If the trend continues, researchers warn, the migration of blacktip sharks could grind to a halt because of the rapidly warming ocean

Blacktip sharks that journey down the Florida coast have declined in number so sharplythat researchers warn one of the largest migrations in US waters could grind to a halt because of the rapidly warming ocean.

A stretch of coastal south-eastern Florida is usually thick with as many as 15,000 blacktip sharks on any given day in February and March as the animals forge southwards from the Carolinas region in search of agreeably warm waters in winter.

However, aerial surveys of the migration last year found this number had slumped by around two-thirds, to 4,000 sharks. Researchers are still collating the 2018 tally but warn it is likely to match or even fall below last year’s level.

“Last year we had very warm waters and very few sharks, and 2018 so far has been exceptionally warm,” said Dr Stephen Kajiura, a shark researcher at Florida Atlantic University. “We just aren’t seeing the numbers; there are remarkably few sharks. It really indicates that their migration is tied closely to temperature and if this trend continues we may not see sharks here any more.”

For the past eight years, Kajiura and his team have used a light aircraft to video the migration of the sharks along a section of Florida coast stretching from Miami Beach to Jupiter Bay. He said that years with unusually warm waters have consistently resulted in sharks choosing not to move south, instead remaining in areas further north that match their ideal temperature of around 69F to 77F (21C to 25C).

Facebook Twitter Pinterest A stretch of coastal south-eastern Florida is usually thick with as many as 15,000 blacktip sharks on any given day in February and March as the animals forge southwards. Photograph: The Guardian

This trend may also be experienced by other sharks, such as the great white, which also migrate south in winter but do not hug the coast like blacktips and have not had their movement as closely studied.

While the eight-year survey period is too short to draw definitive conclusions over the influence of climate change, previous research has shown that some sharks are likely to be pushed polewards as the ocean warms. The world’s seas are around 1.8F degrees (1C) warmer than a century ago, mainly due to a sharp increase in temperature over the past three decades.

Sharks are ectothermic and cannot generate their own body heat, meaning they have to travel to find suitable temperatures. They are also reliant on prey fish that move for similar reasons. The loss of the sharks from Florida could lead to a series of knock-on consequences for a marine ecosystem that has developed with them as major predator.

“Sharks have been coming down here for millennia, so a rapid shift in migration could result in a dramatic change to the ecosystem,” Kajiura said. “It could disrupt the balance of things, you could have an explosion in prey fish numbers, which could cause problems elsewhere.”

Kajiura has faced an additional challenge in his research this year – the election of Donald Trump as president. When Trump makes one of his regular visits to Mar-a-Lago, his Florida resort, air traffic is shut down in the area, leaving shark-counting researchers grounded.

Trump has a known aversion to sharks. Stephanie Clifford, a pornographic actor who goes by the stage name Stormy Daniels, said in a recently published interview that Trump is “terrified” of sharks and would never donate to a charity that helps the marine predators. “He was, like, obsessed,” said Clifford, who claimed she had an affair with Trump. “It’s so strange, I know.”

Kajiura said he found Trump’s distaste for sharks “amusing” despite his research being hindered by the frequent flying bans.

“When I’m flying I can see the sharks right off his beach, they are everywhere around there,” he said. “Blacktips bite more people than any other shark in Florida. I’m half tempted to let them know about all the sharks there but then I thought it’s probably best not to.”