The world's shark populations are experiencing significant declines with perhaps 100 million -- or more -- sharks being lost every year, according to a study published this week in Marine Policy.
"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," explains lead author Boris Worm, professor of biology at Dalhousie.
In the recently published paper, "Global Catches, Exploitation Rates and Rebuilding Options for Sharks," Worm and three other researchers from Dalhousie University teamed up with scientists from the University of Windsor in Canada, as well as Stony Brook University in New York, Florida International University (FIU) in Miami and the University of Miami, to calculate total shark mortality and outline possible solutions to protect the world's shark populations.
"This is a big concern because the loss of sharks can affect the wider ecosystem," said Mike Heithaus, executive director of FIU's 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." Such changes can harm other species, and may negatively affect commercial fisheries, Heithaus explains.
Based on data collected for the latest study, 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 Boris 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."
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. 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.
"The findings are alarming, but there is hope. Existing regulations are a great start but we must ensure they are adequately enforced," said Samuel Gruber of the University of Miami. "In addition, more nations must invest in sustainable shark fisheries management. This means introducing catch limits, trade regulation and other protective measures for the most vulnerable species and those that move across international boundaries."
The key message in this research is sustainability. 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.
The information from this report comes at a critical time, as 177 governments from around the world will attend the March 3-14 meeting of the Conference of the Parties to the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) in Bangkok. CITES is widely considered one of the best tools for protecting vulnerable species from extinction. Hammerheads, Oceanic whitetip, and porbeagle sharks are currently being considered for protection under CITES.
- Boris Worm, Brendal Davis, Lisa Kettemer, Christine A. Ward-Paige, Demian Chapman, Michael R. Heithaus, Steven T. Kessel, Samuel H. Gruber. Global catches, exploitation rates, and rebuilding options for sharks. Marine Policy, 2013; 40: 194 DOI: 10.1016/j.marpol.2012.12.034
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