Overfishing Large Sharks Impacts Entire Marine Ecosystems, Shrinks Shellfish Supply - DI Sea Lab Scientists Conduct Shark Survey in Gulf of Mexico

	Ph.D. student Marcus Drymon readies to release a shark.

Despite a public relations image that has elevated them to tabloid status, sharks play a vital role in the ocean’s food chain.  Fewer big sharks in the oceans mean that bay scallops and other shellfish may be harder to find at the market, according to an article in the March 30 issue of the journal Science, tying two unlikely links in the food web to the same fate.

 A team of Canadian and American ecologists, led by the late, world-renowned fisheries biologist Ransom Myers at Dalhousie University, has found that overfishing the largest predatory sharks, such as the bull, great white, dusky, and hammerhead sharks, along the Atlantic Coast of the United States has led to an explosion of their ray, skate, and small-shark prey species.

“With fewer sharks around, the species they prey upon – like cownose rays – have increased in numbers, and in turn, hordes of cownose rays dining on bay scallops, have wiped the scallops out,” says co-author Julia Baum of Dalhousie.

Co-author Dr. Sean Powers, a Senior Marine Scientist at the Dauphin Island Sea Lab and Assistant Professor of Marine Sciences, University of South Alabama, says, “Shark populations are uniquely susceptible to over-fishing.  Compared to other fish, sharks have very low reproductive rates and are extremely long-lived.”

Here on the Gulf Coast, Dr. Powers and DISL Marine Scientist Dr. John Dindo are in the midst of a long-term survey of Alabama coastal shark populations.  In monthly trips in 4-60 feet of water, the scientists and their teams long-lining sharks for tagging, as well as collecting tissue samples and determining the health and age of the sharks.  With 80 species of sharks in the Gulf of Mexico, 29 of them coastal species, Drs. Powers and Dindo are collecting invaluable data to help determine the status of sharks in the Gulf.
 
 

	Dr. Sean Powers

“By conducting a long-term study of the shark population along the Alabama Gulf Coast we’re trying to determine how their populations have and will change over time and what does the future hold for them.‘
 
“In the Atlantic, the removal of these top predators led to the collapse of the century-old scallop fishery in North Carolina.  We need to find out if we see the same changes in the Gulf of Mexico and evaluate the potential impact of these changes on shellfish populations.’

“The cownose ray example is just one of a growing number of studies that point out that fishery species are connected throughout the foodweb and that management of one species can have consequences for others. Consequently, efforts to manage fisheries need to take into account these multi-species or ecosystem effects,” he concludes.