Why Sharks Are in Double Jeopardy

Sharks and other large animals face risk both from their dollar values among hunters and from shrinking habitats.

Big sharks, along with other large animals both on land and in the sea, are in double jeopardy of extinction due to their large body size and high dollar value, according to new research.

The analysis, reported in the journal Current Biology, reveals under-appreciated risk to marine species similar to that of iconic terrestrial species, such as endangered rhinos that remain on the brink of extinction.

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"We typically assume that if a species is reduced to low numbers, individuals will be hard to find, hunters will stop hunting, and populations will be given a chance to recover," co-author Loren McClenachan of Colby College said in a press release. "But the extreme values of these species mean that without significant conservation intervention, they will be hunted to extinction."

McClenachan, with Andrew Cooper and Nicholas Dulvy of Simon Fraser University, identified a diverse group of more than 100 large marine and land-dwelling species that are targeted for international luxury markets. Examples of such status items can include things like sharkskin boots and ivory jewelry.

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They estimated the value of these species across three points of sale and explored the relationships among extinction risk, value, and body size. They also took into account two mitigating factors: poaching fines and geographic range size.

The investigation showed a threshold above which economic value is the key driver of extinction risk. Although lower-value species -- with "value" again defined by market worth -- are influenced primarily by their biology, the most valuable species are at high risk of extinction no matter their size. Once mean product values are greater than $12,557 per about 2.2 pounds, body size no longer drives risk, the report shows.

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The researchers also uncovered important differences between marine and terrestrial species that point to elevated risk of extinction for sharks and other water dwellers.

Although marine products are generally less valuable on a per pound basis, individual animals are still just as valuable as the most commercially coveted terrestrial species. An individual whale shark, for example, is about as valuable as the most valuable terrestrial species: rhinoceroses and tigers.

"Hunters don't kill kilograms, they kill individuals, so we need to pay attention to these high values of individual animals," McClenachan said.

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The risk to sharks and other marine life isn't reduced for species with larger ranges as it is on land, meaning that they are often in an even worse situation.

"The assumption that large ranges protect species from extinction is based on conservation science done on land -- where animals found in multiple countries have a higher chance of protection in at least one location -- and appears not to apply to marine species, where widespread and little-policed hunting contrasts with tighter controls on land," McClenachan explained.

The authors call for more consideration of trade of marine animals, and ask that researchers pay more attention to differences between terrestrial and marine animals when it comes to conservation.

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"For too long, we have been reading wildlife trade reports with scant recognition of the diversity and value of the marine wildlife trade," Dulvy said.

McClenachan added, "We need to pay attention to fundamental differences between marine and terrestrial species. Conservation science began on land, so it is tempting to assume that underlying principles are the same in the ocean. However, as we found, this is far from the case. If we're not aware of these basic differences, it's impossible to design effective conservation."