A key method of controlling whale populations involves body length, which relates to their age and likely breeding status, particularly for females. The IWC previously set a minimum legal catch length of 38 feet.
For the study, the researchers compared whale length data from catches Japan reported to the IWC with data for the same period from the Soviet Yuri Dolgorukiy factory feet during 1960–1975.
The Soviets and other countries also conducted illegal whaling, but Ivashchenko, also at the National Marine Mammal Laboratory, managed to find formerly secret Soviet whaling industry reports in Russian archives.
Video Slideshow: Wondrous Whales from Drone's Eye View
"The data used in this particular paper were collected by Dmitry Tormosov, who hid 57,000 individual catch records (i.e., forms filled out on the deck of the factory ship) in his potato cellar in Kaliningrad for 30 years until after the Cold War," Clapham said. "We've seen the original data."
Prior to the 1972 implementation of the International Observer Scheme (IOS), which required that an independent inspector be aboard whaling ships, the Soviet fleet was documented as having killed 5,536 female sperm whales, of which only 153 were at or above the minimum legal length. During the same period, Japan killed 5,799 female sperm whales and reported 98.5 percent were of legal size.