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Blacktip sharks are vanishing from PB County waters — why that’s not good

For eight years Stephen Kajiura has flown a stretch of shoreline from Boca Raton to Jupiter during blacktip shark season, meticulously counting each shadow in the blue shallows of the Atlantic.

Squeezed between the narrow continental shelf and swift Gulf Stream current, a unique shark bottleneck parallels Palm Beach County during the annual migration. Tens of thousands of blacktips swim a languid path along the coast, sometimes closing beaches and making national news when captured by a circling drone.

But this year, the shark tally, which is counted frame by frame from aerial video, concerns the Florida Atlantic University researcher.

The peak shark abundance — the largest amount counted on any given flight — was only 2,800.

That’s down from 12,130 in 2011, a 76 percent reduction that has been on a slow dive since the counts began.

“It’s dramatic,” said Kajiura, who runs FAU’s Elasmobranch Research Laboratory. “The numbers have plummeted and this year was the lowest we’ve had so far.”

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Blacktips come to Florida during the winter months seeking warmer waters. Typically, they leave coastal Virginia and North Carolina as snowbirds around September or October, beginning their return north from Florida around March.

But it’s possible warmer water temperatures may be why Kajiura is seeing fewer sharks. He said the lower blacktip numbers correlate directly with higher water temperatures.

In 2011, the mean water temperature along the blacktip’s Palm Beach County route was 73.9 degrees. This year, the mean temperature was 75.3 degrees.

Bottom line: The sharks may be finding their ideal temperature farther north, ending their annual migration, for example, in Vero Beach.

“Temperatures go up, shark numbers go down,” Kajiura said. “Eight years does not show global climate change, but it does show what might be happening on a larger scale and why we might not get those big numbers of sharks like we used to if waters keep warming.”

Video: South Florida man bit by shark, catches shark, says he’ll eat it

While the blacktip migration is a stirring spectacle, many types of sharks call Florida home year-round. Some rogue blacktips even spend their summers in South Florida. They aren’t the ones fishermen complain want a bite out of their catch before they can reel it in.

“Those are bull sharks, hammerheads, and sandbar sharks, all chasing bonita, which are very abundant and very bloody,” said Pete Schulz, co-owner of Fishing Headquarters in Jupiter. “The blacktip are different. They’re just following the bait schools along the beach.”

Kajiura questions what happens if the blacktips aren’t there in quantities high enough so the “annual spring cleaning” of bait fish can occur.

“If you don’t have that influx of top-level predators wiping out smaller fishes, what impact does that have on the ecosystem,” Kajiura said. “There are cascading effects.”

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Some shark researchers are less concerned, citing the eight-year study period as too short of a time frame to draw any long-term conclusions.

Sharks survived the Permian extinction about 252 million years ago, notes Gavin Naylor, director of the Florida program for shark research at the Florida Museum for Natural History.

The Permian extinction is also called “the Great Dying” because most of life on Earth was wiped out.

But not the sharks.

“I think that many things that happen do point to climate change, but sharks have been through probably the most spectacular extinction event in the world,” Naylor said. “I think they are pretty capable of dealing with a lot that’s thrown at them except wholesale targeting by commercial fisheries.”

In Florida waters up to 3 miles off the coast, blacktip harvesting is limited to one shark per person per day, or two sharks per vessel per day. In federal waters, blacktips may be commercially harvested with up to 45 sharks per vessel per trip.

RELATED: This FAU professor will appear on Discovery Channel’s “Shark Week”

And the blacktips’ metronome-like migration — the largest seasonal shark migration in the western Atlantic — makes them any easy target.

“It is heartening to see these large migrations,” Naylor said. “If you pull them out of the water, there would be consequences, but we don’t know exactly what would happen.”

Blacktip sharks have a temperature tolerance of about 59 to 86 degrees, but generally prefer 68 to 77 degrees, according to R. Dean Grubbs, associate director of research for Florida State University’s Coastal and Marine Laboratory.

Where a swarm of sharks linger during migration is likely the temperature sweet spot at that time.

“If we are in a warm year, and things are warming up fast, the blacktips may blast past Palm Beach County, so on any given day, there may be fewer,” Grubbs said. “In a cool year, they may take a little longer to migrate.”

Kajiura makes about 18 shark-counting flights each season, rescheduling trips when President Donald Trump is in town and there are restrictions around his Palm Beach Mar-a-Lago estate.

He said he knows more research is necessary to draw precise conclusions about the connection between South Florida water temperatures and fewer sharks, but “the general trend is very clear.”

“There has been an increase in temperatures over time and a decrease in sharks,” Kajiura said. “And, the answer is, apparently one degree is enough.”

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