Shark count yields surprising numbers

Scientists know that shark populations have declined over the past several decades, but vital baseline information has been missing. A new study is bringing the numbers into focus.

Researchers conducted an eight-year study of a healthy shark population on Palmyra, a remote, uninhabited atoll in the central Pacific Ocean. The pristine ecosystem is part of a marine refuge that extends 50 nautical miles from its shores. No fishing is allowed within its borders, which protect a diverse array of species, including grey reef sharks.

They were surprised to find far fewer sharks than they expected.

gray reef sharks
(Credit: Darcy Bradley/UCSB)

“We estimated a population size of between 6,000 to 8,000 grey reef sharks at Palmyra, which works out to a density of about 20 sharks per square kilometer,” says lead author Darcy Bradley, a postdoctoral researcher in the Sustainable Fisheries Group at the University of California, Santa Barbara.

“Previous research that used underwater visual survey methods estimated a density of between 200 to 1,000 sharks per square kilometer. So while it’s not totally clear how those density estimates would scale up to a population estimate, it is clear that it would end up a lot bigger than our estimate.”

As reported in Scientific Reports, from 2006 to 2014, researchers captured reef sharks across Palmyra and fitted them with numbered ID tags. They also tracked the movement of some of these animals using acoustic telemetry tags, which emit a sound that acoustic receivers located underwater then record.

These sharks don’t avoid hanging out with people

Of the 1,300 tagged reef sharks, 350 individuals were recaptured, making this effort the largest reef shark tag recapture program in the world. In addition to the tag data, investigators recorded information on the sex and size of each animal and the location of its capture and then plugged all the data into an algorithm that estimated the total population size.

The fact that the shark population is smaller than anticipated is not all bad news, Bradley says.

“If a healthy shark population is smaller than we assumed, that means other shark populations are more precarious than previously suggested,” she says. “However, it also means that the recovery goal for shark populations is lower, which makes recovering shark populations somewhat easier.

“Given that the way we manage fisheries and ecosystem health depends on having decent estimates of abundance, we need to continue to improve the way we count things in the ocean.”

Other researchers from UC Santa Barbara and from the Nature Conservancy, Florida International University, and the US Fish and Wildlife Service are coauthors of the study.

Source: UC Santa Barbara