DOLPHIN HABITAT

FRESH WATER DOLPHIN/GANGES RIVER DOLPHIN
What are Fresh Water Dolphins?

The Ganges River dolphin, or susu, inhabits the Ganges-Brahmaputra-Meghna and Karnaphuli-Sangu river systems of Nepal, India, and Bangladesh.

This dolphin is among the four "obligate" freshwater dolphins

Ganges River dolphin has a long thin snout, rounded belly, stocky body and large flippers. Although its eye lacks a lens (this species is also referred to as the "blind dolphin"), the dolphin still uses its eye to locate itself. The species has a slit similar to a blowhole on the top of the head, which acts as a nostril.

The dolphin has the peculiarity of swimming on one side so that its flipper trails the muddy bottom. This behaviour is understood to help it to find food.

Being a mammal, the Ganges River dolphin cannot breathe in the water and must surface every 30-120 seconds. Because of the sound it produces when breathing, the animal is popularly referred to as the 'Susu'.

The movements of the Ganges River dolphin follow seasonal patterns. However, it seems that animals travel upstream when water level rises, and from there enter smaller streams.

Habitat

Once found in 'large schools', the species is found exclusively in freshwater habitat. In Nepal, it inhabits clear water and rapids. In Bangladesh and India, individuals live in rivers that flow slowly through the plains. The Ganges River dolphin favours deep pools, eddy counter-currents located downstream of the convergence of rivers and of sharp meanders, and upstream and downstream of mid-channel islands.

'Susu' shares its habitat with crocodiles, freshwater turtles and wetland birds, many of which are fish eaters and are potential competitors with dolphins.

The diet includes a variety of fish and invertebrates, namely prawns, clams, catfish, freshwater sharks, mahseers (except in India), gobies and carp. Ganges River dolphins normally chase surface dweller fishes and grovel mud dweller fishes in shallow water with the help of their long snout. They cannot chew and usually swallow their prey.

The presence of dolphin in a river system signals a healthy ecosystem. Since the river dolphin is at the apex of the aquatic food chain, its presence in adequate numbers symbolizes greater biodiversity in the river system and helps keep the ecosystem in balance.

Decline of Dolphin Population

In the 19th century, Ganges River dolphins were once found in 'large schools' close to urban centres along the river. Nowadays, groups are considerably smaller, and individuals may also be found alone.

During the last century, thousands of freshwater dolphins rolled and jumped in their natural habitat, which once ranged throughout the Ganges-Brahmaputra-Meghna and Karnaphuli-Sangu river systems of Nepal, India, and Bangladesh, from the Himalayan foothills to the Bay of Bengal. In 1982, the population in India was estimated to be between 4,000 and 5,000 individuals. Today, it is difficult to sight a dolphin in the rivers.

Extensive modifications of this species' habitat have reduced its once abundant population throughout its range. The distribution in the Ganges has diminished by approximately 100 km (62 miles) since the 19th century, with hundreds of kilometres of tributary habitat lost to irrigation barrages. There has been a concomitant reduction in water flow.

Factors responsible for the drop in fish and dolphin population in the lower Ganga

1. Habitat destruction

The biggest threat to dolphin and fish biodiversity water development projects (e.g. water extraction and the construction of barrages, high dams, and embankments on the river) that impound water and impede migration. These projects result in major changes in the flow, sediment load, and water quality of rivers, which affects the quality of waters downstream. The extraction of river water and siltation from deforestation are also degrading the species' habitat. In some cases, habitat alterations have resulted in the genetic isolation of dolphin populations.

2. Traditional fish farming

Traditional fishermen along the Ganga in Bihar finding few fish in the river, rendering fish as nearly unviable livelihood.

3. Selective fishing

Widespread use of non-selective fishing gear kills the dolphins

4. Illegal fishing and nets

The increase in the number of non-traditional fishermen and illegal fishing nets with fine weave. These nets trap very young fish, decimating population. There is a good amount of unintentional killing through entanglement in fishing gear; directed harvest for dolphin oil, which is used as a fish attractant and for medicinal purposes

5. Regular dredging

The National Waterways Act sees the Ganga as National Waterway 1. Regular dredging to maintain the water depth needed for barges destroys fish habitat and interferes with dolphin echolocation. Dredging causes the river to alter its flow, eroding its shores and causing deep fall-offs. Some twenty dredging-related deaths have been reported from one ghat along

6. Pollution

Levels of pollution are a problem, and will only increase. Sources from industrial waste and pesticides; municipal sewage discharge and noise from vessel traffic; and overexploitation of prey have poisoned the dolphins.  Compounds such as organochlorine and butyltin found in the tissues of Ganges River dolphins have a deadly effect on the subspecies.

7. Fisheries bycatch

Bycatch in gillnets and line hooks is also a major source of mortality for this subspecies

8. Directed take

Although the killing of this dolphin for meat and oil is thought to have declined, it still occurs in the middle Ganges near Patna, in the Kalni-Kushiyara River of Bangladesh, and in the upper reaches of the Brahmaputra. In fisheries for large catfish in India and Bangladesh, dolphin oil and body parts are used to lure prey, and Ganges River dolphins are used to this end.

The Vikaramshila Gangetic Dolphin Sanctuary is the only protected stretch of river dedicated to the endangered cetacean, which is under threat from a variety of human activities.