Transmitters attached to sharks help scientists monitor changes in their migration routes.

On a violently pitching boat just outside Singer Island’s surf zone off Florida, Stephen Kajiura and a crew of students set out baited hooks for sharks. Thousands of black tip sharks come within yards of South Florida’s beaches during their winter migrations, and Kajiura, associate professor of biology at Florida Atlantic University, is catching them and implanting acoustic transmitters to study their movements.

The sharks migrate from as far away as North Carolina, coming here for the winter, along with schools of mullet and other fish they eat. Kajiura said it’s important to understand the extent, range and impact of their migrations because of the sharks’ significant impact on South Florida’s marine environment. Kajiura, who has been studying shark migration since 2011, said it is closely tied to water temperature.

“We know that South Florida is the terminus of this annual migration,” he said. “As global temperatures continue to rise, their preferred temperature may be found not here but farther north. Maybe a decade or two down the road, we’re not going to have these tens of thousands or hundreds of thousands of top predators cleaning out the reefs. What’s going to happen then?”

Since December, Kajiura has been flying in a small plane from the southern end of Miami Beach to Jupiter Inlet to survey black tip numbers and locations. His preliminary observation: the densest concentrations run from Dania Beach to Lighthouse Point and then from Boynton Inlet to Jupiter. Few sharks could be seen from Boca Inlet to Boynton Inlet.

A black tip shark hooked for the tracking project.

Black tip sharks, which typically grow to 2m, are among the most common species to bite people, although these are usually non-life-threatening incidents in which the shark clamps down on a hand or foot in murky water and quickly lets go upon realising that it hasn’t bitten down on a fish. Last year, during the late March migration, two people suffered bites. Neither bite was life-threatening.

One morning off John D. MacArthur Beach State Park, Kajiura and his three students set out to hook sharks and fit them with transmitters. Although the boat was rocking steeply, they successfully set out 60 hooks on a quarter-mile line held up by buoys. The hooks were baited with hunks of mackerel, as well as fish heads. After an hour, they began hauling up the line. They caught one. A grey-green shadow could be seen in the water, forming into the menacing shape of a shark about 2m long. But despite catching two sharks, the trip was a failure.

Normally, when they catch a shark, they implant a transmitter immediately and send it on its way. But because the water was so rough, Kajiura decided to head to deeper, smoother water to do the surgery. So they tied the two sharks to the bow and waited until they had hauled up the line before heading into deeper water.

One shark escaped. The other was successfully implanted with the transmitter but swam away so weakly after being released that Kajiura donned flippers and a mask and dove into the water to check on it. After a minute or two, he swam toward the boat, hauling the shark with one arm. A few minutes later, it was dead, killed by the stress of its encounter with people.

“That’s very rare,” Kajiura said. “Ninety eight percent of the time, the sharks get away just fine. We’ve only lost a couple of animals ever.” The shark was placed on the boat to be brought back for analysis of its stomach contents, as well as for students to study its vertebrae, muscle and skin tissue.

Since December, Kajiura has fitted transmitters on about a dozen sharks, so their signals can be picked up by the hundreds of undersea listening posts operated along the East Coast by the Atlantic Cooperative Telemetry Network. His lab, which currently operates listening posts off Singer Island, Lake Worth and South Beach Park in Boca Raton, will soon add new ones off fishing piers in Dania Beach and Juno Beach.

The transmitters give off pings at a very high frequency, so that they are inaudible to other marine organisms. These unique pings are picked up by the receivers, which record the presence of that particular shark.

“We’re interested in what these sharks are doing when they’re here, and we’re interested in their movements,” he said. “Where are they going when they leave? How far up the coast do they go?” — Sun Sentinel/Tribune News Service