July 13, 2001 A new study spearheaded by a University of Colorado at Boulder researcher that indicates male great white sharks roam Earth’s oceans much more widely than females has implications for future conservation strategies for the storied and threatened fish.
Assistant Professor Andrew Martin of CU-Boulder’s environmental, population and organismic biology department said the research team studied differences in two types of DNA collected from tissue samples of 95 male and female great white sharks. The team concluded that while males disperse widely across ocean basins, females tend to stay in a particular region, perhaps returning to their birth site to reproduce.
Great whites are globally distributed in temperate waters and have been observed in nearly every ocean and sea on the planet. In the study, the researchers compared the genetics of great white sharks from the Australia-New Zealand region and South African coastal zones -- two areas with the greatest abundance of the species.
"These are very rare fish because they are perched at the top of the food chain and because they have been fished very heavily," said Martin. "The nice thing about this study is that we were able to overlay genetic differences on the oceanic geography."
A paper on the subject is being published in the July 12 issue of the weekly science journal, Nature. Co-authors included researchers from the University of Aberdeen, the University of Southern Mississippi, the CSIRO Marine Research Laboratories in Australia, the Natal Sharks Board of South Africa, the National Institute of Water and Atmospheric Research in New Zealand and the Department of Conservation in New Zealand.
The researchers looked at both mitochondrial DNA – which is passed down only by the female of the species – and particular genes from the cell nucleus passed on by both parents. The mitochondrial results indicated the "female-mediated " gene flow of great whites between different ocean basins is rare and exhibits significant differences between sharks from the Australia-New Zealand region and the South African region.
The second gene assessment, from the cell nuclei of both sexes, showed little differences between shark populations from the Australia-New Zealand region and the South African region, indicating males ranged much more widely than previously believed by researchers, said Martin.
"The results were somewhat of a surprise," he said. "Many of us thought both male and female sharks were moving all over the place, but the mitochondrial DNA results indicate there was a significant divergence -- probably two to three million years ago – between the sharks from Australia and New Zealand and those from South Africa."
Although population estimates are difficult to make, great white shark catches in California declined from more than 60,000 pounds in 1984 to less than 1,200 pounds in 1991. "Management practices need to take into account the importance of breeding grounds and the connections of widely separated populations," Martin said. "A globally integrated plan of regional management would be the ideal situation."
Tissue samples from the fish were collected over a 10-year period from sharks caught in nets of commercial fishermen and live great whites that were biopsied using a "pole dart" by researchers. In addition, some samples came from novel experiments off the coast of California that involved the recovery of shark blood and gum tissue following great white bites to unmanned surfboards mounted with cameras, he said.
Martin speculated the females might move around less because they invest a large amount of energy in birthing. "If these fish have been successfully birthing young in an area with a large population of seals, for example, it would make sense to continue to produce young in that area if the seal population remains high."
The new findings indicate the population biology of great white sharks may be more similar to that of marine mammals than to other fish, Martin said. One hypothesis is that there may be learned behavior being passed down by female sharks.
"This brings sharks out of the ‘monster realm’ and positions them more in line with the behavioral patterns of marine mammals," he said. "Both face pressures of survival and both seem to have found similar ways to solve them."
Although the study was restricted to Australia, New Zealand and South Africa, the researchers suspect female great whites from other parts of the globe may have similar mitochondrial DNA differences that suggest long-term isolation of particular female populations.
Great white sharks have been known to reach lengths of 20 feet and weigh more than four tons. Females may birth up to nine live pups, although they reproduce infrequently. Their preferred diet is sea lions and seals, and they can go without food for up to three months.
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