Thousands of blacktip sharks ordinarily swim languidly off the South Florida coast. But this year the shark count is down substantially and warmer water temperatures may be the reason.
A researcher at Florida Atlantic University says a recent tally he did off Palm Beach County during the sharks' annual migration had a high of only 2,800 blacktip sharks. That's down substantially from the high of more than 12,000 sharks in 2011, according to Florida Atlantic University researcher Stephen Kajiura.
The drop is dramatic, Kajiura said.
"The numbers have plummeted and this year was the lowest we've had so far," Kajiura said.
The lower blacktip shark numbers correlate with higher water temperatures. In 2011, the mean water temperature along the blacktip shark's Palm Beach County route was 73.9 degrees. This year, the mean temperature was 75.3 degrees, he said.
"Temperatures go up, shark numbers go down," Kajiura said. "Eight years does not show global climate change, but it does show what might be happening on a larger scale and why we might not get those big numbers of sharks like we used to if waters keep warming."
The researcher tells The Palm Beach Post that the sharks may have gone a little further north, near Vero Beach, Florida.
Blacktip sharks have a temperature tolerance of about 59 to 86 degrees, but generally prefer 68 to 77 degrees, according to R. Dean Grubbs, associate director of research for Florida State University's Coastal and Marine Laboratory.
Kajiura worries about what the impact will be in South Florida if the sharks move further north.
"If you don't have that influx of top-level predators wiping out smaller fishes, what impact does that have on the ecosystem," Kajiura said. "There are cascading effects."
But Gavin Naylor, director of the Florida program for shark research at the Florida Museum for Natural History, said sharks are survivors.
"I think that many things that happen do point to climate change, but sharks have been through probably the most spectacular extinction event in the world," said Naylor, referring to Permian extinction more than 250 million years ago when something killed 90 percent of the planet's species. "I think they are pretty capable of dealing with a lot that's thrown at them except wholesale targeting by commercial fisheries."