007. The distinctively pink Amazon river dolphin is over 2.5 metres long. The equally large Ganges river dolphin has a widespread habitat, and is found in the main bodies and some tributaries of the Ganges and the Brahmaputra.

Closely related to the Ganges dolphin, the Indus river dolphin is the State aquatic animal of Punjab. Here, it is found in the river Beas and its Harike wetlands in the Tarn Taran district. The Environment Ministry study could locate only three of these dolphins, indicating their precarious existence. Only about 1,800 survive in the Indus in Pakistan.

Adapting to muddy water

Dolphins and toothed whales have a prominent, fatty mass on their foreheads called the melon. It acts as a lens that focuses sound, and is very important in echolocation. Our river dolphins prefer waters that are muddy, with low salt content. An unusual feature of the Ganges and Indus river dolphins is their poor eyesight. Navigation and feeding is done by echolocation, where ultrasound waves, in the form of distinctive clicks, are emanated from their vocal chords, and the melon on the forehead is used to sense the echoes of these waves bouncing off nearby objects. These dolphins also show a tendency to swim on their sides, using the fins on the sides of bodies to feel along the bottom of riverbeds in their search for food.

In our river dolphin species, the eye is barely a centimeter across, and has a thick cornea and no eye lens. The retina has very few cells for registering light, and the optic nerve that carries visual sensations to the brain is thin, barely a filament. Visual perception seems restricted to detecting light, and the direction from which light is coming. Comparisons of brain regions involved in sensory perception in these river dolphins with the marine bottlenose dolphin shows that the sight areas are unusually small, and the hearing areas much larger. This indicates a reliance on their biosonar for echolocation. In experiments, Indus river dolphins can detect a 4 mm ball bearing suspended on a nylon thread, and quickly head for it.

Human danger to river dolphins comes from the use of their oil in remedies for conditions ranging from muscular strains to arthritis. Overfishing deprives their food supply, and they are snared in fishing nets as unwanted bycatch. Chemical pollutants pose another danger.

Despite increasingly refined counting methods, river dolphin population trends remain unclear – they could be rising or falling. Either way, their numbers are critically low. We must foster greater public awareness of these remarkable creatures.

(The article was written in collaboration with Sushil Chandani, who works in molecular modelling. sushilchandani@gmail.com)