Video: Why are tens of thousands of sharks swimming off the Florida?

In an annual natural spectacle, tens of thousands of sharks are congregating off the coast of Florida.

Researchers have been tracking the beasts as they wend their way south to warmer climes, capturing aerial footage as they meander through the turquoise waters.

Many shark species are known to migrate, but this is the first time scientists have specifically studied the migration of blacktips, and they are keen to use the opportunity to play down some of the fears surrounding the animals.

“This is a particularly compelling migration, because it happens so close to shore in such clear waters, and because it happens in such a popular winter destination for people,” says lead researcher Stephen Kajiura of Florida Atlantic University, in a telephone interview with The Christian Science Monitor.

“We know that other shark species migrate, but you don’t have that same visceral connection with people able to observe so nearby.”

The team has been taking to the skies and doing aerial surveys of the migration since 2011, but only in the past couple of years have they actually jumped into boats and started fitting transmitters to the sharks.

In the southern reaches of Florida, they have found only male sharks. Moreover, they have learned that the creatures, when returning north after their migration, go much farther north than previously thought, all the way up to Long Island.

Even though there is evidence to suggest blacktip sharks can be found as far north as Massachusetts, Dr. Kajiura explains that this refers to individuals whisked off track by gulf stream currents, rather than the general population.

“It looks like there’s a correlation between global warming and their expanding range,” Kajiura tells the Monitor. “They’re moving further north to find their ideal temperature.”

While blacktip sharks are “widespread in warm temperate, subtropical and tropical waters,” according to the International Union for Conservation of Nature and Natural Resources, there is little hard data on migration patterns.

That said, Conservation Officer John Richardson of the UK’s Shark Trust explains in an email interview with the Monitor that female Blacktip sharks also migrate along the eastern US coastline:

“Beginning in May, female Blacktip Sharks migrate along the US Atlantic and Florida Gulf coasts to nursery areas, where juveniles are born and remain until surface water temperatures decline below approximately 20°C and then migrate to warmer waters near southern Florida and the Florida Keys over winter months.”

But what exactly is it that drives Kajiura’s army of males to head south? The professor says that while temperature is certainly a factor, it is only part of the equation.

“The temperature decreases, so perhaps that drives the blacktip sharks’ prey further south, leading the predators to follow. But also correlated with changing temperatures is the shifting day length.

“It’s impossible to tease it all apart.”

The IUCN lists blacktip sharks as “near-threatened”, but Mr. Richardson of the Shark Trust points out that “the Northwest Atlantic sub-population is considered to be Vulnerable on the IUCN Red List of Threatened Species,” one step closer to facing extinction.

“As well as being a commercially important species, it is also valued in the Northwest Atlantic recreational fishery,” says Richardson. “The Blacktip population in the Northwest Atlantic has been heavily fished, and their inshore distribution also makes them susceptible to habitat destruction.”

This is one reason why Kajiura is so keen to highlight the opportunity presented by such a visible migration: it promotes understanding.

Indeed, for those concerned about the potential danger, it would be worth noting that no blacktip shark bite has ever been fatal, according to the National Oceanic and Atmospheric Administration.

“The sharks are not out to get you, and if they wanted to bite you, there’d be ample opportunity,” says Kajiura. “But in this clear water, they can easily see you’re a human, not a fish. Besides, these sharks are skittish, and they’ll likely swim away, even if you try to get close.”