Why springtime is shark time in coastal Florida waters

BOCA RATON, Fla. — For at least one day, shark research was interrupted by President Trump.

An executive order was not to blame, and no doubt the president was completely unaware of the interference, but Stephen Kajiura, professor of biological sciences and director of Florida Atlantic University's Elasmobranch Research Labratory in Boca Raton, was forced to alter a shark study now in its seventh year because of Monday's airspace restrictions over Trump's Mar-a-Lago estate.

Since 2011, and largely thanks to funding from the Colgan Foundation, Kajiura and his students have been conducting a shark survey along Florida's coastal waters. Flying at about 500 feet above the water along the beach from Boca Raton Inlet to Jupiter Inlet each week, Kajiura has been recording high-definition video of sharks in the water. The video is brought back to the lab, and the sharks are counted.

One by one. Thousands of them. Unfathomable numbers of sharks.

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The 5- to 6-foot-long sharks are right there in the water, where swimmers, snorkelers, boogie boarders, surfers, paddle boarders, tourists and kite boarders are enjoying a day at the beach.

But that's a good thing, Kajiura said. With all those sharks and all the people nearby, one would think the number of shark bites would number in the hundreds.

Dense sharks

"During the months of February and March, people swimming at a Palm Beach County beach are no more than 60 feet away from a shark," Kajiura said. "It's amazing the density of sharks we see and there are extremely few interactions with people."

How dense? According to data Kajiura and colleague Shari Tellman published in the journal article "Quantification of Massive Seasonal Aggregations of Blacktip Sharks in Southeast Florida," the maximum number of sharks counted was 12,128 in an area measuring a little more than 29 square miles. That breaks down roughly to an average of 415 sharks per square mile, but in a rectangular "box" that extends no more than 600 feet off the water line at the beach.

This year, the blacktip sharks are late, though. In all likelihood, they are still somewhere in the waters along the Treasure Coast and Space Coast, and possibly points north.

"Sometime in mid- to late January is when we usually see the sharks come down into Palm Beach County," Kajiura explained, "but this year, they seem to be late in arriving. They should be here any day now. They spend about three months here in shallow waters offshore kind of milling around before they begin heading back north again."

Kajiura and others in the shark studying business are not quite sure why the sharks come, or what they specifically do while they are in South Florida, but they know why they leave. It's to follow the bait migration as it begins moving back north.

"South Florida is the southern terminus of this migration, somewhere between Boynton Beach and Jupiter," Kajiura said. "We don't see them off Miami or farther south, really. I'm really not sure why not, it's the same habitat, really, but we tend to see hundreds there instead of thousands. They may not migrate that extra 70 miles due to food preference."

Kajiura said water temperature has a lot to do with where the sharks go and when they migrate. By March and April, South Florida waters are nearly empty of blacktip sharks as they begin to head north again.

"The reason they are late may be the water temperature," he said. "It's still a little warm for this time of year compared to previous years, which were 2 to 3 degrees cooler. Blacktip sharks have a tight thermal preference for temperature range, usually between 70 to 77 degrees."

Toothy critters

Bites draw a lot of media attention. Most shark bites are unprovoked, and 2016 was a pretty typical year for shark vs. human interaction, said George Burgess, curator of the International Shark Attack File.

Worldwide, there were 81 shark bites in 2016, down from the record 98 set in 2015. As is typically the case, Florida led the way with 32 of the 53 bites occurring in U.S. waters, and nearly half of those occurred in Volusia County.

Burgess has said the event of a shark bite is more a function of human behavior than it is of shark behavior.

"A shark attack is a human phenomenon,” he said. “Sharks are a natural part of the ecosystem. The ocean is a foreign environment to humans, and when we enter the sea, we’re entering a wilderness.”

In the past two months, there have been seven bites in Florida, including three in Brevard County. Swimmers are advised to stay in groups, not wander too far from shore, swim at guarded beaches, avoid being in the water during darkness and twilight hours, and avoid entering the water in places where fishing is taking place or where fish are actively being preyed upon.

'Snowbirdy sharks'

Scientists are working on several projects to understand coastal shark movements in future years. Matt Ajemian, who works out of Harbor Branch Oceanographic Institute at Florida Atlantic University in Fort Pierce, has been working on tagging studies with staff at the Smithsonian Marine Institute in Fort Pierce to understand movements and migrations of cownose rays.

The rays — at one time considered a pest to shellfish-harvesting operations in the Chesapeake Bay area so much so that bowfishing tournaments sprang up to control them — migrate to the Treasure Coast area each winter. Researchers are placing acoustic tags in the rays to track their movements.

But while doing so, Ajemian also is keeping an eye out for bull sharks in the waters of the Indian River Lagoon. Researchers also are tagging the sharks to understand more clearly how long the young bull sharks remain in the estuary, where they travel and when.

"We know with the bull sharks there is believed to be a Florida population and another population that is more 'snowbirdy,'" Ajemian said. "What are the population connections?"

Ajemian said the bull shark work also can lead to understanding what happens when there are harmful discharges from Lake Okeechobee pouring through the St. Lucie and Caloosahatchee rivers.

Kajiura is also taking his research in a different direction. Soon, he said, his team will tag blacktip sharks with satellite tags, kind of like the satellite tagging program OCEARCH has been doing with great white sharks such as Katharine, Mary Lee, Lydia and more. The 10 transmitters could identify where the sharks are in real time, as the OCEARCH tags do.

"We’re going to put satellite transmitters on blacktips this year and see if it works," he said. "The antenna on the transmitter has to break the surface of the water, so it relies on the shark coming up to the surface. I simply don’t know if they are going to do that often enough to give us meaningful data."

Sharks will soon be gaining more followers. And that is a good thing for the sharks.

Shark bites in Florida

7 in eastern Florida counties in past two months (Brevard 3; Volusia, Indian River, St. Lucie, Martin, 1 each) 
32 in Florida in 2016
30 in Florida in 2015
23.8 is five-year average

Source: International Shark Attack File

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