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Tiger sharks migrating farther north due to warming oceans, study finds

Researchers analyzed nine years of tracking data from satellite-tagged tiger sharks, and nearly 40 years of tag and recapture information from NOAA. They also used sea-surface temperature data gathered by satellite.

(CN) — Tiger sharks in the North Atlantic are swimming farther north and doing so earlier in the year due to warming oceans, according to a study published Thursday by the journal Global Change Biology.

This potentially affects the entire food chain and humans, researchers on the project said, though the consequences of the shift weren't a subject of the study.

“The consequences of changing the ecosystem are rarely positive," said Ben Kirtman, professor of atmospheric sciences at the University of Miami. "You have to assume we had this delicate balance and now we are messing with this delicate balance.”

Tiger sharks are cold-blooded, meaning they rely on the environment to regulate their temperatures. With their ideal temperature being about 80 degrees, the sharks can be found off Florida and the Bahamas during winter, but travel farther north during the warmer months, said Neil Hammerschlag, a marine ecologist, director of the University of Miami's Shark Research and Conservation Program and lead author of the study.

"They are like snowbirds," he said.

The sharks are apex predators, meaning they have few natural predators and are at the top of their food chain. They eat almost anything they can capture, including many kinds of fish, sea turtles and birds.

“They play important roles in maintain checks and balances in the food web. If ocean warming causes tiger sharks to shift where and when they are spending their time, this can have rippling, cascading, effects to other species in the ecosystem," Hammerschlag said.

The scientists found that during the past decade, with ocean temperatures the warmest on record, tiger shark migrations extended farther north by about 250 miles for every one-degree Celsius increase in water temperature above average. They also migrated about 14 days earlier to the waters off the northeastern United States.

“The study is sort of confirming some things we’ve already suspected: as temperatures warm, it's not just going to affect weather and climate," Kirtman said. "It’s going to affect things that are living in the ocean.”

By swimming north, the sharks leave areas off the coast of the southeast United States protected from commercial fishing, potentially leaving them vulnerable.

"It is increasing their chance of being exploited by fishing," Hammerschlag said. “Until really recently tiger sharks were spending a lot of time in those marine protected areas and getting conservation benefits."

To reach their findings, the researchers used multiple datasets. They analyzed nine years of tracking data from satellite-tagged tiger sharks, and nearly 40 years of tag and recapture information from the National Oceanic and Atmospheric Administration Cooperative Shark Tagging Program. The scientists also used sea-surface temperature data gathered by satellite.

Multiple lines of evidence and data sources showed how sharks were responding to ocean warming, Hammerschlag said. The study included scholars from multiple disciplines.

The data also shows the structure of warming in the Atlantic, Kirtman said. Satellite data has shown the Gulf Stream to be warming, and the data from the sharks confirms that. Other hot spots for warming include off Cape Hatteras, Long Island and Cape Cod.

"Since 1850 we are seeing warming of something like 4 degrees Fahrenheit. That’s a lot when you think of the beings that live in the ocean," he said. "Four degrees is a lot for them to handle."

There are consequences in a changing climate that we don't normally think about, Kirtman said. The changes to the sharks will affect the food web, and eventually people.

"There's going to be these surprises as the climate changes, he said. "There are things happening that we haven’t anticipated, and this is along that line."

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