100 million sharks killed each year: study

An estimated 100 million sharks are killed every year due to aggressive overfishing, threatening to push some species to extinction, a new study has found.

"Sharks have persisted for at least 400 million years and are one of the oldest vertebrate groups on the planet. However, these predators are experiencing population declines significant enough to cause global concern," said lead author Boris Worm, professor of biology at Dalhousie University.

"This is a big concern because the loss of sharks can affect the wider ecosystem," said Mike Heithaus, executive director of Florida International University (FIU) School of Environment, Arts and Society and co-author of the paper.

"In working with tiger sharks, we've seen that if we don't have enough of these predators around, it causes cascading changes in the ecosystem, that trickle all the way down to marine plants," Heithaus said in a statement.

Such changes can harm other species, and may negatively affect commercial fisheries, Heithaus said.

Based on data collected, shark deaths were estimated at 100 million in 2000 and 97 million in 2010. The total possible range of mortality is between 63 and 273 million annually.

The biggest culprit in the significant population decline is a combination of a global boom in shark fishing -- usually for their valuable fins -- and the relatively slow growth and reproductive rates of sharks.

Because adequate data of shark catches is lacking for most of the world, the wide range of possible mortality is based on available data of shark deaths and calculated projections for unreported, discarded and illegal catches.

But even with the uncertainty there is little question that sharks are being caught faster than they can reproduce.

"Sharks are similar to whales, and humans, in that they mature late in life and have few offspring," said Worm.

"As such, they cannot sustain much additional mortality. Our analysis shows that about one in 15 sharks gets killed by fisheries every year. With an increasing demand for their fins, sharks are more vulnerable today than ever before," he said.

While some sharks are receiving protection through national and international agreements, the team of researchers suggests legislation should be expanded to a greater number of species, according to the study published in Marine Policy.

Imposing a tax on the export and import of shark fins could also help curb demand and generate income for domestic shark fisheries management, according to the study.