Imprimir Republish


On the shark trail

Fin fragment analysis maps genetic structure of the hammerhead shark

SEAWATCH.ORGSphyrna lewini: three different populations in the Western AtlanticSEAWATCH.ORG

By analyzing mitochondrial DNA from shark fin fragments, a US group, which includes a Brazilian researcher, was able to identify the geographic region in the ocean in which hammerhead sharks from the globally fished Sphyrna lewini species were captured.

The study analyzed part of the mitochondrial DNA sequence extracted from fins sold in the marketplace. One of the main objectives of the study, published in the journal Endangered Species Research, is to propose a moratorium or a restriction of the capture of this type of hammerhead shark, currently at risk of extinction due to predatory hunting and the illicit  trade in fins.

These fins fuel a clandestine market, particularly in Asia, where they have high commercial value. Shark fin soup is considered a delicacy among the Orientals and is highly appreciated at ceremonies such as weddings and banquets.

“We managed to associate all the fins analyzed with a given area of the ocean where this species of hammerhead shark lives, based on the similarity between the genetic composition of the fins and the genetic stocks of the individuals from these waters,”  said Danillo Pinhal, a PhD candidate at the Biosciences Institute of Paulista State University (Unesp), Botucatu, to Agência FAPESP.

The research was coordinated by Demian Chapman, from Stony Brook University, New York, and by Mahmood Shivji, director of the Guy Harvey Research Institute of Nova Southeastern University, in Florida, USA. Pinhal joined the team to do part of his doctorate, on the “Genetic-populational structure of the Sphyrna lewini hammerhead shark (Elasmobranchii: Sphyrnidae) using molecular markers of microsatellites,”  with FAPESP aid, in the form of a grant. The project is covered by the Regular Aid to Research Project program, coordinated by professor Cesar Martins, from the Biosciences Institute of Unesp.

According to Pinhal, the aim of his doctoral research is to understand the populational dynamics of the Sphyrna lewini species along the Brazilian coast in even greater detail. “I am doing a fine-scale analysis, comparing a large number of individuals from several sites on the coast of Brazil. I also use samples from the Caribbean and the Gulf of Mexico and microsatellite genetic markers, besides mitochondrial DNA, to compare the genetic composition of the animals along the Brazilian coast in relation to these areas and to better evaluate the contemporary evolution of the species,” he explained.

Besides the fins, the study also compared the DNA from 177 samples of S. Lewini individuals from the wilds of the Atlantic and determined the genetic variability of the existing stocks. These analyses enabled the identification of three different stocks in the Western Atlantic, comprising the northern part of the Atlantic (USA and Gulf of Mexico), the central part (Caribbean) and the South Atlantic, which encompasses the entire Brazilian coast.

One of the possible explanations for genetic differentiation within a given area might be the presence of nurseries along the coast, comprised of shallow waters that are calm and rich in food. “The females select such areas because there is a segregation between newborns and male adults, as the latter don’t live in them; therefore, they are not predators of their own offspring. This is one of the species? strategies,” he said.

The high loyalty of the females to the place of birth might be another reason for the presence of different stocks of the species in the Western Atlantic. “We not only found that the tendency toward philopatry, meaning the chance of females returning to the place of birth to have their offspring, is far greater than we had previously thought, but we also realized that there is great differentiation between the populations in sites that are relatively close to each other,”  he pointed out.

According to him, these characteristics could be responsible for the genetic differentiation in these areas. Furthermore, the continuity of his doctoral research also aims to better understand the movements of the males. “We have shown that there is some differentiation among the populations of S. lewini hammerhead sharks not only in the different oceans, as had already been published, but also, on a more minor scale, in areas that are relatively close to each other. We observed that within a given ocean, in this case, the Atlantic, there are populations that are quite genetically distinct,”  said Pinhal.

the study highlights that hammerhead shark species are the target of predatory fishing. The fins can cost 20 to 30 times more than a kilogram of shark meat. The chief center in the world trade in shark’s fins is Hong Kong, but the practice is disseminated throughout the world.

“The problem is the lack of inspection of fishing and of the traded catch. The fishermen resort to finning, a method whereby the animal is captured and, once its fins have been cut out, is thrown back into the water, alive. However, as it can no longer swim, it is in agony until it dies,” Pinhal told us. In the published article, the researchers concluded, based on an analysis of the control region for the mitochondrial DNA (mtCR), that 21% of the fins analyzed came from the Western Atlantic, where the hammerhead shark has already joined the list of endangered species, according to IUCN, the International Union for the Conservation of Nature. According to the biologist, it is the first time that a study was able to determine the geographic origin of a fin using DNA.

According to several studies, the global decline of the hammerhead shark has already turned it into a species in danger of extinction. “On the Brazilian coast, as we have found that there is a shortage of fish, fishermen have been capturing( sharks) and are also making use of their meat. One can say that the practice of finning is on the way to becoming extinct, as they are trying to make use of everything due to the declining catch, in other words, from traditional fishing of other species that were commonly sold,”  said Pinhal.

According to him, the DNA analyses are making a significant contribution to the understanding of the distribution and health of the critically endangered marine species. “Sharks are particularly sensitive to over-fishing because they grow slowly, take a long time to mature sexually and have low relative fecundity, biological characteristics that bring them closer to mammals,”  he highlighted.

The researcher stressed that the work aims to provide mechanisms that can enable the adoption of measures for suitable husbandry and conservation of these species of fish. The group plans to use the data published in the article to propose protective measures at the Convention on International Trade in Endangered Species, to be held in March in Qatar, in the Middle East.

Scientific article
CHAPMAN, D. et al. Tracking the fin trade: genetic stock identification in western Atlantic scalloped hammerhead sharks Sphyrna lewini. Endangered Species Research. In press.