Ganges and Indus Dolphins (Platanistidae)
Ganges and Indus dolphins
Small gray dolphin with long beak, exposed interlocking teeth and tiny eyes; broad flippers and flukes
Females 8.2 ft (2.5 m) and males 6.6 ft (2.0m); 185.2 lb (84 kg)
Number of genera, species
1 genus; 1 species
Rivers and tributaries
Indus River in Pakistan; Ganges River drainage in India, Bangladesh, and Nepal
Evolution and systematics
This is the most primitive group of all river dolphins. It is closely related to five extinct families that were widely distributed during the Oligocene (34–24 million years ago [mya]) and Miocene (24–5 mya). Two fossils exist from the middle to late Miocene. Zarhachis and Pomatodelphis were found in marine environments in North America and Europe, but paleontological data are too scarce to establish when these marine ancestors first entered rivers. It is hypothesized that they inhabited the estuarine regions created during the rise of the sea level in the middle Miocene and survived in rivers as the waters regressed in the late Miocene. Different hypotheses have been advanced about their phylogeny. Placement between other river dolphins and Ziphiidae (beaked whales) or between Ziphiidae and Physeteridae (sperm whales) has gained considerable support from genetic and morphologic data.
Classification of this species at the family level is the least controversial of all river dolphins. It is the only species of family Platanistidae. It is grouped under superfamily Plastanistoidae with five fossil families: Prosqualondotidae, Squalondotidae, Squalodelphinidae, Waipatiidae, and Dalpiazinidae.
Although Platanista from the Indus and Ganges drainages have been proposed to be different species, namely Platanista gangetica and Platanista minor (or indi), based on morphologic and biochemical analysis, currently they are considered a single species, Platanista gangetica. While differences in tail length between the Indus and Ganges dolphins has led some authors to consider two subspecies, P. g. gangetica and P. g. minor, genetic analysis has not resolved this issue.
The taxonomy for this species is Plantanista gangetica (Roxburgh, 1801), Hooghly River, Ganges River Delta. Other common names include: English: Blind river dolphin, susu; French: Plataniste du Gange, plataniste de l'Indus, sousou; Spanish: Delfín del Ganges, delfín del Indo.
The primitive appearance of the Ganges and Indus dolphin is unlike that of any other dolphin, even other river dolphins. The snout is elongated, about one-fifth of the body, and widens towards the tip. The anterior teeth are larger and exposed, especially close to the tip. The dorsal fin is merely a small hump close to the rear of the bulky body. However, the flippers and
fluke are relatively large. On top of the head, there is a longitudinal ridge. The blowhole is a longitudinal slit in contrast to a horizontal opening typical in dolphins. A wattle, forming several folds, adds to the species' ungainly appearance. Uniquely, the external ear sits below eye level. The eyes are tiny, smaller than the ear opening. The optical apparatus is underdeveloped and is thought to perceive only shades rather than images; hence, the name blind river dolphin. The skull is extremely asymmetrical compared to most odontocetes and has prominent facial inflections unseen in other dolphins. The neck is very long and, because of unfused vertebrae, flexible. The brain has the simplest cerebral cortex among odontocetes. Coloration is gray or brown, occasionally with a pinkish belly.
Currently, the species is found in the Ganges/Brahmaputra/Megna and Karnapuli River systems and their tributaries in India, Bangladesh, and Nepal, and in Pakistan in the Indus River system. Previously, its range extended further upstream into several tributaries. In the Ganges River, the species no longer occurs beyond the Bijnor Barrage (gated dam), completed in 1984 with a loss of a 62-mi (100-km) segment of their habitat. In the Indus River, it does not inhabit the tributaries above Chasma, Trimmu, Sidhnai, and Islam Barrages, built from 1927 to 1971. Its southern range also has shrunk; the lower limit in the Indus River is the Kotri Barrage. Reduced precipitation may drastically affect their distribution, forcing the dolphins to leave smaller tributaries during the dry season.
