"That means sharks are getting more aggressive about finding alternative food sources, and are more likely to come closer to shore where they mistake swimmers for prey," said Condrey.

Condrey formed his hypothesis after he and and LSU graduate student Kevin Barry studied the daily stomach contents of blacktip sharks off the Louisiana coast for three years to learn what and how often they ate.

The researchers found the rapid growth rate of blacktip sharks needed to be supported by a constant supply of food, primarily menhaden fish.

While the Gulf Coast had a large stock of menhaden with 50 percent of the adults reproducing under current fishing conditions, Condrey said the Atlantic menhaden population has dwindled to a 3 percent reproduction rate.

Menhaden, which are about 5 to 7 inches (13 cm to 18 cm) long and are similar to herring or anchovies, are a big component in chicken feed and have been at the center of an overfishing debate on the East Coast.

Condrey said shark attacks last summer in the Virginia Beach area may have been connected to the decline in menhaden.

"Virginia Beach is right next to what is considered a primary nursing ground for menhaden, which is Chesapeake Bay. The reduction in availability of menhaden for Atlantic sharks may have resulted in a change in their foraging strategy," he said.

"It occurred to me that we might be overlooking the obvious," Condrey said.

There were 76 unprovoked shark attacks worldwide last year, down from 85 attacks recorded the year before, according to the International Shark Attack File (ISAF) that is based at the University of Florida's Museum of Natural History.

The organization said the number of shark attacks in a given year is directly correlated to the amount of time that humans spend in the sea.

"As the world population continues its upsurge and interest in aquatic recreation concurrently rises, we realistically should expect increases in the number of shark attacks and other aquatic recreation-related injuries," ISAF said.

However, the group said there are several steps swimmers can take to minimize the chances of a shark attack. Their advice:

* Stay in groups as sharks are more likely to attack a lone individual.

* Avoid the water during darkness or twilight hours when sharks are most active.

* Don't wear shiny jewelry because it reflects light that resembles the sheen of fish scales.

* Avoid bright colored clothing as sharks see contrasting colors particularly well.