Dusky Sharks Not Safe at Home

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Having a familial hometown can prove a dangerous survival strategy.

Dusky shark mothers live in the same region generation after generation, which puts them in danger of overfishing, said a global collaboration of scientists who studied the DNA of two species of shark from the waters of United States, Australia, South America, Asia, New Zealand and southern Africa.

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"This research shows that adult  females faithfully give birth along the continental region where they were born. If fished too much, the population will collapse, and it is extremely unlikely that it will be replenished from immigration of sharks from another region," said Demian Chapman, leader of the group of researchers, in a press release by Stony Brook University.

But a genetic tracking technique could help scientists and conservationists figure out where these ocean-going predators face the greatest pressure from humans fishing for the main ingredient of shark fins soup.

"We know very little about the shark fin trade, but by using DNA-zip coding we can identify source populations that are contributing most to the trade, and prioritize them for management," Chapman said.

By studying the mitochondrial DNA of dusky (Carcharhinus obscurus) and copper sharks (Carcharhinus brachyurus), the scientists developed a map of shark populations.

"By analyzing part of the genome that is inherited solely through the mother, we were able to detect differences between sharks living along different continents – in effect, their DNA zip codes," said Chapman.

Discovery News first reported on this DNA tracking system in 2009, when Chapman used the technique to study the scalloped hammerhead shark. In that study, he found that shark fins in Hong Kong had come all the way from the Western Atlantic.

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The dusky shark is disappearing from those same Western Atlantic waters.

The shark was once common, but has decreased by 80 percent, according to a recent population study. Besides living in the same area generation after generation, the shark is also slow to mature. It takes about 20 years for a shark to reach breeding age. Even when it does become fertile, they only reproduce every three years and the pregnancy takes two years.

"We, therefore, really need to establish sampling programs of fins on their way to Asia or in the markets to regulate the global trade before many more populations suffer the fate of the dusky shark in the United States," Chapman said.

"Here in the United States, it took only a few decades to nearly wipe out our dusky sharks, and it will probably take a few centuries for their stocks to be replenished," said Martin Benavides, lead author of two studies published about the sharks in Endangered Species Research and research assistant at the Institute for Ocean Conservation Science.

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"Our results dash any hopes that dusky sharks from other areas of the world will replenish the depleted U.S. stock. The sight of a dusky shark swimming off our shores will be a rare experience for generations to come," said Benavides.

The researchers analyzed the DNA from 400 sharks to compile their data.

The dusky shark is listed as "Endangered" in the Western Atlantic by the International Union for Conservation of Nature.

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IMAGE: The dusky shark (Wikimedia Commons).

IMAGE 2: An assortment of dried shark fins in the process of being shipped from Fiji to Hong Kong. (Wikimedia Commons).

IMAGE 3: The copper shark (Wikimedia Commons).