At the first hint of warm temperatures, they begin to head toward South Carolina.
They could be looking for a mate.
Or a tasty morsel of fresh seafood.
But they're not tourists from up north.
They're lemon sharks.
The apex predators flock to the waters off the Carolinas' sea islands from April to late October, but scientists aren't sure why.
Steven Kessel, a researcher from Cardiff University in Wales, traveled to Hilton Head Island from Miami last week to try to answer that and other questions about the species.
Working with captain Chip Michalove of Outcast Sport Fishing Charters on Hilton Head, Kessel and a research assistant fished the waters of Port Royal Sound for the sharks so they could attach acoustic transmitter tags to track their migrations.
The researchers and the charter boat's customers cast heavy-duty fishing lines into the choppy waters between one and three miles from shore.
Kessel hoped to land at least two lemons to learn, among other things, where they mate, where they give birth, and why lemons from Florida are in the Carolinas during the summer.
"I know my sharks are coming here from Florida, but I don't think they're giving birth this far north," Kessel said from the deck of the Outcast. "I think by the time they get to South Carolina, they've dropped their pups in either north Florida or Georgia. If we can figure all that out, then we will be able to document almost their entire biological life cycle."
GOING AFTER LEMONS
On Monday, anglers stood at the stern, watching as a line jerked on the surface.
It didn't take long to get a strike.
They hauled to the surface a lemon that was nearly 8 feet long, 200 pounds.
Kessel, research assistant Ornella Weideli, a master's program student from the Swiss University of Basel, and the crew muscled the shark to side of the boat. A water hose was inserted in its mouth to prevent respiratory problems.
Moving quickly, Kessel drew blood, took a small DNA sample from a fin and injected a tiny chip beneath the skin that can be read by a handheld electronic scanner, the same technology veterinarians use to identify household pets.
The scientist then made an incision a little longer than an inch and inserted a small acoustic transmitter tag, which sends signals to nearly 400 underwater listening monitors installed by a variety of research groups from Florida to the Carolinas.
The entire process took no more than 20 minutes but left the crew breathless, Michalove said.
"All my groups were happy to have the scientists on board," he said. "You get the fisherman side and the scientists' perspective. They've never seen someone pull an 8-foot shark aboard, slice it open and then sew it back up before, all while it's awake and pretty wiry. I've seen some pretty wild stuff out there, but I was pretty impressed."
THE PERFECT SPOT
The group fished in an area Michalove calls the "shark hole."
That's where Illinois dentist Stephen Lieson hauled in a monster, 380-pound, state-record lemon last summer on the Outcast, a catch that got Kessel's attention.
He had a hunch it migrated from the Jupiter, Fla., area, where Kessel and other researchers have tagged nearly 50 lemons.
Not long after the record catch, Michalove snagged another large lemon. This one had a National Marine Fisheries Service tag, indicating it was from the group Kessel had been studying in Florida.
Michalove contacted the service, which wrote to Kessel.
It's that sort of communication that aids research efforts, Kessel said.
"We rely on local knowledge -- it's the first step to any study," he said. "So when Chip and others who fish here day in and day out catch lemon sharks, it matches up with my hypothesis of seasonal migration."
Kessel and Michalove's crews caught about 20 sharks of eight species, including hammerheads and blacktips, and took DNA samples from them. They inserted acoustic tags into three lemons, each one about 8 feet long.
Kessel says the lemons can help researchers understand the species' life cycle, which in turn, could advance conservation efforts.
The work of Kessel, the Swiss Shark Foundation and colleague Dr. Samuel Gruber -- professor emeritus at the University of Miami, who has been studying lemons for more than 40 years -- helped the sharks win protected status in Florida last year, Kessel said.
Now, Kessel hopes the Port Royal Sound study will provide insight into the species' life history to help better protect them along the entire south Atlantic coast.
Kessel believes the migrating lemons probably give birth in northern Florida or Georgia in rivers or close to shore where they are protected from other sharks.
"Obviously sharks don't respect state boundaries," he said. "The most pressing question is where, exactly, are they giving birth? If we can answer that, we may be able to better protect their nursing habitats."
But if you're a lemon in Florida, why come north?
"I'd say it's likely a physiological factor that they're coming here. They're cold blooded so they follow the warm water north and then are pushed south into Florida in the fall," Kessel said. "Maybe they're following the bait source or it could be a factor of reproduction. There's still a lot of unanswered questions, which is why we're continuing to research."
Follow reporter Cassie Foss at twitter.com/LcBlotter.
Researcher studying the sounds of area's waters, July 17, 2011
Dentist lands a toothy record -- South Carolina's biggest lemon shark, July 23, 2010