The strange world of river sharks

Worldwide there are more than 500 recognised species of sharks and that number is continuing to grow as new species are discovered lurking in the depths of the world’s oceans. Encompassing everything from the well known great white and hammerhead sharks, to little known oddities such as the deep-sea dwarf lanternshark or the enormous plankton feeding whale shark; these fish are the undisputed masters of the sea.

Yet each year, directly and indirectly, over 100 million sharks are killed leading to 30% of species being listed as threatened. These monstrous predators have proven no match for indiscriminate trawling and long line fishing. Combined with the perceived threat to swimmers and the fashionable Chinese shark fin soup, the future is bleak for a group of fish that has roamed the oceans unhindered for more than 400 million years.

Surprisingly, the oceans are not the only place that sharks roam. There have been whispers, glimpses and a mere handful of specimens that point to the existence of sharks inland. These little known cousins are the river sharks, and the race is on to save them before they vanish.

So what are river sharks, and why are they special? First, a quick lesson on fish biology: rivers are a very different place than the oceans, so for fish that evolved in a salty environment it is extremely difficult to transition to fresh water. In fact with a lack of salt, most fish would take on too much water through osmosis and eventually die, this explains why freshwater and salt-water species tend to stay separate. So although a few dozen shark species have been found a short way up rivers, those found to live there permanently are likely to be different river adapted species.

There are currently three known species of river shark all from the genus Glyphis, with up to three more species in the process of being described by scientists. Their range is uncertain, but all seem to originate from Southeast Asia, New Guinea and Australia, relying on coastal lagoons and tidal rivers. One is listed as endangered and two as critically endangered.

First up is the Speartooth shark (Glyphis glyphis), which first became known in 1839 from a single stuffed specimen. This remained the sole known record until the late 20th century when a population was discovered in Australia. A recent assessment of 38 different river systems concluded that the global population must be fewer than 2500 individuals and is likely to be declining. Closely related, is the New Guinea River Shark (Glyphis garricki), which is thought to be even rarer at less than 250 surviving individuals and occupies a similar area. The third and final species (for now) is the Ganges shark (Glyphis gangeticus), perhaps the rarest of them all. First identified in 1867, none were recorded for over a century, with a handful of photographs and unconfirmed specimens occurring in recent years.

The key threats to these little known sharks are illegal and unregulated fishing, habitat degradation and pollution, which is especially problematic in the Ganges. Because rivers cover a much smaller area than the open ocean, these fish are likely to have historically small populations and be particularly vulnerable to these pressures. Whilst it is also unknown how far up rivers these fish may travel, there are also concerns about the harmful impact of dams on these movements.

In some areas these little known sharks are reputed to be ferocious man-eaters, but most attacks are probably confused with bull sharks, an oceanic species that can spend periods of time surviving in fresh water. Indeed, bull sharks may be a major obstacle to conservation, because many river shark museum specimens and literature records are in fact much more common species that have been misidentified.

To get an idea of quite how rare these fish are, we spoke to Jonathan Davis a shark expert from Texas Parks and Wildlife. ‘I have been studying sharks for years, specifically bull sharks that transition into freshwater from salt water. I have travelled all over the world and studied other sharks as well, but I’ve never actually encountered a river shark.’

There is however some good news. Recent conservation efforts implemented by the Australian government have focused on Kakadu National Park; a protected area in Australia’s Northern Territory thought to be important for pupping (reproduction) in some species.

It is hoped a strategy to reduce fishing pressures and increase awareness will also benefit the sawfish, a closely related (and equally odd) group of threatened fish facing similar threats. Yet the main obstacle to conservation is simply that we know so little. What do they eat? How often do they reproduce? What is their range? Do they migrate? All questions that we need to answer, and quickly, before these ancient fish disappear forever.


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