April 28, 2011

DNA Helps Scientists Track Origin Of Shark Fins

An international team of scientists have used DNA to determine that groups of dusky sharks and copper sharks living in different coastal regions across the globe are in fact separate populations of each species.

Both groups are heavily exploited for the shark fin trade, which claims tens of millions of animals every year to produce the Asian delicacy: shark fin soup. Many of these fish are declining rapidly as a result of pressures of fishing for their fins.

The dusky shark is classified by the International Union for Conservation of Nature (IUCN) as endangered in its Western Atlantic habitat, because its population is below 20 percent of what it was twenty years ago.

The new studies, led by the Institute for Ocean Conservation Science at Stony Brook University, show that the genetic differences among populations of these sharks are large enough for scientists to be able to track the origin of these threatened animals.

The research appears in two scientific articles:

"Global phylogeography of the dusky shark, (Carcharhinus obscurus): implications for fisheries management and monitoring the shark fin trade," is published in the online edition of the journal Endangered Species Research.

"Phylogeography of the copper shark (Carcharhinus brachyurus) in the southern hemisphere: implications for the conservation of a coastal apex predator," will be published in the journal Marine and Freshwater Research.

The main goal of the studies was to identify what genetic differences existed among the regional groups of dusky and copper sharks and establish how many distinct populations there are. Their second objective was to determine if the population differences were great enough to allow scientists to reconstruct their contributions to fin trade in the future.

Like most large sharks, these species are widely distributed throughout the world's oceans, but are mainly restricted to coastal regions during reproduction.

"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 Dr. Demian Chapman, leader of the research team and assistant director of science of the Institute for Ocean Conservation Science.

"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 Chapman.

And this is exactly what is happening along the US Eastern Seaboard, where populations of the dusky shark are rapidly declining and could likely be listed on the Endangered Species List. While once common in the ocean waters off the coast of the US, a recent stock assessment of dusky sharks along the US East Coast and the Gulf of Mexico showed an 80 percent drop even though the species has been under protection since 2000.

The recovery of the species is extremely slow because the average age of maturity is 20 years and its reproductive cycle only occurs once every three years. Pregnancy in these animals lasts two years and their litter size is relatively small (3 to 14 offspring).

"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 both studies and research assistant at the Institute for Ocean Conservation Science.

"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," he said.

Chapman added that very little is known 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."

"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," he said.

It was extremely difficult for experts, for many years, to determine the origin of these fins and if they were from threatened species. A 2009 study by Chapman, used DNA testing to trace scalloped hammerhead shark fins from the Hong Kong market all the way back to the sharks' geographic origin and found many came from collapsed Western Atlantic populations.

The new research findings show that this type of testing can also be used to trace the origins of the fins of both dusky and copper sharks.

"As apex predators at the top of marine food webs, it is essential for ocean health that we take steps, such as monitoring and regulating the fin trade, to protect these large sharks," said Dr. Ellen K. Pikitch, executive director of the Institute for Ocean Conservation Science.

The research projects were conducted by scientists from the United States, Australia, South America, Asia, New Zealand and South Africa. Scientists analyzed mitochondrial DNA in nearly 400 sharks sampled from all over the world's oceans.


Image 1: The copper or bronze whaler shark (Carcharhinus brachyurus) is a large, coastally oriented top predator that is vulnerable to overexploitation. Credit: Robert Nyman

Image 2: The dusky shark (Carcharhinus obscurus) is classified as "Endangered" in the Western Atlantic by the International Union for Conservation of Nature as its population is below 20 percent of what it was two decades ago. Credit: NOAA

Image 3: This is an assortment of dried shark fins in the process of being shipped from Fiji to Hong Kong. Tens of millions of fins are exported from nations around the globe to Asia, where they are used to make shark fin soup. Scientists have found that "zip codes" in the fin's DNA can reveal where the fin came from and thus contribute to improved shark conservation. Credit: Institute for Ocean Conservation Science


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