Bull Sharks In Rivers

Saltwater and freshwater have different specific gravity, or salinity, or however you want to refer to the chemical concentraions of the two different bodies of water; simply put, osmosis is a huge factor in an organism's ability to live in water.

Osmosis can be defined as the movement of water through a membrane due to the differences in concentrations of two solutions. This is why you always hear people say it is a bad idea to drink saltwater because it will actually dehydrate you. The saltwater is at a higher concentration than the cells in our bodies, so due to the natural forces of osmosis, water will leave our "less concentrated cells" and enter the "higher concentrated saltwater" until they become equal (physical laws of science love to be at equilibrium). The result is the loss of water from our cells which is no good.

OK, so how does this relate to sharks?

The forces of osmosis means that animals must either be adapted to freshwater or adapted to saltwater. Saltwater fish live in the ocean, and freshwater fish live in rivers, lakes, etc. Marine fish must go against the laws of physics in order to retain water to avoid dehydration due to osmosis, while freshwater fish must go against the laws of physics to prevent the absorption of too much water.

So then how the heck does a saltwater Bull Shark survive in freshwater?

Bull sharks can survive in freshwater rivers due to a feature called osmoregulation. Osmoregulation is the ability of an organism to maintain a constant concentration of water in their body even when the surrounding environment would cause it to either gain or lose water. Remember how I was saying fish have to be adapted to either freshwater or saltwater due to the large differences in salinity? The ability of bull sharks to undergo osmoregulation means they can survive in both saltwater AND freshwater!

Bull sharks may breed in freshwater, and there are advantages to it. Freshwater habitats such as estuaries and rivers have features that are suitable for smaller sharks to survive. There is cover (or shelter) and there is food.

The young 3 foot Bull shark in the image was caught in Australia's Brisbane River and is believed to be approximately 18 months old. It is also thought to be unlikely that a shark of such young age traveled all the way up the river. Instead, it is believed that the shark was born up river.

Genus: Carcharhinus
Species: leucas

Size: Up to 12 feet
Weight: Up to 500 lbs

Geographic Distribution: Map shown below. Bull sharks inhabit coastal regions and have been reported over 1,000 miles inland in South America's Amazon River, and even all the way up the Mississippi River in the United States!

Habitat: The Bull shark inhabits somewhat shallow coastal regions around 100 feet or less and even enter estuaries, bays, and river mouths. Bull sharks breed in estuaries, and even up river, which allows the juveniles to thrive in the protected habitat and feed off of fish living in those waters.

In a very historic event that seems like it has been taken out of a fairy tale, several people were killed after being attacked by a large shark (eye witnesses say about 10 feet in length) in Matawan Creek. Matawan Creek is actually a tidal creek with both saltwater and freshwater.

It was said, at that time long ago, that the culprit was a Great White shark. However, knowing what we know about sharks today, it is much more likely that it was in fact a Bull shark that had traveled up the river. A Great White would not theoretically be able to survive in the described environment, although it is said that the conditions on that particular day contained unusually high salinity in the creek which would make it more believable that a Great White may have survived in the water for a very short time. The culprit of the Matawan Creek shark attack remains a mystery.