A new genetic analysis of Antarctic minke whales concludes that population of these smaller baleen whales have not increased as a result of the intensive hunting of other larger whales -- countering arguments by advocates of commercial whaling who want to "cull" minke whales.
Antarctic minke whales are among the few species of baleen whales not decimated by commercial whaling during the 20th century, and some scientists have hypothesized that their large numbers are hampering the recovery of deleted species, such as blue, fin and humpback, which may compete for krill.
This "Krill Surplus Hypothesis" postulates that the killing of some two million whales in the Southern Ocean during the early- and mid-20th century resulted in an enormous surplus of krill, benefiting the remaining predators, including Antarctic minke whales.
But the new analysis, published in the journal Molecular Ecology, estimates that contemporary populations of minke whales are not "unusually abundant" in comparison with their historic numbers. Using a novel genomic approach, the scientists were able to calculate the long-term population size of Antarctic minkes as roughly 670,000 individuals -- which is similar to estimates of current population size from sighting surveys.
"Some scientists involved in the International Whaling Commission have suggested that Antarctic minke whales have increased three-fold to eight-fold over the last century because of the lack of competition for krill," said Scott Baker, a whale geneticist at Oregon State University and associate director of the Marine Mammal Institute at OSU. "But until now, there has been little evidence to help judge what historic populations of minke whales actually were.
"Our study clearly shows that minke whales today have a great deal of genetic diversity, which reflects a long history of large and relatively stable population size," he added.
Along with Kristen Ruegg and Steve Palumbi of Stanford University, Baker and OSU postdoctoral fellow Jennifer Jackson analyzed genomic DNA from 52 samples of minke whale meat purchased in Japanese markets. The whales had been killed during Japan's controversial "scientific whaling" program in the Antarctic. By amplifying and sequencing a large number of genes, the scientists were able to estimate the historic range of population sizes necessary to produce and maintain the levels of genetic diversity found in the individual minke whales they tested.
"This genomic approach is a significant advance over most previous studies, which have examined diversity using only a handful of genetic markers," Baker said.
Funding for the research was provided by a grant from the Lenfest Ocean Program and the Marsden Fund of the New Zealand Royal Society.
The Southern Ocean is one of the world's largest and most productive ecosystems and in the 20th century went through what Baker called "one of the most dramatic 'experiments' in ecosystem modification ever conducted." The elimination of nearly all of the largest whales -- such as the blue, fin and humpback -- removed a huge portion of the biomass of predators in the ecosystem and changed the dynamics of predator-prey relationships.
Blue whales were reduced to about 1-2 percent of their previous numbers; fin whales to about 2-3 percent; and humpbacks to less than 5 percent. "The overall loss of large whales was staggering," Baker said.
"It is possible that the removal of the larger whales would have meant more food for minkes," Baker said, "but we don't know much about the historic abundance of krill and whether the different whale species competed for it in the same places, or at the same time. It is possible that there might have been enough krill for all species prior to whaling."
The scientists also say that current minke whale populations may be limited by other factors, including changes in sea ice cover.
"The bottom line is that the Krill Surplus Hypothesis does not appear to be valid in relation to minke whales and increasing hunting based solely on the assumption that minke whales are out-competing other large whale species would be a dubious strategy," Baker said.
- Kristen C. Ruegg, Eric C. Anderson, C. Scott Baker, Murdoch Vant, Jennifer A. Jackson, Stephen R. Palumbi. Are Antarctic minke whales unusually abundant because of 20th century whaling? Molecular Ecology, 2010; 19 (2): 281 DOI: 10.1111/j.1365-294X.2009.04447.x
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