Warming waters may be shifting blacktip shark migrations

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By Dinah Voyles Pulver | The Daytona News Journal

DAYTONA BEACH — A researcher at Florida Atlantic University who studies shark migration off Florida’s east coast suspects thousands of blacktip sharks aren’t migrating as far south in the winter as they used to because of warming water temperatures along the coast.

That could spell more sharks off Central Florida, said Stephen Kajiura, a researcher and professor in the University’s department of biological sciences and director of its elasmobranch research laboratory. 

Kajiura surveys for blacktips between Miami and Palm Beach County once a week when the sharks are migrating, using an assortment of methods including a boat, a plane, drones and acoustic monitoring devices. 

Typically, Blacktip sharks spend summers off Georgia and North Carolina, Kajiura said. That’s where they mate and give birth in bays and estuaries. 

“Then, as the water starts to cool down, (below 71 degrees) these sharks start to move south,” he said. But with water temperatures in the Atlantic “getting progressively warmer, we’re seeing fewer and fewer sharks.” 

Historically large groups of sharks have traveled as far south as Broward County. In prior years, researchers reported as many as 15,000 sharks could be seen in the clear water off the coast of South Florida. 

“Last year, we saw a dramatic decline in the number of blacktip sharks that migrated south,” he said. “In fact, it was so low that we estimated the population to be about one-third of what we have seen in previous years.” 

Although he hasn’t surveyed the shark populations off the Central Florida coast, he surmises with fewer of the top level predators in South Florida, more and more of them are lingering in Central Florida. 

Central Florida is already known for having a high number of shark bites, with Volusia County known as the shark bite capital of the world. And as blacktips are one of the two leading suspects for bites, an increased number of sharks could mean an increased number of bites, Kajiura said. 

Shark bites in Volusia County have historically peaked in the spring and fall, when sharks are migrating north and south. 

An increase in shark bites was seen in 2016, with 15 bites reported, but that was well below the record of 24 bites in 2008. Only nine bites were reported in 2017. The University of Florida’s International Shark Attack File, which investigates bite reports, has said several factors can influence the number of bites, including weather, the local economy and the number of swimmers in the water. 

The two species most often blamed for bites in Central Florida are blacktips and spinners. Both are smaller species. Blacktips can reach lengths up to 6 feet. 

Kajiura is interested in the ecological impacts if the blacktips continue to show up in fewer numbers in South Florida. 

“When you lose that influx of predators, there might be cascading effects at other levels,” he said. “If you don’t have these big boys coming in to eat bait fish, then the bait fish populations might explode.” 

He hopes to see additional research about sharks and their impacts long the coast. 

“I wish I had collected this data in the 1980s, or I wish I had it from 50 years ago,” he said. Instead, the research he’s doing now will be the baseline for future research, he said. “We need these studies now to inform us how things are going to be changing in another 20, 30 or 40 years.”

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