Another year of fewer shark bites is being attributed to changes in blacktip shark migration, something that may change the ocean as we know it.
Fewer sharks are hugging southeast Florida’s shoreline as they make their annual migration, a significant absence noted in a new report on shark bites and a potential signal of an ailing ecosystem.
Florida Atlantic University scientists believe warmer coastal waters — 1.8 degrees in the past decade — between Boca Raton and Jupiter have thrown the sharks off course, halting their journey north of the area or pushing them farther out to sea.
FAU shark expert Stephen Kajiura began tracking the blacktips’ yearly sojourn a decade ago by meticulously counting each raisin-shaped shadow in photos taken during flights over the region as the toothy travelers headed south for the winter.
PHOTOS: Shark off Okaloosa Island draws beachgoers’ attention
The decrease in numbers between 2011 and 2019 has been nearly 60 percent, from a peak shark abundance of 12,128 to 4,955 last year.
“These animals play an important role in the ecosystem and lots of sharks is indicative of a healthy ecosystem,” Kajiura said. “If you are missing the boys at the top, then suddenly you have the potential for things to be very out of whack.”
Blacktips eat baitfish — weeding out the sick and unhealthy — in a food chain that starts with zooplankton munching on phytoplankton.
SHARK PHOTOS: Great white shark caught on the Navarre fishing pier
“A huge number of sharks sweeping and cleaning house every year is a good thing,” Kajiura said.
The annual migration of blacktip sharks
Blacktip sharks begin plying the waters of Southeast Florida at the end of January with the highest concentrations appearing in February. Their voyage often makes national news when captured in aerial footage. The sight of hundreds of sharks languidly cruising the beach can be unnerving and awe-inspiring.
VIDEO, PHOTOS: Shark swarm off Port St. Joe
While Kajiura has been flying the coast since 2011, he also has physical tracking devices on about 100 blacktip sharks tagged off Palm Beach and South Carolina.
Previous studies on blacktips showed that their return north during the summer months typically ended near Cape Hatteras, N.C., but Kajiura said half of his tagged sharks are traveling as far north as Long Island.
Their preferred temperature is about 72 to 77 degrees, Kajiura said.
GALLERY: Sharky’s new shark
“When the water is too hot, they’re not here. When the water is too cold, they’re not here,” he said.
The dearth of blacktip sharks near the coast was included in the annual shark bite report released this month by the University of Florida’s International Shark Attack File.
Shark bites were “unusually” low for the second consecutive year in 2019, with 64 unprovoked attacks worldwide. That’s about 22 percent lower than the most recent five-year average of 82 incidents per year.
Florida boy gets knocked off surfboard by shark
Florida still leads the nation, with 21 unprovoked bites in 2019
Gavin Naylor, director of the Florida Museum of National History’s shark research program, said changes in the migration patterns of blacktip sharks could be the reason for the decline. Blacktips are blamed for most bites in Florida.
“It’s not that there’s a decrease in sharks, it’s that the aggregations are less dense than when the waters are cooler,” Naylor said. “It could be they are all over the shelf farther offshore and that’s not where people surf or swim so they’re not getting bitten.”
PHOTOS: Shark Fishing in the Gulf of Mexico aboard Shock N Ya’ll
Florida maintained its top spot in the shark attack record with 21 unprovoked bites in 2019, about 34 percent lower than the state’s five-year average.
But Naylor said two years of data is not enough to say fewer shark bites is the “new normal.”
In Florida, 2017 saw the highest number of bites in the past decade with 31, including five in Palm Beach County. That was followed in 2018 with just 16 bites statewide, including two in Palm Beach County.
Neither Kajiura or Naylor would pin the blacktips’ migration shift on climate change. While both agree waters are warmer, a decade of data isn’t enough to directly connect the two, they said.
Mild changes in the ocean waters’ affects the blacktips’ migration
Still, 1.8 degrees is enough to affect a blacktip shark’s movements, said Florida International University assistant professor Yannis Papastamatiou, who works in the school’s department of biological sciences.
Blacktips are cold blooded, which means their metabolisms are tied strongly to water temperature. White sharks, another winter visitor to Florida waters, are warm blooded, and can handle a wider range of temperatures.
Papastamatiou is less convinced there could be a widespread ecological collapse without the blacktip sharks.
“Overall we have a pretty poor understanding of the impacts sharks have on their ecosystems,” he said. “However, we are talking about a significant decline in the number of predators visiting these waters so there may well be some impacts.”
Kajiura is hoping for a cold winter to see if there is an increase in sharks along the coast.
“That would really demonstrate that it’s not just a coincidence,” he said.