Stanford DNA study: Hunting minke whales on grounds of overabundance not justified

By Louis Bergeron

Claiming the population of Antarctic minke whales boomed after World War II, Japan's scientific whaling program has been "sampling" increasing numbers of them each year on the grounds that reducing the number of minkes actually benefits the Antarctic ecosystem.

Meat from these "sampled" whales ends up for sale on the shelves of Japanese grocery stores.

The Japanese position is rooted in the belief that the minke population is booming. But a new analysis of the whales' DNA by a team headed by Stanford researchers concludes otherwise. There is no evidence of a significant increase in the population of minke whales, the researchers said. Their research demonstrates that the current population of Antarctic minke whales is within the historical norm of the species over the last 100,000 years.

"Based on our genetic analysis, average Antarctic minke whale populations over the past 100,000 years have been around 670,000," said Stephen Palumbi, professor of biology, director of Stanford's Hopkins Marine Station and senior fellow at the university's Woods Institute for the Environment. "That number easily falls within the range of current population estimates for the whales, as determined in studies by the International Whaling Commission," he said. Palumbi is the senior author of a paper describing the work, published in the journal Molecular Ecology.

The controversy over the numbers of Antarctic minke whales is rooted in the early 20th century, when commercial whalers killed an estimated 2 million large baleen whales in the Southern Ocean around Antarctica. Blue, humpback, fin and sei whale populations all plummeted.

According to some estimates, those whales, if they were still around, would annually eat as much as 150 million pounds of krill, a tiny, shrimp-like crustacean that is a dietary staple of all baleen whales. Some researchers have suggested that with so many large whales gone, the reduced competition for krill has created an opportunity for the minke to flourish.

Earlier theory untested

If the Antarctic minke whale population has boomed, then their large numbers might be inhibiting the recovery of other whale species that had been overexploited. That idea has been heavily promoted by the Japanese Ministry of Foreign Affairs as one of the main justifications for Japan's annual haul of minke whales. But until now, no one had any scientific data to either support or refute the theory of a minke population boom.

"You can look at a claim of minke whales being super abundant as being a testable scientific hypothesis that has been sitting there for a very long time, without being tested or refuted," Palumbi said. "So we decided to work out the ways of testing that hypothesis."

To determine whether the current population of minke whales represents a boom, Palumbi's team, including postdoctoral researcher Kristen Ruegg, had to compare it to population numbers over tens of thousands of years. Trying to estimate the population of any animal species that far back in time is obviously a challenge, but by looking at the variability in modern DNA, researchers can do just that.

A type of genetic mutation called a "silent mutation" is a minor change to an individual's DNA that has no effect on its ability to survive. These silent mutations are passed on to subsequent generations and add to a population's genetic diversity over time. Because the mutations occur at a predictable rate, the accumulation of mutations – the variability – in modern individuals can be used to work backward and estimate what the population size had to have been at a given point in the past for the number of mutations in the modern whale's DNA to accumulate.

"We have done a lot of work over the last couple of years on pinning that mutation rate down, estimating the variability in rate across many genes and applying it to a well-accepted model of how DNA evolves," Palumbi said. "Those advances have come out in a series of papers over the last couple of years, and so we were ready to use this to test the minke boom hypothesis."

But the first challenge in doing the analysis was simply obtaining some DNA. Mounting an expedition to the frigid waters of the Antarctic is neither easy nor inexpensive, but there is one place where minke whale DNA can be obtained with relative ease – the seafood markets of Japan. Whale meat is sold in these markets to help defray the cost of the Japanese scientific whaling voyages.

Palumbi's team has been going to Japan to obtain minke meat for the last 15 years, collaborating with Oregon State University scientist Scott Baker, and has developed a network of buyers who visit fish markets throughout central Japan for them.

Mobile molecular lab

"They bring the meat to us and we set up a little molecular lab in our hotel room in Tokyo and extract and copy the DNA there in the hotel," he said. The team then brings back a "gazillion" copies for analysis.

The average population of 670,000 minkes that Palumbi's team estimated with their DNA analysis falls well within the population estimates by the International Whaling Commission for the late 20th century. The commission supervised several field surveys that estimated the Antarctic minke whale population was about 608,000 between 1978 and 1984, and roughly 766,000 from 1985 to 1991.

Palumbi said the Japanese estimate the current minke population at 760,000. "They put that number on every package of minke whale meat that they sell," he said.

An unpublished 2006 report of another population survey supervised by the commission suggests that the minke population may actually be declining, rather than booming or remaining steady.

None of the estimates support the notion of a late 20th-century population explosion of Antarctic minke whales when compared with Palumbi's historical estimate.

"The hypothesis of a population explosion resulting from krill abundance has no validity," Palumbi said. "We can refute it, and that failed idea is no reason whatsoever to be hunting minke whales."

Palumbi said the larger goal of the study was to provide accurate information to use in managing the Antarctic minke whale population. "What any kind of management needs is scientific data," he said.

"If you don't test a hypothesis, if you just assert that something is really important and then move on to your management plan from that standpoint, then not only is that management plan likely to be wrong, but you have no way of improving it over time," he said.

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