Bull Sharks, Carcharhinus leucus, In Coastal Estuaries

Tuesday, August 24, 2010

Nature Notes
by Bob Thomas

There is a persistent rumor that there are sharks in coastal estuaries such as Lake Pontchartrain – and the rumor is true.

We now know that each summer as coastal waters warm the Pontchartrain and Barataria-Terrebonne Basins are invaded by huge numbers of bull sharks (Carcharhinus leucus), one of the species considered potentially dangerous to humans. But there is no need for fear. The invaders are immature with most being 4-5 feet long, and the rare report of a 6-footer.

Bull sharks that enter the local estuaries are born (yes, they are born alive – not hatched from eggs) outside Louisiana’s barrier islands, and it appears that they make a beeline for their nursery grounds – the fresher and highly biodiverse estuaries. Evidence suggests that some may return for a couple of summers.

There are two principal reasons they do so. One is that they have no natural predators in the estuaries. In fact, they immediately become the top predator in those habitats. Their main nemesis is a gill net set by humans. Nothing in their multi-million year evolution prepared them for this intervention into their habitat.

Their second reason for entering the estuaries is that there is an abundance of food for animals their size. Evidence collected by the ecological team led by Dr. Marty O’Connell, professor and research scientist at the University of New Orleans, shows that their favored foods, hardhead (Arius felis) and gafftopsail (Barge marinus) catfish, enter the estuaries at the same time. Although they have this preference, they will eat many other estuary species as encountered.

Safety from large predators and a full stomach are powerful incentives.

Bull sharks are well known invaders of freshwater areas. The most inland specimen captured in the United States was taken in the Mississippi River near Alton, Illinois in 1937, over 1800 miles upstream from the Gulf of Mexico. They enter freshwater systems all over the world, and are especially known for their presence in the Zambezi River, the Amazon found near Iquitos, Perú some 2500 miles from the mouth, the Ganges, and Lake Nicaragua.

In fact, scientists believe that a large bull shark was responsible for the vicious attacks, purportedly by a great white shark (Carcharodon carcharias), in a New Jersey creek that inspired the book and movies about the escapades of Jaws.

In recent decades, shark numbers have plummeted, largely due to people slaughtering them out of fear. Jaws still has a horrifying effect on human attitudes about sharks. As a result, all sharks are viewed as man-eaters, even though only a handful of species are known to attack humans and then only on rare occasions. Scientists believe that most, if not all, attacks on humans are a case of mistaken identity. The attacks are often associated with an abundance of the sharks’ normal food, and the shark, usually a small one, slashes out and inadvertently bites a human. Another stimulus is a human moving about in murky water. The bull shark simply attacks the blurry moving object.

That said, the bull shark is known to unpredictably become aggressive and attack humans. Since they can reach 12 feet in length and weigh up to 500 or more pounds, their attacks may do considerable damage or be lethal.

A devastating impact on shark populations is the finning fishery. Fishing boats hook thousands of them to supply the world shark-fin cuisine market. Sharks of many kinds are hauled to the surface, dorsal fins are removed, and the sharks are released to die a slow death.

Being the apex (top) predators of the sea makes sharks very important ecologically.
Occupying the top of the food pyramid in the marine domain, sharks, like other apex predators such as lions, jaguars, and polar bears, give birth to relatively few young (6-10 per year) that take years to become sexually mature. When they are killed in vast numbers, it is no wonder that their populations nose-dive and either do not recover or remain at unnaturally low densities.

When apex predators in any ecosystem are removed in large numbers, the ecological consequences are dire as the apex predators’ food populations tend to expand rapidly in the absence of control. The result is ecosystem devastation or collapse.

We all dream of a renewed and healthy coastal system of estuaries. An important characteristic of such an ecosystem is a balance among its components – and bull sharks are key to maintaining that balance.

Also published in Delta Journal, The Times Picayune, April 6, 2008.


Chris Davis, current graduate student at                                         Jenny Wolff, undergraduate student at the
the University of New Orleans, and a typically                                 University of New Orleans, with a Lake
sized Lake Pontchartrain bull shark,                                               Pontchartrain bull shark. Summer 2009.
Carcharhinus leucus, sporting a green Floy                                     Photo by Chris Schieble.
tag bearing a number that helps scientists
gather data on growth and movements.
Summer 2009.
Photo by Chris Schieble.


Sunny Brogan, former graduate student at
the University of New Orleans, with another
Lake Pontchartrain bull shark. Note the green
Floy tag just below the dorsal fin.
Summer 2009.
Photo by Chris Schieble.