Sharks play an essential rol in the health of our oceans

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A fresh view on fresh water sharks!

by Dorien Schröder, Dutch Shark Society 

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We have talked before how the over 500 species of sharks exhibit such a great amount of diversity in the way they look, feed and their habitats. But few people know that sharks aren’t limited to the ocean, they can also be found in estuaries, rivers and lakes!

There are 43 elasmobranch species that have been reported in fresh water. This includes some species of stingray, sawfish, skates, dogfish and sandbar sharks. The ability to enter fresh water is usually limited because the blood of elasmobranch is at least as salty as their environment through the accumulation of urea. However, elasmobranchs found in fresh water show a significant reduction of urea in their blood. They also decrease salt excretion through the rectal glands to conserve their salt balance.

The most famous of the fresh water sharks is the bull shark. They are normally found in the coastal areas of warm oceans and are known to swim up rivers. The Brisbane river in Australia is said to have a population of 500 sharks living there and some were reportedly seen swimming the streets after the flooding in 2011. In South America one was found 4,000 kilometer up the Amazon River, near Iquitos in Peru. The bull shark doesn’t only tolerate fresh water, but also extremely high salt contents, such as in the St Lucia Estuary in South Africa. Young bull sharks are normally born in fresh water and as they age their tolerance for salt water increases and they move towards estuaries and coastal waters, only to move back up river to reproduce. There are less predators in the rivers, as most other sharks cannot live there, giving the young bull sharks a better chance of survival.

Less well known are five rare species in the genus Glyphis, the Borneo river shark (named after shark scientist Sarah Fowler), Ganges shark, Northern river shark, Speartooth shark (image) and Irrawaddy river shark. They can be found in South East Asia and Australia. They are true fresh water sharks, meaning they never go into the ocean. They are all most likely to be viviparous, giving birth to live young, like the bull shark. Not much is known about these sharks as the murky conditions in which they live doesn’t lead to many sightings and few specimens have been caught. All are endangered because of the small population sizes.

 

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Speartooth shark

 

A short clip of a Glyphis spec., caught in a river, tagged and released:

 

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