Susu, Sisu, Shushuk,
The Ganges River dolphin (Platanista gangetica) is a species of toothed whale found in South Asia. It is also known by the name susu (popular name) or "Sisu" (Assamese language) and shushuk (Bengali). The Ganges river dolphin has been recognized by the Government of India as its National Aquatic Animal and is the official animal of the Indian city of Guwahati. Its first occurrence, within the Hooghly River, was documented by William Roxburgh.
The Ganges River dolphin has a rectangular, ridgelike dorsal fin and females tend to be larger than males. These dolphins are usually tan, chocolate brown, dark grey, or light blue. They have an elongated, slender snout with sharp and very pointed teeth, similar to most river dolphins. The river dolphin has a rounded belly which, combined with their rectangular dorsal fin, makes them look particularly stocky in build compared to other dolphins. Their flippers and tail flukes are large and broad. They have a large melon head used for echolocation because they cannot see well. Their eyes are usually small due to the cloudy water.
These dolphins live along the Ganges-Brahmaputra-Meghna and Karnaphuli-Sangu river systems of Bangladesh and India, and the Sapta Koshi and Karnali Rivers in Nepal. They favor deep pools, eddy countercurrents located downstream of the convergence of rivers and of sharp meanders, and upstream and downstream of midchannel islands.
Ganges River dolphins prefer to spend time singly or in pairs. They are shy in nature and are not known to do acrobatic maneuvers near boats. They may occasionally gather in small groups of up to 30 individuals where the best fishing spots are. Ganges River dolphins are active throughout the day. They swim almost constantly with only brief periods of sleep, which add up to seven hours per day. They swim on their sides when in shallow water. River dolphins generally surface with the rostrum, head, and dorsal fin breaking the water and rarely breach or raise the tail fluke, though surface activity can vary based on age, distance from shore, and time of day. Diving may last as long as 8 minutes among adults and subadults; dives of newborns and juveniles are not as long. Ganges River dolphins have poor eyesight and rely on echolocation to find prey. When hunting at the surface, dolphins listen for the movements of schooling fish which are then herded with spins, side-swimming, and lobtailing. Echolocation signals are not frequently used at the surface, since many fish at this level can hear ultrasound. At the mid-surface level, the dolphins use more echolocation clicks to find prey hidden in clutter and vegetation as far as 20 m (66 ft) away. They flush out bottom-dwelling prey by digging around. The prey is held in their jaws and swallowed. During the monsoon, Ganges River dolphins tend to migrate to tributaries of the main river systems. Some individuals may swim along with their beaks emerging from the water, and they may "breach"; jumping partly or completely clear of the water and landing on their sides. Ganges River dolphins are highly vocal and most typically communicate with the help of echolocation sounds such as clicks, bursts, and twitters.
Ganges River dolphins are carnivores (piscivores). They feed on a variety of shrimp and freshwater fish, including carp and catfish.
Ganges River dolphins do not have a specific mating season. Births appear to be most frequent between December and January and between March and May. Courtship and mating behavior for this species has been documented from March to May when the water level is lower; multiple males chase one female and this typically ends with one of the males earning the right to mate. Females give birth to a single calf after the gestation period which usually lasts 8-12 months. The calf will stay with its mother for one year and become reproductively mature at about 10 years of age.
The most serious threat to Ganges River dolphins comes from human activities. These animals have been adversely affected by human use of river systems in South Asia. Entanglement in fishing nets as bycatch can cause significant damage to local populations, and individuals are taken each year by hunters; their oil and meat are used as a liniment, an aphrodisiac, and as bait for catfish. Poisoning of the water supply from industrial and agricultural chemicals may have also been a contributing factor to population decline, as these chemicals are biomagnified in the bodies of the dolphins. Perhaps the most significant issue is the building of more than 50 dams along many rivers, causing the segregation of populations and a narrowed gene pool in which dolphins can breed. An immediate danger for the populations in the National Chambal Sanctuary is the decrease in river depth and the appearance of sand bars dividing the river course into smaller segments, as irrigation has lowered water levels throughout their range.
According to the IUCN Red List, the total population size of the Ganges River dolphin is around 5,200 individuals. Currently, this species is classified as Endangered (EN) on the IUCN Red List, and its numbers today are decreasing.
Ganges river dolphins play an important role as top predators in their river ecosystem. They prey on a range of fish and crustacean species, thus controlling their populations.
