River dolphins are cetaceans, which means that they related to whales and porpoises. These mammals live only in rivers and waterways of Asia and South America. There are five species of river dolphins belonging to three different families. In Asia the Baiji or Chinese river dolphin (Lipotes vexillifer ) lives in the lower part of China's Chang Jiang or Yangtze River. On the Indian subcontinent, Indus river dolphins (Platanista indi ) originally were found throughout almost the entire length of the Indus River, but now are limited to a stretch of that river in the Sind Province of Pakistan. Ganges river dolphins (Platanista gangetica ) are found throughout the Ganges River, although their numbers are decreasing. In South America the Amazon river dolphin (Inia geoffrensis ), also called the boto, is found primarily in the Amazon and Orinoco Rivers of Brazil and Venezuela. The South American franciscana river dolphin (Pontoporia blainvillei ), unlike other river dolphins, is a marine (ocean) dolphin that lives in shallow water close to shore from central Brazil south to Argentina.
The baiji live in China's Yangtze River, one of the busiest waterways in the world. The Chinese have recognized the baiji river dolphin for centuries. The name "baiji," which means "white dolphin," has been found in a Chinese dictionary from 200 b.c. This dolphin is also called the Chinese river dolphin or whitefin dolphin.
Baiji coloring ranges from pale to dark blue-gray on the back and sides. The belly is white or gray-white. The dolphin has a triangular dorsal fin and a snout that continues to grow as it ages.
Newborn calves measure 32–35 in (about 0.8 m) in length and weigh from 6 to 11 lb (2.7 to 5 kg). Adult females are somewhat larger than the males. Adult length ranges from 6.5 to 8 ft (2 to 2.4 m). Adults weigh from 220 to 355 lb (100 to 160 kg).
Indus river dolphins live in a short stretch of the Indus River in Pakistan. Most dolphins live in an area between two dams built during the 1930s. The dams have limited their migration to other parts of the river. Ganges river dolphins live in the Ganges, Meghna, and Brahmaputra rivers in western India, Nepal, Bhutan, and Bangladesh. They are also found in the Karnaphuli River in Bangladesh.
Indus and Ganges river dolphins share many similarities. They are both called "susu." The word sounds like a sneeze and resembles the sound of a dolphin breathing. They are also called blind river dolphins, because they are the only dolphins that cannot see. These dolphins navigate by echolocation, producing sound waves, then interpreting the echoes these sound waves generate.
The Indus and Ganges river dolphins range in color from gray-blue to brown. Both have a hump on their back instead of a dorsal fin. They swim on their side, and females sometimes carry their young on their back.
Newborn Indus and Ganges river dolphins weigh about 17 lb (7.7 kg) and measure 28–35 in (about 0.8 m) in length. Adults weigh 155 to 200 lb (70 to 90 kg). Their length ranges from 5 to 8 ft (1.5 to 2.5 m).
Amazon river dolphins live in rivers in Brazil, Bolivia, Colombia, Guyana, Ecuador, Peru, and Venezuela that are part of the Amazon and Orinoco watersheds. They are also called boto or pink river dolphins. Adults range in color from blue-gray to pink. The pink color becomes more intense when these dolphins are more active and dims when they are less active. The Amazon river dolphin as a hump on its back instead of a dorsal fin.
Newborn calves weigh about 15 lb (7 kg) and are about 31 in (0.8 m) long. Adults weigh from 185 to 355 lb (85 to 160kg). Adult females range in length from 5 to 7.5 ft (1.5 to 2.4 m). Male dolphin length ranges from 6.5 to 9 ft (2 to 2.7 m).
According to the Whale and Dolphin Conservation Society, Amazon river dolphins have entered local South American folklore. One legend says that male dolphins captured women and took them underwater. Some women explained their pregnancies by saying that dolphins fathered their babies.
Franciscana are classified as river dolphins, although they live in the Atlantic Ocean. They were originally thought to move from fresh water to salt water during their lifecycle, but scientists have now determined that they live their entire life in the sea. Franciscana are found in the shallow coastal waters off eastern South America, primarily in the La Plata estuary. They are also known as La Plata dolphins.
Franciscana have gray-brown backs. Their color may lighten during the winter or as they age. Some older dolphins are primarily white. In relation to their size, franciscana have the longest beaks of any dolphin.
Newborn franciscana are 28 in (0.7 m) long and weigh from 16 to 19 lb (8.6 kg). Adult length ranges from about 4 to 6 ft (1.2 to 1.8 m). Adult franciscana dolphins weigh from 65 to 115 lb (29 to 52 kg).
The river dolphin population is decreasing throughout the world, often through stresses caused by development. Some dolphins are killed by chemical pollution . Other dolphins become entangled in fishing nets and die, and hunting has led to declines in the populations of all river dolphins except franciscana.
The damming of rivers divides populations and prevents migration to better environments. This has caused a decline in the population of baiji, Ganges, and Indus river dolphins. The lack of adequate food supply (prey depletion), has caused declines in franciscana, baiji, and Ganges dolphins. Development has destroyed or polluted much of the habitat where dolphins live. Loss of habitat threatens baiji, franciscana, and Ganges river dolphins.
Baiji are the most rare cetacean. The population of baiji dolphins is in the dozens, according to the World Conservation Union (IUCN). The Chinese government declared the baiji a protected animal in 1949, and people were punished for intentionally killing baiji. However, accidents and development still caused many deaths.
In 1975, China declared the baiji a national treasure, and subject to more protection. In 1996, the IUCN rated baiji as one of the 12 most endangered species in the world. In 2000, fewer than 100 individuals were believed to survive in the wild. Increasing development and the building of dams along the Yangtze River is likely to kill the few animals remaining.
In 1998, the IUCN estimated that the population of Indus river dolphins was less than 1,000. In 2000, the Indus dolphin was considered critically endangered and threatened with extinction . In addition, in 2000 the IUCN estimated that only a few thousand wild Ganges river dolphins remained. It is also considered an endangered species.
South American river dolphin population has not been depleted as much as the river dolphin population of Asia. Although the exact population of Amazon river dolphins has not been estimated, they are believed to have to the largest population of all species of river dolphin. In 1996, the IUCN considered Amazon river dolphins to be vulnerable to population decreases because of increased dam building along the Amazon River. According to the IUCN, inadequate verifiable data exists about the population of franciscana dolphins. Lack of data has made protection and management of this dolphin population difficult.
[Liz Swain ]
Blair, Cornelia, Mark Siegal, and Nancy Jacobs, eds. Endangered Species 1998. Wylie, TX: Information Plus, 1998.
Carwardine, Mark, ed. Whales, Dolphins and Porpoises. New York: Check-mark Books, 1999.
Day, Trevor. Oceans. New York: Facts on File, 1999.
"River Dolphins." Environmental Encyclopedia. . Encyclopedia.com. (January 21, 2019). https://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/river-dolphins
"River Dolphins." Environmental Encyclopedia. . Retrieved January 21, 2019 from Encyclopedia.com: https://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/river-dolphins