About Dolphins, Associated Species And Their Habitats
Ganges river dolphins are one of six freshwater dolphins in the world (know who the other species are, look at how they differ from their closest relative – the Indus river dolphins). However, they are the oldest among these, having evolved 16-23 million years ago (have a look at their ancestral fossil records). They are blind (have at a look at their eye structure) and occupy the waters in the Indian subcontinent – the Ganges-Brahmaputra-Meghna-Kanaphuli-Sangu river systems.
Biology, Ecology and Evolution of Dolphins
Size class
Females size range between 2.5 – 2.78 m, males with a maximum of 2.2 m. Neonates size range 63 – 71 cm and grow to about a meter in 1 year. Adults weigh about 150-170 kg. Have sharp pointed teeth that flattens with age.
Reproduction and Gestation
Females size range between 2.5 – 2.78 m, males with a maximum of 2.2 m. Neonates size range 63 – 71 cm and grow to about a meter in 1 year. Adults weigh about 150-170 kg. Have sharp pointed teeth that flattens with age.
Biology, ecology and evolution of dolphins
Size class
Reproduction and Gestation
Communication
Habitat
Prey
Current status and distribution
Distributed across Ganga and Brahmaputra
Split into subpopulations
Dropped from thousands to 1800 by 21st century
The Gangetic Dolphin ranges into most of the large tributaries in the Ganga Basin: the Chambal, Ramganga, Yamuna, Gomti, Ghaghara, Rapti, Son, Gandak and Kosi, besides the main channel of the Ganga.
In the Brahmaputra valley, it ranges into the major tributaries such as the Tista, Adadhar, Champamat, Manas, Bhareli, Subhansiri, Dihang, Dibang, Lohit, Disang, Dikho and Kulsi rivers.
Dolphins in the main channel of the Ganga were split into two subpopulations in 1975, when the Farakka Barrage was commissioned. Today they occur in 5 subpopulations within India, the Brahmputra, Bijnor–Narora, Narora–Kanpur Barrage, Kanpur Barrage – Farakka, and Farakka to Ganga Sagar. On the other hand, surveys have confirmed extirpation of populations from many of the tributaries in the last 10-20 years, viz., Son, Ken, Betwa, Punpun and Barak.
The Gangetic Dolphin ranges into most of the large tributaries in the Ganga Basin: the Chambal, Ramganga, Yamuna, Gomti, Ghaghara, Rapti, Son, Gandak and Kosi, besides the main channel of the Ganga. In the Brahmaputra valley, it ranges into the major tributaries such as the Tista, Adadhar, Champamat, Manas, Bhareli, Subhansiri, Dihang, Dibang, Lohit, Disang, Dikho and Kulsi rivers. Dolphins in the main channel of the Ganga were split into two subpopulations in 1975, when the Farakka Barrage was commissioned. Today they occur in 5 subpopulations within India, the Brahmputra, Bijnor–Narora, Narora–Kanpur Barrage, Kanpur Barrage – Farakka, and Farakka to Ganga Sagar. On the other hand, surveys have confirmed extirpation of populations from many of the tributaries in the last 10-20 years, viz., Son, Ken, Betwa, Punpun and Barak.
What was once thought to be abundant with an estimated population of tens of thousands (though this estimation included the Indus river dolphins), the numbers of these Ganges river dolphins were reported to lie somewhere between 4000 and 5000 towards the end of the 20th century. This was later reported to have dropped to 1800 individuals including the population in the river tributaries by the beginning of the 21st century. The dolphin populations have been reported to have had a decline of 1/3rd of its population in just 4 generations.
Threats and concerns of dolphins and other species
Exploitative fishery
Unsustainable fishery has wiped out multiple species from the riverine habitats. These include the Mahseer that once inhabited the Barak river of Assam. Today these practices are continuing to threaten many species. E.vacha is an endangered species which is in high demand that they poach dolphins to bait these with the dolphin oil.
Direct hunting
Local consumption of meat is predominant in many parts of these river systems and span across multiple species. While Gharial was reported to have become locally extinct due to direct hunting, poaching still is rampant today with reports of turtles, dolphins, poisoning birds including the recent venture into killing Ruddy Shelducks.
