The Gulf Of Mexico: A Shark's Nursery?

Research that began as a study of the bycatch of a small fish has developed into a study of sharks using the northern Gulf of Mexico as a nursery.

Janaka de Silva's dissertation research centers on the bycatch of the Gulf of Mexico menhaden fisheries. Menhaden is a small, saltwater fish used in poultry feed and cooking oils that can grow as large as 10 to 12 inches long, weighing half of a pound.

His findings are particularly relevant since national efforts to restore over-fished sharks have not considered this bycatch in the menhaden fishery, Condrey said.

Bycatch is the incidental harvesting of non-target species.

After spending weeks on menhaden fishing boats, the LSU researchers noticed that the menhaden nets contained a large number of adolescent sharks on an irregular basis. During one incident, the researchers counted as many as 145 sharks in one set.

While it would be normal to have a couple of sharks in the nets, de Silva and his major professor, fisheries expert Richard Condrey, wondered why there was such a large number of sharks periodically in the nets.

De Silva hypothesized that the shark bycatch is the result of sharks using the northern Gulf of Mexico as a nursery in which menhaden is an important source of food. He was invited to present his findings at an international symposium in Sri Lanka in December.

Condrey said that de Silva's research is important because people who are making decisions concerning sharks are focused on the Atlantic Ocean's and Gulf of Mexico's direct shark fishing industry and national efforts to restore over-fished sharks have not considered them as bycatch in the menhaden fishery. But if the Gulf of Mexico is a birthing and nursing ground for sharks, then the governing boards will have to consider the importance of this area and its bycatch.

"I've lived in Louisiana all my life. And when I saw a shark that was a meter (3 feet) long, then that's a big shark to me because that was all that I saw living here in Louisiana. But my personal hypothesis is maybe it's the biggest shark that I've seen because Louisiana is the pupping ground (nursery) for sharks. Sure those sharks will grow and get bigger, but if our hypothesis is true, they won't do that off the coast of Louisiana."

Most fish populations include a lot of young fish and a smaller number of mothers and fathers, but sharks are different. They have a longer life span, and it can take several years for one shark to reach sexual maturity. Even then, most sharks give birth to one or several offspring. "In contrast to most fish there are nearly just as many young sharks as there are adult sharks. So you can't really say which age group is the safest to take from," de Silva said.

One fisherman catching 145 sharks once would probably not have devastating effects on the shark population, but overall, if a large number of these event occur, "I think this would definitely have an impact on the shark population.

"And to restore shark populations, all the fishermen might have to do is be aware when the possibilities of catching a shark might be high. In the long run this could be best for the sharks and the fishermen," de Silva said.

But Condrey is cautious about knee jerk measures to protect sharks. "This creature has been a major predator on Earth for 400 million years; who knows what will happen if its population is shifted? We need to address this question with as much information and data as possible. But this information needs to be out there, and that is why Janaka is presenting this paper in Sri Lanka," Condrey said.

"The Gulf is a perfect place for young sharks to grow. The waters don't attract the larger 15 to 20 foot sharks that one would see in the deeper waters, and the food supply, menhaden, is bountiful. I think sharks spend the early part of their life cycle growing and living within several miles off Louisiana's Gulf Coast," de Silva said.

Sharks have fascinated humans for hundreds of years. Sharks were on earth 100 million years before the first dinosaur, and the shark is often called the perfect predator.

While Condrey realizes that he is not a shark biologist, his years of studying marine life make him believe that de Silva's hypothesis has some validity.

"Even if we prove that a large number of adolescent sharks are part of the menhaden bycatch, we still have more information to gather and harder questions to answer," Condrey said.

Sharks are considered apex predators, which means that they are at the top of the ocean's food chain. Condrey has generally cautioned against rash movements to selectively harvest any species without considering the ecological consequences.

"This is especially true for sharks. These creatures have been in the waters for more than 400 million years, I don't think we can accurately say today what would happen if some sharks were totally protected while others were not. That might have devastating effects on the ecosystem," Condrey said.

De Silva emphasized that any attempt to protect or control the shark population would need to have international support since a shark's life cycle can take it across national borders, with the northern Gulf of Mexico being one important stop.

De Silva was invited to present his findings because of his inventive approach to a problem with international implications. The symposium marked the beginning of an on going process of promoting meaningful development in the third world, Condrey said.