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As oceans grow warmer, blacktip sharks are spending more time farther north

SACHA PFEIFFER, HOST:

Ocean temperatures off the coast of southern Florida are reaching record highs this summer, and that warming trend is affecting some migrating sharks which are in search of cooler waters. Yvonne zum Tobel with member station WLRN has more.

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YVONNE ZUM TOBEL, BYLINE: Like snowbirds, blacktip sharks migrate to South Florida every winter to enjoy the warmer waters.

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ZUM TOBEL: Known for the spatter of black on the tips of their fins, the sharks weigh about 50 pounds and can get 6 feet long.

BETH BOWERS: They're prey to a lot of larger sharks. So they're pretty much - behave like prey, you know? They're very skittish.

ZUM TOBEL: So they're not the biting type, says Beth Bowers with Florida Atlantic University Shark Lab in Boca Raton. She's been studying blacktip sharks for the past eight years. Research from the lab shows the number of blacktips coming to South Florida during the winter months is down. That's because the ocean's temperature is up.

BOWERS: So between Boynton and Palm Beach, it used to be a really high density of about 2,200 sharks per square kilometer. But it starts going down over the years.

ZUM TOBEL: About a decade ago, they counted over 12,000 blacktips migrating to South Florida waters. And the most recent survey shows numbers below 2,000.

BOWERS: And so our question was, where do they go after they leave here?

ZUM TOBEL: Bowers attached acoustic telemetry tags to the sharks, which uses sound to track their movements. She found that more sharks hang out off the coast of Central and North Florida these days before they move further north to their mating grounds off the coast of South Carolina. They used to show up there at the end of April.

BRYAN FRAZIER: Whereas this year I think we saw the first ones in March, which is much earlier.

ZUM TOBEL: That's Bryan Frazier, a shark researcher with the South Carolina Department of Natural Resources.

FRAZIER: They are coming much earlier, and they're staying longer or migrating back through later than they have in the past. And, of course, that's directly tied to water temperature.

ZUM TOBEL: Blacktips have been found even further north - as far north as Long Island, N.Y., during the summer months, says Tobey Curtis. He's with the National Marine Fisheries Service.

TOBEY CURTIS: So we know the blacktips are around. We know fishermen catch them from time to time.

ZUM TOBEL: He says blacktip sharks are becoming more common off Long Island in the summer and are just another shark species to look out for.

For NPR News, I'm Yvonne zum Tobel in Boca Raton. Transcript provided by NPR, Copyright NPR.

NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.

Yvonne Bertucci zum Tobel