Navigation vessels
Navigation vessels particularly threatens the Ganges river dolphins. Though rare instances of vehicle collisions are reported from boat propellers they are not uncommon. Dolphins rely on reception of their sonar clicks to navigate, forage and communicate. The noise created by these vessels which overlaps with the spectrum of these dolphins, disturb acoustic environment affecting day to day activities of these acoustically dependent species.
Dams and barrages
Cordoning off rivers to deviate water to other parts of the state or to create reservoirs has had irreversible repercussions by isolating populations of different species and much more. While much of the Brahmaputra mainstem is free of this, the Ganga river has 20+ dams and barrages. One of the oldest barrages, the Farakka barrage, has been reported to increase silt deposition before the barrage causing a reduction in land area of the Sunderbans – causing damages to an entire ecosystem.
Human activities
Water extraction for irrigation use, disposable of sewage wastes, mechanized sand mining for extraction of river sand are just some of the things that affect the river ecosystem. Introduction of non-native species from commercial fisheries, that potentially invade the natural river systems are still in practice.
Fish, birds, turtles, otters and more
Fish
Dolphin habitats are also home to multiple other species. The Hilsa fish, Tenualosa ilisha, is an anadromous fish that has large scale upstream migration from oceans and spawn in the Ganges river systems. The Sperata aor is a commercially important fish and is also a potential feed of these dolphins. Eutropiichthys vacha is Endangered in India (CAMP 1998) and Critically Endangered in Bangladesh (IUCN Bangladesh 2000), a catfish species, inhabits these freshwater. Similarly, Setipinnia phasa, a potamodromous fish found in the waters of Bengal and Orissa is known for its environmentally induced reproductive plasticity.
Birds
In the Brahmaputra mainstem, more than 17000 individuals were encountered during a single survey in 2018. These were water birds of 46 species with three Endangered, two Vulnerable and 4 Neat Threatened according to the IUCN. The sand banks of the Chambal rivers are also nesting grounds for the the Indian Skimmer. The Brahmaputra water also are a source for multiple wetlands such as the Maguri in Assam etc., that support a whole range of birds.
Turtles
The Ganges and Brahmaputra river systems are one of the richest turtle habitats in the country country. Out of the 24 recognized species, these rivers inhabit 19 of them, most of which are not found anywhere else. These include some of the rarest softshell species too.
Otters
The small-clawed otter and the smooth-coated otters are diurnal animals that largely inhabit regions with minimum human presence. While both these species are considered vulnerable, the sand banks of much of the Brahmaputra banks are marked with their tracks and spraints of at least one of them, in good numbers.
Other species
The only other apex predator that co-occurs with the Ganges river dolphins is the 38 million year old species – the Gharial. The gharial inhabited much of the Ganges-Brahmaputra in India, however, with depletion in fish sources due to exploited fishing and direct hunting, today only about 200 gharials are left in about 2% of where they used to found before.
Water and air quality
River Ganges and Brahmaputra are the two primary habitats of Gangetic river dolphins in India and, both the rivers have different sources of pollution. In River Ganges, water pollution mainly includes untreated sewage and industrial discharge (leather, paper and pulp, fertilizer, pharmaceutical, sugar, jute and textile), which generally release wastes directly into rivers. The widespread use of fertilizers and pesticides for pests, weed and vector-borne control also creates severe water-quality problems. The status of the Brahmaputra river in Assam is much better than in comparison to the river Ganges, but the pesticide concentration have also been reported in the Brahmaputra. During 2005-2010, 5039 metric tons of DDT were used for malaria control and tea estates in Assam. Everyday activities like sand mining and dredging activities may also reintroduce the settled pollutants back into the water body and increases water turbidity. Assam is known as the Oil capital of India, and the refining and distillation of petroleum are one of the critical causes of PAH contamination in the environment. The exposure to high concentrations of pollutants caused by pesticides and sewage discharge (organic load causing algal bloom) in the river system causing mortalities of other aquatic species, such as fishes, that constitute the major food of dolphins. However, at lower concentrations, pollutants can also lead to the accumulation of pollutants in aquatic species.