sheens in the wood

There is a persistent rumor that there are sharks in Lake Pontchartrain – and the rumors are correct.  Years ago, an angler would occasionally land a large shark along the lake front, and a photo usually found its way to the local newspaper.

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

Bull sharks that enter the lake are born (yes, they are born alive – not hatched from eggs) outside Louisiana’s Chandeleur Islands, and it appears that they make a bee-line for their nursery grounds – the Lake Pontchartrain Basin estuaries.  There are two principal reasons they do so.  One is that they have no natural predators in the estuary.  In fact, they immediately become the top predator in the lake ecosystem.  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 lake is that there is an abundance of food for animals their size.   Safety and a full stomach are powerful incentives.

Dr. Bruce Thompson, marine scientist with LSU’s Coastal Fisheries Institute, has studied bull shark populations in coastal Louisiana for the last couple of decades.  He has collected juvenile bull sharks throughout the lake, but has never been able to verify the presence of a mature individual.  According to Dr. Thompson’s records, there has never been an attack on a human in the Lake Pontchartrain Basin.

Recent studies by Dr. Martin O’Connel, of UNO’s Pontchartrain Institute for Environmental Sciences, yielded 18 bull sharks between 2000-2003, all taken off Goose Point near the mouth of Bayou LaCombe.

Bull sharks are well known invaders of freshwater areas.  The infamous Zambezi sharks of Africa are bull sharks.   This species is also the shark found in Lake Nicaragua, rivers in Gambia, and the Ganges.  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).  In the summer of 2004, an immature male was caught in a gill net in the Red River just north of Simmesport, Louisiana.  In fact, scientists believe that a large bull shark was responsible for the vicious attacks in a New Jersey creek that inspired the book and movie Jaws.

Being the top predators of the sea makes sharks very important ecologically.  In recent decades, their numbers have plummeted.  This is largely due to people slaughtering them out of fear (Jaws had a horrifying effect on human attitudes about sharks).  All sharks are viewed as man-eaters, even though only a handful of species are known to occasionally attack humans.  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 bites a human.  The result is usually a minor injury, but if the shark is large, a fatal blow may be delivered – even if there is only one bite before the shark flees.    

Another devastating impact on shark populations is the finning fishery.  Fishing boats hook thousands of them to supply the world shark-fin cuisine market.  The shark is hauled to the surface, the dorsal fin is cut off, and the shark is released to die a slow death.

Being at the top of the food pyramid in the marine domain, sharks, like other top predators such as lions, jaguars, and polar bears, bear relatively few young 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 don’t recover or remain at unnaturally low densities.

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