Sharks are changing where they swim, breed and hunt along American coasts, say experts

Their changing behavior could have profound impact on the ecosystem and economy.

Feeling hot lately? Well, so are sharks.

And over the last decade, they’ve been changing age-old habits to keep up with the rising heat — habits that could have far-reaching consequences on the ecosystem as well as our economy, scientists have found in two different studies conducted in the U.S.

This week, the American Meteorological Society released its State of the Climate report, an annual look at how the planet’s seas and atmosphere are faring. Not only was 2017 one of the hottest years on record, greenhouse gases and sea levels were at record highs and sea surface temperatures touched levels rarely observed before. Glaciers continued to shrink, coral reefs were dying out fast, and heat waves, hurricanes and wildfires struck again and again.

And while it may take decades or centuries before humans understand how to adapt to an ever-hotter planet, researchers have discovered that sharks are already reacting to higher water temperatures by rapidly changing when and where they swim, breed and hunt along American coasts.

Every year since 2011, Stephen Kajiura, a researcher at the Florida Atlantic University, and his team have flown a plane over a strip of the Florida coast to observe the migration patterns of black tip sharks, a species that grows up to 6.5 feet in length and is found from New England to the Florida Keys, and the Gulf of Mexico.

Thousands of these sharks swim along the Eastern Seaboard every winter in search of warmer climes and amass just off the Florida coast, where the clear water and light-colored, sandy sea-floor makes it easy to spot and count them from the air.

What Kajiura’s team found was startling — the number of black tip sharks has been dropping rapidly, from about 12,000 sharks in 2011 to below 3,000 in a recent count.

“It’s a dramatic change for such a short period of time,” Kajiura told ABC News. “And there’s a strong correlation to warmer waters, which suggests that in the future, fewer and fewer sharks will be seen.”

The average water temperature went up from 23.3 C (73.9 F) in 2011 to about 24.7 C (76.5 F) in 2017, he said. “An increase of over a degree may not seem like much, but it could have a profound effect on the marine ecosystem.”

That’s not to say that the black tip sharks are dying. On the contrary -- using their remarkable ability to adapt, the sharks are moving northwards to coasts like that of North Carolina, where waters are now warm enough in the winter, but not too warm as near Florida, Kajiura said. They may also be following a shift in distribution of their prey — going where the food goes.

And this could spell bad news for both local fish and fisheries.

“When top level predators like these enter new waters, there are cascading effects on the entire ecosystem,” he said. “It could affect the populations of fish being farmed, which would have an economic impact on people whose livelihood depends on it. The plankton smaller fish feed on could multiply. Already, we’ve been hearing anecdotes that local fishermen have noticed the number of bait fish like mullet or menhaden going down.”

Maintaining a healthy balance between populations of sharks and other fish is vital to the U.S. economy. According to the National Marine Fisheries Service, ensuring this balance is important for commercial and recreational use and integral to coastal communities.

While conducting a separate study, Charles Bangley, a post-doctoral fellow at the Smithsonian Environmental Research Center, accidentally found that the population of juvenile bull sharks in North Carolina had increased. Bangley had been conducting a survey in Pamlico Sound, North Carolina, a giant, shallow lagoon where a few bull sharks, which can tolerate waters with lower salinity, hang around every year near the mouth of an inlet and feed on shellfish and smaller fishes.

“It was during this work that we stumbled on the evidence for the increase in juvenile bull sharks,” he told ABC News.

“Around 2010 and 2011, we noticed that juvenile bull sharks went from being kind of rare, sporadic catches to showing up regularly every year. And then in 2012 there was a huge increase in them. A total of 70 bull sharks were captured between 2013 and 2016.”

“It's not like bull sharks have never been there before,” Bangley added. “But the way they use the estuary seems to have changed from being a place where the adults wander in to feed, to being a place that they give birth in and which functions as a nursery habitat for their young.”

Much like black tip sharks, attacks on humans by bull sharks are rare. In fact, Bangley described the ones he found in the estuary as “pretty cute,” mostly under 4 feet in length.

“For me the serious aspect of this is not the sharks themselves but the kind of environmental change that they represent,” he said.

“If these sharks are shifting how they use the environment -- they’re the upper tier of the ecosystem, everything below them and around them will also be changing. And are we going to run into these mismatches where these sharks are shifting in but some other species that haven’t had to deal with them previously haven't shifted out yet?”

While both these studies were conducted on the East Coast, a paper published in Nature Climate Change in 2012 predicted that the habitat of sharks in the Pacific too would be similarly affected.

“We used data gathered by 4,300 electronic tags placed on 23 marine species, including tuna, sea birds, blue whales, turtles, and of course, sharks, and plugged it into climate models to map where their habitat is likely to go,” Elliott Hazen, a research ecologist with the National Oceanic and Atmospheric Administration who led the study, told ABC News.

“We looked at salmon sharks, we looked at great white sharks, we looked at blue sharks and mako sharks. What we found was that the habitat of all these sharks would shift northward by more than 600 miles over the next 100 years.”

What about the threat of shark bites? Attacks on humans are extremely rare, all three experts emphasized. Sharks are at far greater danger from humans than humans are from them. But that doesn’t mean attacks, often by accident, will never occur. As sharks move to areas where humans traditionally don’t expect them to be, encounters between swimmers or surfers and sharks are likely to increase, said Kajiura.

And great white sharks have been spotted more frequently in places as far north as the coast of New York, where some reports suggested they were even breeding. Last month, two children were bitten by sharks in the waters near Fire Island.

On the West Coast, more and more great whites and whitecap sharks will likely reach the offshore waters of the northeast Pacific, including British Columbia in the next few decades, William Cheung, associate professor at the Institute for the Oceans and Fisheries at the University of British Columbia, told ABC News.