Unsustainable fishing: Dal researchers on the global shark decline

Upwards of 100 million sharks or more are killed each year, according to a new study by Dal researchers — and that number that does not bode well for the future of the ocean predators.

In a new paper published in Marine Policy, Biology’s Boris Worm and three Dal colleagues — Brendal Davis, Lisa Kettemer and Christine Ward-Paige — teamed with scientists from the University of Windsor in Canada, Stony Brook University in New York, Florida International University (FIU) in Miami and the University of Miami. Together, they completed a calculation of total shark mortality worldwide using available data.

"Sharks have persisted for at least 400 million years and are one of the oldest vertebrate groups on the planet,” says Dr. Worm, the lead author on the paper. “However, these predators are experiencing population declines significant enough to cause global concern."

The math is, indeed, concerning. The researchers estimate an average exploitation rate for sharks of between 6.4 and 7.9 per cent. That means that of the estimated number of sharks in the ocean, 100 million of them or more are killed each year. Based on what we know about 62 different shark species, sharks have an average rebound rate of 4.9 per cent each year. The bottom line: we are killing more sharks than there are young sharks being born.

And the numbers could be even worse than those numbers suggest: while total shark deaths were estimated at 100 million in 2000 and 97 million in 2010, the total possible range of mortality could be between 63 and 273 million annually.

"Sharks are similar to whales, and humans, in that they mature late in life and have few offspring,” adds Dr. 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."

More protection needed

That’s due to a global boom in shark fishing, particularly for their valuable fins. 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.

While some sharks are receiving protection through national and international agreements, the researchers suggest legislation should be expanded to a greater number of species. Another solution they suggest is a tax on the export and import of shark fins to curb demand and generate income for domestic shark fisheries management.

"Because of the role sharks play in the sustainability of marine ecosystems, the researchers insist that protective measures must be scaled up significantly to avoid further depletion and possible extinction of some of the world’s top predators,” concludes Dr. Worm.

• Link: Worm Lab at Dalhousie