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The rare shark of the Ganga


Highlights:
- The Ganges shark, often mistaken for the more common bull shark, is a critically endangered shark species.
- Unlike many other species of sharks, the Ganges shark is regarded as a true river shark and is only found within the middle and lower reaches of freshwater, inshore marine, and estuarine ecosystems.
- The Ganges shark remains under-researched and with challenges like low population size, long gestation period and small litter sizes that prevent deeper study about it.

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The Ganges shark, Glyphis gangeticus, is listed as a critically endangered species on the IUCN red list. The species is so rare, that after a single sighting in 2006, the species was not seen again until 2016, when it re-emerged at a local Mumbai fish market.

Typically found in the Ganga, Hooghly, Mahanadi, and Brahmaputra rivers of India, the distribution of the species recently expanded when, as the result of various genetic studies, the Borneo river shark (G. fowlerae) and the Irrawaddy river shark (G. siamensis) were reclassified as part of the Ganges shark species. The habitat range of the species is now thought to include rivers in Pakistan, Myanmar, Borneo, and Java.

Given the species’ relatively conventional appearance, it is often mistaken for the more common bull shark. However, unlike the bull shark, the Ganges shark has two spineless dorsal fins (with the second being half the length of the first), an anal fin, and a broad, rounded snout that is significantly shorter than the width of the mouth.

Reaching up to 178 cm in length, the Ganges shark is typically characterised by a uniform grey or brown colour and has no distinguishing markings of any kind. The eyes are also exceptionally small and tilted, with nictitating eyelids that may be indicative of a physiological adaptation to living in largely turbid rivers with poor visibility. Additionally, the species has two sets of teeth, one upper set that is broad, serrated, and triangular in shape and a lower set that is long and protruding, with underrated cutting edges.

Because this species is part of the Carcharhinidae family of sharks, they are ‘requiem species’ characterised as migratory, live-bearing sharks that live in warm waters, much like the more common tiger shark and grey reef shark. However, unlike many other species of sharks, the Ganges shark is regarded as a true river shark and is only found within the middle and lower reaches of freshwater, inshore marine, and estuarine ecosystems.

The species is also thought to travel by as much as 100 km in either direction of its place of birth. However, this migration is generally not considered to be for breeding purposes, since new-born individuals have been found in the Hooghly River, suggesting that female sharks actually give birth in freshwater.

Although their feeding habits are largely unknown, it is presumed, from their backward tilted eyes and slender teeth that the species trawls for small marine fish and stingrays along the bottom of the river. Individuals will then use olfactory cues and electroreception to attack prey species, from below, that are back-lit by the sun.

While Ganges sharks are thought to be violent, blood-thirsty man-eaters, the brutal attacks associated with the species are more likely the result of confusion with the much more common bull shark. This is likely due to the fact that both species share the same habitats since bull sharks are known for their long-distance migrations into freshwater systems. However, unlike the bull shark, which has much stouter teeth, the Ganges shark has relatively long, sharp teeth that are much more suitable for impaling fish than dismembering mammalian species, making it unlikely that Ganges sharks would actually attack humans. This is also incredibly unlikely given the species low population numbers.

The Ganges shark remains under-researched and with challenges that prevent deeper study about it. First, the species is an apex predator and as such, is characterised by generally low population sizes, as well as long gestation periods, delayed maturity, and small litter sizes. These attributes make the species extremely susceptible to extinction, even at low levels of population decline. Additionally, the relatively narrow habitat range of the species makes it even more susceptible.

Factor in overall declines in population size as a result of overfishing, poaching, habitat destruction, increases in river use and pollution, and you have a species that is on the brink of extinction.

The Ganges river basin, which the species inhabits, is densely populated and home to more than 400 million people. It is also one of the most heavily polluted water systems in the world and is regularly inundated with raw sewage, litter and industrial waste.

Accordingly, local shark populations have been decimated by pollution, as well as harvesting of the species for meat, oil, and fins that are typically sold in local markets.

The shark is protected under Schedule 1 of the Wildlife (Protection) Act, 1972 and capture, killing or sale of it is punishable by law. But with the lack of research, it is uncertain whether the protection of the Ganges shark under the Act has actually had any really impact on the species’ overall population size. Additionally, there are no provisions within the act that protect the species against abrupt population crashes, making it critical that locals develop a better understanding off the species and its benefits to the local environment.

Like many apex predators, Ganges sharks are thought to help regulate the size of prey populations and provide a level of ecological stability within the river’s larger ecosystem. It is also possible that a reduction in apex predator numbers could lead to an explosion in mesopredator populations, resulting in the possible extinction of local prey species.

This story was first published on Mongabay, click here to access it…