Meet The River DolphinsThere are four main families of river dolphin:
Franciscana (La Plata River Dolphin)
The Franciscana or La Plata Dolphin (Pontoporia blainvileli) is scientifically grouped in the river dolphin family (its neck vertebrae are not fused and the beak is very long).
However, the Franciscana do not really live in freshwater at all. They live in salt-water estuaries and near-shore marine environments in South America.
The baiji or Chinese river dolphin (Lipotes vexillifer) was the sole member of the Lipotes family and lived in the Yangzte River of China for some 20 million years. It is now extinct and so tragically as entire family of mammals is now gone forever.
The baiji’s extinction is entirely due to human impacts such as the ongoing degradation of its habitat, heavy ship traffic, pollution, human land uses as well as unsustainable fisheries and bycatch. It is the first large mammal to be driven to extinction by people in recent times.
The baiji’s demise can only mean that the Yangzte river ecosystem and the people living there are in very serious trouble. The Yangtze river basin has a population of approximately 400 million people – that is about one in 17 of all people on the planet! Sadly the baiji’s story is an early-warning sign of more extinctions to come, unless action is taken to stop history repeating itself both in China and in other parts of the world.
The Yangtze is still home to a second freshwater cetacean, it is the world’s one and only freshwater porpoise - the finless porpoise (Neophocaena phocaenoides asiaeorientalis). It is endangered. Like the baiji it is only found in this one river and like the baiji its numbers are in rapid decline. The same things that drove the baiji to extinction are leading to the demise of the finless porpoise – habitat degradation and bycatch.
The Platanista family includes the Ganges river dolphin of Nepal, India and Bangladesh and the Indus river dolphin of Pakistan. There is an ongoing debate amongst scientists about these dolphins – are they separate species or subspecies of the same species?!
One thing is certain, the Ganges and Indus River Dolphins are identical in appearance. The body is a grey -brownish colour and stocky. They have a small triangular lump as opposed to a dorsal fin. Their flippers and tail are large in relation to the body size, which is about 2 to 2.2 meters in males and larger (2.4 to 2.6 m) in females.
These dolphins have long, slender beaks (noses) characteristic of all river dolphins. The teeth are visible in both the upper and lower jaws even when the mouth is closed. The snout thickens towards its end. These river dolphins do not have a lens in their eyes and so they are blind, although they can probably detect the intensity and direction of light. They use echolocation to hunt and navigate.
They feed on a variety of fish, including carp and catfish and shrimps. Both Ganges and Indus river dolphins are usually encountered in ones and twos or in loose aggregations; they do not form tight, obviously interacting groups.
The Ganges River dolphin was formerly distributed throughout the Ganges-Brahmaputra river system of Bangladesh, India and Nepal, and possibly Bhutan. Although it still has a fairly extensive range, its distribution has contracted, and its abundance has declined dramatically in some areas. Currently it is found in the Ganges-Brahmaputra-Meghna and Karnaphuli-Sangu river systems of Bangladesh and India. A few individuals survive in Nepal in the Karnali River and possibly the Sapta Kosi River.Its habitat is severely fragmented, and additional barrages continue to be built. Further reductions in the extent of occurrence and area of occupancy of this dolphin are expected. It is almost certainly declining in numbers and will continue to do so as habitat degradation shows no sign of abating.
The Indus river dolphin is found only in the Indus river of Pakistan. It is classed as endangered by IUCN. It is one of the world’s rarest animals and is the rarest river dolphin (now that the baiji is extinct). Estimated overall numbers of animals surviving maybe as few as 1100 individuals and numbers are thought to be still declining. Historically it ranged from the Indus delta upstream to the Himalayan foothills. Currently the distribution of the Indus River dolphin is severely fragmented and dramatically reduced in extent. An estimated 99% of the Indus River dolphin population occurs in only 690 linear km (430 mile) segment of the river. Currently the Indus River dolphin is limited to three subpopulations in the Indus mainstream located between the Chashma and Taunsa, Taunsa and Guddu, and Guddu and Sukkur Barrages.
The main reason for the decline of the Indus River dolphin was the construction of numerous dams and barrages, starting in the 1930s, that have fragmented the population and reduced the amount of available habitat. Another severe threat to the survival of the Indus River Dolphin is probably the increasing withdrawal of water. Dolphins no longer occur in the lower reaches of the Indus because upstream water extraction leaves downstream channels virtually dry for several months each year.
The Inia family includes the boto or Amazon river dolphin (Inia geoffrensis) and the Bolivian river dolphin (Inia boliviensis) which live in the Amazon and Orinoco river basins of South America – in Peru, Colombia, Venezuela, Brazil, Ecuador, Bolivia and Guayana.
The Inia family includes the Amazon river dolphin (Inia geoffrensis) and the Bolivian river dolphin (Inia boliviensis) found only in Bolivia!