These dolphins occupy rivers and tributaries that run through hills (up to 820 ft [250 m] above sea level in Nepal) and plains, some with turbulent rapids and sharp meanders. River bends, mid-channel islands, or convergences of tributaries create eddy countercurrents, a preferred habitat for dolphins. Dolphins are found both in shallow and deep water and appear to favor 10–30 ft (3–9 m) depths. Water temperatures are 46.4–91.4°F (8–33°C). Dolphins have been seen at the mouths of the rivers that flow into the Bay of Bengal and are thought to disperse between the Ganges/Bramahputra/Meghna and Karnaphuli/Sangua systems along the coast. This may occur during the monsoon when a freshwater plume from the river extends into coastal waters.
A remarkable behavior is the dolphins' side swimming, with their tail slightly higher than the head, thought to be an adaptation to very shallow waters. Aerial behaviors are uncommon, leaping being performed mainly by calves. Surfacing usually occurs beak first, followed by the melon. Only the front of the body is exposed. This is a very vocal species, which produces pulsed sounds rather than whistles. For navigation and foraging, they use echolocation in place of vision. Mostly solitary, their mean group size is fewer than three individuals, although groups of 25–30 have been observed.
Feeding ecology and diet
They feed predominantly on benthic species, including reports of catfish, herring, carp, gobies, and mahseers. Invertebrates such as prawns and clams have also been found in their
stomach contents. In captivity, individual daily consumption varies from 1–3.3 lb (500–1,500 g). In the Brahmaputra River, they often feed in association with the river tern (Sterna aurantia), sightings of which may be used to help locate dolphins.
Sexual maturity is estimated at 10 years. Estimates of gestation range 8–11 months. Neonate length is estimated at 3 ft (1 m). Lactation may last from two months up to one year. Calving appears to occur throughout the year. Information on other reproductive parameters and mating behavior is scarce.
River dolphins are among the world's most threatened mammals. P. gangetica is the second most vulnerable river dolphin, being classified as Endangered. In the Indus River, it has lost a significant portion of its historical range. Subpopulations in Nepal and the Karnaphuli River in Bangladesh are believed to be close to extinction. In the Indus and Ganges River systems, respectively, it is estimated that only a few hundred and several thousand occur. Perhaps the worst threat is posed by nearly 100 water development projects such as dams, barrages, embankments, and dikes. The dams reduce downstream flow and, hence, eliminate periodic enrichment during flooding, reducing riverine productivity. Dams also disrupt seasonal migrations and spawning habitat of fishes. Over-fishing further aggravates this loss of prey. In addition, dams split dolphins into smaller groups, potentially reducing genetic diversity and compromising the long-term viability of populations.
Hunting of dolphins is another threat. Tribal people in the Bramahputra River, Nepal, and in parts of Bangladesh continue to hunt dolphins for meat and oil. Although the direct hunt has decreased following implementation of protective regulations in 1972, enforcement is ineffective and many fishermen are unaware of the laws. By-catch occurs mainly in gill nets and mosquito nets or kapda jal (very fine-meshed nets that are illegal). It is estimated that 90–160 dolphins are caught annually in monofilament gillnets in Sirajganj, a town near the Jamuna River. It is unclear whether these catches truly are accidental since the meat and oil are used as fish attractants.
The high human population density in this region, combined with poverty, also causes acute pollution problems from untreated sewage and agricultural run-off. There are few tox-icological
studies, but high concentrations of heavy metals were found in a river dolphin neonate from Bangladesh.
The Asian River Dolphin Committee has proposed better consideration of dam locations and monitoring of their impacts. It also recommended creation of artificial eddy countercurrents and "Managed Resource Protected Areas," where fisheries are conducted in a sustainable manner.
Significance to humans
Its oil has been valued as medicine for a variety of diseases (e.g., arthritis, rheumatism), as an aphrodisiac, and as an ointment for humans and livestock in India, Bangladesh, and Pakistan. It is also used as a fish attractant, and the meat is consumed in some regions. A common practice of fishermen in the Ganges and Bramahputra Rivers is to hang pieces of dolphin meat on the side of the boat and sprinkle the water with a mixture of oil and minced meat. In a site where 15–20 boats target dolphins for oil, it was estimated that about 20 dolphins are required annually for a fishery that operates only two months per year. There is a considerable demand for dolphin oil, especially in the catfish fishery in northeast India. Recent research shows, however, that fish scraps, freely available to fishermen, are equally effective as catfish bait. Thus, educating fishermen to use fish oil rather than dolphin oil may be a promising conservation measure.
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Paula Moreno, MS