Indus river dolphin

The Indus river dolphin (Platanista minor) is a species of freshwater dolphin in the family Platanistidae. It is endemic to the Indus River basin in Pakistan and Beas River in northwestern India. This dolphin was the first discovered side-swimming cetacean. It is patchily distributed in five small, sub-populations that are separated by irrigation barrages.

From the 1970s until 1998, the Ganges River dolphin (Platanista gangetica) and the Indus dolphin were regarded as separate species; however, in 1998, their classification was changed from two separate species to subspecies of a single species. However, more recent studies support them being distinct species. It has been named as the national mammal of Pakistan and the state aquatic animal of Punjab, India.

Taxonomy
The Indus river dolphin was described in 1853 by Richard Owen under the name ''Platanista gangetica, var. minor'', based on a dolphin skull, which was smaller than skulls of the Ganges river dolphin.

The Indus and Ganges river dolphins were initially classified as a single species, Platanista gangetica. In the 1970s, they were considered to be distinct species, but again grouped as a single species in the 1990s. However, more recent studies of genes, divergence time, and skull structure support both being distinct species.

The Ganges river dolphin split from the Indus river dolphin during the Pleistocene, around 550,000 years ago.

Description
The Indus dolphin has the long, pointed nose characteristic of all river dolphins. The teeth are visible in both the upper and lower jaws even when the mouth is closed. The teeth of young animals are almost an inch long, thin and curved; however, as animals age the teeth undergo considerable changes and in mature adults become square, bony, flat disks. The snout thickens towards its end. The species does not have a crystalline eye lens, rendering it effectively blind, although it may still be able to detect the intensity and direction of light. Navigation and hunting are carried out using echolocation. The body is a brownish color and stocky at the middle. The species has a small triangular lump in place of a dorsal fin. The flippers and tail are thin and large in relation to the body size, which is about 2–2.2 m in males and 2.4–2.6 m in females. The oldest recorded animal was a 28-year-old male 199 cm in length. Mature adult females are larger than males. Sexual dimorphism is expressed after females reach about 150 cm; the female rostrum continues to grow after the male rostrum stops growing, eventually reaching approximately 20 cm longer.

Distribution
The Indus river dolphin presently only occurs in the Indus River system. These dolphins occupied about 3,400 km of the Indus River and the tributaries attached to it in the past. But today, its only found in one fifth of this previous range. Its effective range today has declined by 80% since 1870. It no longer exists throughout the tributaries, and its home range is only 690 km of the river. This dolphin prefers a freshwater habitat with a water depth greater than 1 meter and that have more than 700 meters squared of cross-sectional area. Today this species can only be found in the Indus River's main stem, along with a remnant population in the Beas River. A population can be found in the Harike Wetland located in Punjab, India.

Since the two originally inhabited river systems – between the Sukkur and Guddu barrage in Pakistan's Sindh Province, and in the Punjab and Khyber Pakhtunkhwa Provinces – are not connected in any way, how they were colonized remains unknown. The river dolphins are unlikely to have travelled from one river to another through the sea route, since the two estuaries are very far apart. A possible explanation is that several north Indian rivers such as the Sutlej and Yamuna changed their channels in ancient times while retaining their dolphin populations.

Behaviour and ecology
It is thought that Indus river dolphin swims on its side to efficiently navigate shallow waters during the dry season.

Threats
The Indus river dolphin has been very adversely affected by human use of the river systems in the subcontinent. Entanglement in fishing nets can cause significant damage to local population numbers. Some individuals are still taken each year and their oil and meat used as a liniment, as an aphrodisiac and as bait for catfish. Irrigation has lowered water levels throughout their ranges. Poisoning of the water supply from industrial and agricultural chemicals may have also contributed to population decline. Perhaps the most significant issue is the building of dozens of dams along many rivers, causing the segregation of populations and a narrowed gene pool in which dolphins can breed. There are currently three sub-populations of Indus dolphins considered capable of long-term survival if protected.

Conservation status
The Indus river dolphin is protected under Appendix I of the Convention on the International Trade of Endangered Species which prohibits the commercial international trade of the species (including parts and derivatives). It is listed as Endangered on the IUCN Red List, and by the U.S. government National Marine Fisheries Service under the U.S. Endangered Species Act. It is the second most endangered cetacean in the world. As of 2017 it is estimated that there are only about 1,800 individuals remaining (up from 1,200 estimated in 2001). A demonstrable increase in the main river population of the Indus subspecies between 1974 and 2008 may have been driven by permanent immigration from upstream tributaries, where the species no longer occurs.

It is threatened by extensive fishing that reduces their prey availability. Accidentally entangling in fishing nets causes fatalities. Deforestation along the river basins is causing sedimentation which degrades the dolphin's habitat. Another factor for its decline is the construction of cross-river structures such as dams and barrages causing more isolation of the already small sub-populations. A major threat is human induced water pollution through industrial and human waste, or agricultural run-off containing high amounts of chemical fertilizers and poisonous pesticides.

Studies suggest that a better understanding of this species ecology is needed in order to develop good conservation plans. Regular monitoring is necessary to assess the population's status and factors causing its decline. A satellite tagging effort was begun in 2022.