Like fishermen, whale hunters sometimes alter the details of their catch. In the 1960s, Soviet Union (USSR) whalers illegally killed almost 180,000 cetaceans, but reported taking far lower numbers. Now, it seems that Japanese whalers in the North Pacific also manipulated their numbers around this time, according to a new study. The finding, which comes as Japan is readying to hunt whales for what it says are research purposes, raises new concerns about the country’s current endeavors; it also may invalidate several past studies on whale demographics and conservation, the authors say.

“It’s really nice investigative work,” says Andrew Brierley, a marine ecologist at the University of St Andrews in the United Kingdom. “They’ve shown that the Japanese whalers added a fudge factor to make it appear that the whales they took were at the legal size [for hunting], when in all likelihood, they probably weren’t.”

The false Japanese data have only come to light because of the previous discovery of the USSR’s fake reports. Both countries—along with all other whaling nations—began submitting catch statistics, including the sex and length of killed whales, to the International Whaling Commission (IWC) in 1946. The organization, which oversees the global conservation and management of whales, specified where each country could hunt and the number and species of whales that they could take. Member nations also agreed to carry biologists on board whaling vessels to collect the catch data.

But in 1948 the USSR went rogue and began hunting whales illegally. Over the next 30 years, its whaling industry killed an estimated 178,811 more cetaceans than it reported to IWC. Fortunately, four Soviet biologists kept secret, correct records, which were declassified in the 1990s.

In 2007, Yulia Ivashchenko and Phillip Clapham, cetacean experts at the National Oceanic and Atmospheric Administration’s Alaska Fisheries Science Center in Seattle, Washington, joined their colleagues and one of the former Soviet (now Russian) biologists to adjust IWC’s records using the real Soviet whaling data. They discovered that the USSR hadn’t only engaged in illegal killing, but it had also ignored whaling regulations, particularly those pertaining to the sperm whale, Physeter macrocephalus.

Because this species had been heavily hunted for its oil since the late 1700s, IWC ruled that only sperm whales longer than 11.6 meters could be killed. (Male sperm whales may reach 20 meters in length, and females attain a maximum size of only 14 meters.) But the USSR whalers brushed aside this regulation, falsifying female lengths or recording females as larger males. “They did it so there wouldn’t be a discrepancy between the catch figures and the [whale] oil they collected,” Ivashchenko says, explaining that male sperm whales produce more oil.

While interviewing the Russian scientists, Ivashchenko got a tip. “Several people said, ‘Oh, the Japanese did the same thing.’ But there was no evidence.” Finally, in 2002 a retired Japanese whaling station manager admitted that the country’s coastal whaling stations had regularly falsified their data, even after the 1986 whaling moratorium went into effect.

To find out if the country’s pelagic fleets had also turned in fake data, Ivashchenko and Clapham turned to the now correct Soviet reports. They focused on the two countries’ catches of sperm whales, because the Japanese and Soviets were often hunting the marine mammals in the same area at the same time. In their real records, the Soviets reported having a “hard time to find legally-sized sperm whales,” Ivashchenko says. “Yet the Japanese were getting their quota each year—even though they were spending less time hunting. So, come on! These numbers are just not real.”

The scientists’ close comparison of the two countries’ 1968 and 1969 sperm whale hunts revealed just how unreal the Japanese reports were. Over those 2 years, the Japanese whaling fleets in the North Pacific reported harpooning 1568 females. Of these, 1525—or 97.3%—were listed as being at or above IWC’s minimum length requirement. In contrast, the real Soviet data show that their whalers (who were hunting in the same area as the Japanese) killed 12,578 females—yet only 824 (or 6.6%) were the legal size. The doctored Japanese records report that their fleet even managed to catch 141 females in 1969 that were 12.5 meters or longer; that same year, the Soviets harpooned 5680 females, and only two of them were such a large size.

After 1972, when IWC required whalers to have independent observers on board, the Japanese reported catching far fewer large female sperm whales or those of the minimum size, Ivashchenko and Clapham report today in Royal Society Open Science.

Discovering that the Japanese whaling data for sperm whales is likely fabricated opens a can of worms, says John Frizell, a spokesman with Greenpeace in the United Kingdom. “Current population estimates and conservation depend on accurate historical data.” And, notes Clapham, “these fake data are in the IWC catch database; researchers use it. How many studies are now invalid because of this?” He, Ivashchenko, and others also suspect that if the Japanese whalers falsified their sperm whale catch data, then they likely submitted inaccurate reports on humpback and fin whales, too.

So far, no one in Japan has stepped forward to help correct the data as the Russian biologists did. Joji Morishita, the director-general for the National Research Institute of Far Seas Fisheries in Tokyo and a delegate to IWC, says that he “welcome[s] any scientific work for the purpose of correcting past catch reports” in an email to Science (which he stresses represents his views and not necessarily those of his agency or of Japan's Institute of Cetacean Research, which oversees its whaling programs.) But he seems to think that the new paper only indicates “there was misreporting by the USSR in the past.”

Frizell notes that the Japanese delegation gave similar responses at the recent IWC meeting in San Diego, California, where Ivashchenko presented an earlier version of the paper. “This stonewalling over something that happened over 40 years ago raises concerns on whether we can trust Japan’s current whaling data,” he says.

He’s not alone in his suspicions. C. Scott Baker, a conservation geneticist at Oregon State University, discovered in 2001 that the Japanese regularly underreport the number of whales killed as bycatch. He notes that “the 1986 moratorium on commercial whaling did not put an end to Japanese whaling or the misreporting of catches.”

And that means that if the Japanese eventually persuade IWC member nations to reinstate commercial whaling, then there “must be absolutely independent observers on factory ships and in the marketplace,” Clapham says. “We know what happens without observers. People cheat.”