River Sharks - The Riverbed Predators
Sharks are generally known as the kings of the ocean. Most documentaries keep reminding us that sharks prefer swimming out in the deep blue sea. And while this may be true for your great white and others, it appears some rare types of sharks do indeed prefer swimming closer to our habitat.
Yes, river sharks do exist. And I am not just talking about the bull shark, who is notorious for swimming up tropical rivers, but instead about a handful of shark species whose actually habitat are rivers.
Why river sharks have a reputation for attacking humans
Shark attacks in rivers are not unheard of. In India, river sharks are even known as "Man Eaters", although this reputation might be largely thanks to the bull shark, whose habitat happens to include India's Bombay, Cochin and Hooghly rivers.
Indeed, with small eyes and teeth, river sharks are believed to be primarily fish-eaters. Incidents with humans could be explained to their hunting style. River sharks have upward tilting eyes, which suggests that they swim along the river floor whilst stalking the water above for their pray. With the waters that they inhabit often being cloudy and therefore providing limited visibility, it is understandable that sometimes humans swimming along the surface are mistaken for fish. Thanks to the river sharks small teeth these attacks rarely end fatally, in comparison to the larger bull shark, whose broader dentition is far more lethal to us humans.
How come there are so few of them?
Due to fishing pressure and changes in their habitat, river sharks have become an endangered species. Sharks are slow breeders, and their long gestation, slow growth, delayed maturity and small litter size makes them particularly vulnerable to environmental changes, not giving them the chance to adapt. Sport angling and gill netting has also had huge detrimental effects on their numbers. To make matters worse a lot of the time they are not recognised for what they are. Explaining how often fishermen catch them and eat them, while only keeping the jaws as souvenirs.
So what do we know about river sharks?
Well, for starters six different species of river shark have been identified, and another four are believed to be swimming out there somewhere. Every type of river sharks seems capable of tolerating low to reduced salinity environments.
River sharks may have an inbuilt mechanism that helps separate themselves from bull sharks. This separation is thanks to the river shark's ability to adapt to waters with very low dissolved oxygen content when compared to bull sharks. Young river sharks especially benefit from this separation, as bull sharks often feed on other sharks' off-spring.
As far as size goes it has been established that adult Ganges Shark, a species of River Sharks, are around 180cm long, while newborn are between 55 and 65cm long. The size at birth or maturity is still unknown for other River Shark species.
Which muddy rivers should I not go swimming in?
As mentioned a few times, river sharks are extremely rare. Still, here are the six documented types of river sharks and their known locations:
Glyphis gangeticus (Müller & Henle, 1839)
Known Habitat: Hooghly-Ganges River Systems, West Bengal, India. Karachi, Pakistan.
Glyphis glyphis (Müller & Henle, 1839)
Known Habitat: Borneo, New Guinea, Queensland, Australia.
Irrawady River Shark
Glyphis siamensis (Steindachner, 1896)
Known Habitat: Irriwaddy River, Rangoon.
Glyphis Species A
Only two specimen found
Known Habitat: Bizant River, Queensland, Australia. Alligator River System, Northern Territory.
Glyphis Species B
Only one specimen found
Known Habitat: Borneo.
Glyphis Species C
Only nine specimen found
Known Habitat: Adelaide River, East, West & South Alligator River.
Angler Capturing a Shark:
Example of a bull shark: