After 15 years of research in the waters of the South Atlantic and Indian Oceans, scientists from six countries today unveiled the largest genetic study of humpback whale populations ever conducted in the Southern Hemisphere. The results of the analysis appear today in PLoS One, an interactive open-access journal for scientific and medical research.
"Humpback whales are perhaps the most studied species of great whale in the Northern Hemisphere, but many of the interactions among Southern Hemisphere populations are still poorly understood," said Dr. Howard Rosenbaum, director of the Wildlife Conservation Society’s Ocean Giants Program and lead author of the study. "This research illustrates the vast potential of genetic analyses to uncover the mysteries of how humpbacks travel and form populations in the southern ocean basins."
Mother and calf humpbacks breach together. (Photo by M. Leslie courtesy WCS)
By analyzing DNA samples from more than 1,500 whales, researchers can now glimpse the population dynamics and relatedness of Southern Hemisphere humpback whales as never before, and help inform management decisions in the politically charged field of whale conservation.
The findings so far have revealed that the highest rate of gene flow between populations is between whales that breed on either side of the African continent, with an estimated one or two whales every year swimming from one ocean to join the whales in another breeding ground.
A lower rate of gene flow was found between humpbacks breeding on opposite sides of the Atlantic - one population along coastal Brazil and the other along the coast of southern Africa. While no individual whales have been detected traveling across the southern Atlantic to both breeding grounds, genetic similarities show a slight degree of interaction.
An examination of humpback whale songs between these two populations are similar, another hint at interchange between the two groups, most likely in the whales’ feeding grounds in Antarctic waters, the scientists speculate.
A breeding population, which inhabits the northern Indian Ocean off the Arabian Peninsula, numbers fewer than 200 whales and is the most distinct in terms of genetics and migratory behavior. Unlike the other humpback populations, it is non-migratory and only distantly related to the nearest group of humpbacks, which breed off Madagascar and the eastern coast of Southern Africa. As a small, insular group, this population is unique and therefore a conservation priority, the scietists say.
Scientists from the Wildlife Conservation Society, Columbia University and the American Museum of Natural History, all based in New York City, contributed to the research project. Other contributors to the study include: the Instituto Baleia Jubarta and the Faculdade de Biociências PURCS of Brazil; France's Association Megaptera and Université de La Rochelle; the Agence Nationale des Parcs Nationaux of Gabon; the Environment Study of Oman; and the University of Cape Town; University of Pretoria; and Marine and Coastal Management of South Africa.
So little is known about southern ocean basin humpbacks that researchers initially used old whaling records for insights into whale population boundaries.
One set of charts, "The Distribution of Certain Whales as Shown by Logbook Records from American Whale Ships,"was compiled by Charles Townsend of the New York Zoological Society, now known as the Wildlife Conservation Society. These charts recorded the locations of more than 50,000 whale captures between 1761 and 1920.
According to the charts, many humpback whales were captured in the Gulf of Guinea, southeastern African and northeastern Madagascar, the same locations where humpbacks congregate today.
"Townsend was attempting to identify distribution and possible boundaries between whale populations or breeding stocks," observed Rosenbaum. "We’re still trying to answer the same question with molecular technology in concert with whaling logbook records."
Researchers observe whales in Madagascar (Photo Julie Larsen Maher © WCS)
Over the course of their 15 year study, researchers collected skin samples from 1,527 whales from 14 sampling sites from the southwestern and southeastern Atlantic Ocean, and the southwestern and northern Indian Oceans.
The scientists sampled the tissue of living whales with biopsy darts fired from crossbows. The scientists say their darts harmlessly bounce off the marine mammals as they surface to breathe. Samples came also from skin which is continually sloughed off by the animals and collected by the research teams.
The samples were brought to the lab at the American Museum of Natural History Sackler Institute for Comparative Genomics and examined through a technique called polymerase chain reaction, which amplifies specific regions of DNA that can be used to inform researchers about gene flow between populations.
"Understanding the needs of humpbacks and other whale species can be challenging in terms of direct observations of these animals in the wild. Molecular technology gives us a window into the lives of whales that can help us understand the ecological forces shaping their movements and distribution," said Rosenbaum.
"We can also use our findings to inform management decisions for a species that is only now beginning to recover from centuries of commercial whaling."
The humpback whale is a baleen whale that grows up to 50 feet in length. The species has long pectoral fins and a head with knobs on the top and lower jaw. The humpbacks are known for lofting themselves completely out of the water and for their songs, typically sung by males that scientists believe is a mating behavior.
The slow-swimming humpbacks were hunted commercially until the International Whaling Commission protected the species globally in 1966.
Current estimates for humpback whale numbers are incomplete. The IWC's latest estimate of 34,000 - 52,000 Southern Hemisphere humpbacks south of 60 degrees South latitude in summer dates from 1997-1998.
The scientists on this study say that while these whales are recovering, total population sizes may only be a small percent of the original global population.
This study was financially supported by The Eppley Foundation For Research, the Flora Family Foundation, and the Lenfest Ocean Program.
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