Shark-Swarming Season
The largest shark migration in U.S. coastal waters occurs every winter off Florida during the peak of tourist season
- Michael Lipske
- Animals
- Jan 27, 2014
EVERY WINTER, THOUSANDS OF HUNGRY BLACKTIP SHARKS migrate along the Atlantic coast of Florida, often swimming within a few feet of shore. Shari Tellman’s reaction? “I would love to put on a snorkel and fins and go out in the middle of them.”Tellman is not a thrill seeker, however. Having studied blacktips, she knows that the sharks, which average about 5 feet in length and are named for the black markings on the tips of their fins, usually have eyes only for the schools of herring and other small fish they feed on. The largest annual shark migration in U.S. coastal waters, it coincides with Florida’s winter tourist season, though many people happily splashing in the surf may never realize they could be surrounded by predators.
As a graduate student in biology at Florida Atlantic University, Tellman regularly monitored the shark migration along the Palm Beach County coast from a small airplane during the past three winters. Last February, during the peak of the migration, she photographed as many as 12,000 sharks—both blacktips and slightly larger spinner sharks—during a single flight.
Each year, the sharks head for bays and estuaries along the southeastern U.S. coast to mate and give birth. Female blacktips and spinners bear live young (as many as 10 among blacktips and up to 15 among spinners) every other year. The juveniles reach maturity in shallow nursery grounds away from the adults.
Stephen Kajiura, head of the shark study and an associate professor in biological sciences at Florida Atlantic University, worries about the impact of commercial fishing on the sharks during migration, when so many of the creatures are concentrated in a relatively small area. He also is concerned about climate change because the shark migration correlates closely with water temperatures.
“Sharks seem to prefer a temperature of 22 to 24 degrees C [about 73 degrees F], which typically is found in more northern latitudes during summer and in South Florida during winter,” he says. “As global water temperatures increase, warmer water will be found at increasingly higher latitudes. So sharks may no longer migrate as far south and we might not see the large seasonal influx of these top-level predators.” Such a change, he says, eventually could wreak havoc on the local ecosystem.
On occasion, the sharks themselves have been the ones wreaking havoc. Last year’s migration coincided with spring break—when thousands of college students descended on the Florida coast—so beaches were closed in some areas. Over the years, blacktips have been blamed for about 20 percent and spinners for 16 percent of unprovoked shark attacks in Florida, according to the International Shark Attack File maintained by the Florida Museum of Natural History. None of the attacks were fatal.
However, Tellman thinks that at least some of those incidents may not have been attacks in the usual sense of the word. In certain situations, such as in more turbid waters farther north on the Florida coast, sharks sometimes may have trouble distinguishing the fish they’re chasing from a swimmer or surfer splashing in the water. That, she says, is more of a mistake made in the heat of the hunt rather than an attack.
Tellman sees the annual migration as something to celebrate rather than fear. “The fact that the sharks are around and in good numbers means our ecosystem is in balance,” she says. “So in that respect, it’s good news, definitely good news, not bad news.”
Writer Michael Lipske is based in Washington, D.C.
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