The Amazon River Dolphin or boto, is a freshwater river dolphin endemic to the Orinoco, Amazon and Araguaia/Tocantins River systems of Brazil, Bolivia, Peru, Ecuador, Colombia, Guyana and Venezuela. They share their habitat with some incredible animals including manatees, giant otters, spectacled caimans, Orinoco crocodiles, anacondas, turtles,giant freshwater fish including the biggest in the world - the pirarucu, and the infamous piranhas.
Botos are the largest of all river dolphins (1.8 – 2.5m long). They have a hump on their backs rather than a well-defined dorsal fin and a long narrow beak lined with teeth. Botos have 25-30 peg-like front teeth for catching prey and molars for crushing their food at the back of their mouths. They use echolocation to hunt and catch their prey - they eat fish (particularly catfish) and crustaceans and crabs. Their flexible neck vertebrae - unique to river dolphins - means they can move their necks up and down and from side to side - this gives them much greater manoeuvrability in the flooded forest and enables them to enter and swim amongst the roots and branches hunting and catching the fish that feed from seeds and berries.
Their unusual pink colouration is a beautiful and striking contrast to the often brownish coloured water and green forest. The calves are often a greyish colour and often they become pinker with age. Their unique colour is thought to be due the presence of lots of blood vessels close to the surface of the skin. The calves are grey when born and the dolphins seem to get pinker with age.
Botos tend to give birth to their calves during times of the highest water levels (May/June) and the start of the water levels falling (July). This is coincident with abundant fish availability. The pregnancy is slightly longer than a human one as it lasts for 10 – 11 months and each mother gives birth to a single calf. The calf is born tail first under the water and its mother helps it to the surface to take its first breath. The can swim straight away and stays very close to its mothers and regularly suckles milk. Females may give to a single calf every 4-5 years.
The overall population size is currently unknown, although Fundacion Omacha have been carrying out a series of river surveys throughout South America over the last 3 years in an attempt to rectify this. River surveys have shown that densities of dolphins does vary between different river systems.
The Bolivian river dolphin
Inia. g. boliviensis occurs only in Bolivia, it is isolated from other botos or Inias by a geographical barrier – the 400 km of waterfalls and rapids. These stretch from Porto Velho on the Madeira River in Brazil upstream to Riberalta on the Beni River in Bolivia. The botos in Bolivia have been separated from other botos for such an extended period that they are now genetically distinct and have been classified as a different species of river dolphin. This clearly has huge consequences for river dolphin conservation. Bolivia is very proud of this special and unique river dolphin living only in Bolivia and has made it an emblem of Bolivia.
There are also other species of dolphin that have populations which solely live in fresh water – the Irrawaddy dolphin, finless porpoise (see above) and the tucuxi (sotalia).
Tucuxi or Sotalia (Sotalia fluviatilis)
Perhaps rather surprisingly there is another dolphin species that lives in South American rivers and lakes of the Amazon and Orinoco basins. Botos and sotalia can often be found alongside each other in South American rivers and lakes. However, they are not closely related genetically. Botos are true members of the Inia river dolphin family. Sotalia are classed as members of the oceanic dolphin family (Delphinidae) and they look more like typical marine dolphins.
These Amazonian riverine sotalia are small dolphins that grow up to 1.56 metres in length, and weigh about 60 kilograms. The sotalia is light grey to bluish-grey on the back and pinkish to light grey on the tummy. The dorsal fin is triangular and may be slightly hooked at the tip.
Sotalia’s range in the Amazon and Orinoco overlaps with that of the boto, but unlike the boto they do not enter the flooded forest during high water, they remain in the deeper water channels and lakes.
Sotalia are vulnerable to the same threats that apply to Inia, including fisheries entanglement, habitat deterioration and fragmentation of populations by dam construction and so efforts to protect river dolphins need to include sotalia.
Irrawaddy Dolphins (Orcaella brevirostris)
Irrawaddy dolphins are grey and look like small belugas (white whales of the Arctic) in appearance, but are most closely related to the orca.
Although sometimes called the Irrawaddy river dolphin, they are not true river dolphins. Irrawaddies are oceanic dolphins that live in brackish water near coasts, river mouths and in estuaries. There are also a few established sub-populations entirely restricted to freshwater rivers in Asia, including the Mekong in Laos and Cambodia, the Ayeyarwady River in Myanmar, and the Mahakam River in Indonesia. All three are critically endangered. There are also populations in Songkla Lake, Thailand and Chilka Lake in India; both are critically endangered.
They have a large melon and a blunt, rounded head, and the beak is indistinct. They have a small, blunt, triangular-shaped dorsal fin. The flippers are long and broad. Adults grow up to 2.3m (8ft) long and weigh 130kg (287lb) kg.
Threats include bycatch in monofilament gillnets used extensively in all rivers. Capture for captive display in aquariums in Asia is also a major threat. These unique small riverine subpopulations of Irrawaddy dolphins throughout Asia are all in serious trouble and all are under threat of extinction.