(from "Kujira to Inbou" (Whales and Plots), by Yoshito Umezaki, 1986)

Like the growth rings of trees, the layers observed in a section of the earplugs of whales are the most reliable source for determining the age of whales. A dark layer developed in the breeding season and a light layer developed in the feeding season form a set for one year. Also, the distance between these growth layers becomes suddenly narrow when whales become sexually mature. This is because lateral growth of the external auditory meatus changes suddenly. Therefore, by examining the earplugs, we can determine at which age whales have become sexually mature.

In the case of Southern Hemisphere minke whales, it was found that sexual maturity had changed to a younger age - from twelve to thirteen years old in the 1940s to eight years old in 1970. Since the commercial harvest of minke whales started in the early 1970s in the Antarctic Ocean, this change can be interpreted as meaning that the environment for their growth had been improved due to the reduction of blue whales. If sexual maturity comes at a younger age, many whales have calves at a younger age, and the number of births per female increases, and this becomes a cause of increase of the population.

To avoid admitting this, anti-whaling scientists insisted that the interpretation of the growth layers of the earplugs was incorrect. Several Japanese scientists made judgments about the age determination of minke whales and their results matched within two years old. On the contrary, anti-whaling scientists insisted that the results contained an error of 50%. This means what was judged as a 10-year old whale by Japanese scientists could be 5 or 15 years old, and what was judged as a 40-year old whale could be 20 years old or 60 years old. To the question of "On which evidence do you so insist?", they just repeated: "There's no evidence but we believe it."

In reality the figure of 50% was a key. Using such a high value of error and doing the computation, denying the decline in the age of sexual maturity can be easily obtained as a result. In other words, they wanted the figure of 50% error to obtain this result. Therefore it was necessary to use an error value of 50% for the interpretation of age.

Japan got another unscientific refutation on the determination of the age of sexual maturity. In the case of minke whales, they are ready to become parents when they grow to about 8 meters long. The Japanese scientists analysed how many years it took to grow to this length, based on the growth curve. The result almost coincided with that of the earplugs analysis. It became clear that minke whales became 8 meters long at twelve to thirteen years old in old times, but they became the same length at 8 years old in the late 1960s.

Anti-whaling scientists denied this result because "different results can be obtained by different statistics". But they did not submit any scientifically acceptable evidence to support their insistence.

In the case of the pregnancy rate, anti-whaling scientists adhered to an insistence which ignored whale biology. It was insistence on the decline of the pregnancy rate of North Pacific sperm whales. The data used were true. Since the data were submitted by Japan, Japanese scientists do not deny the source data itself. What the Japanese scientists argue is that the data were obtained in different areas.

With 40 degrees north latitude as the border, sperm whales spend the breeding season in the south and spend the feeding season in the north. Female whales which get pregnant in the south soon migrate to the north, but females which are not pregnant remain in the south. Therefore, the pregnancy rate is high in whales taken in the north, and it is low in the south. Anti-whaling scientists compared the two datasets and insisted that the decline of the pregnancy rate showed the depletion of the sperm whale population.

One pregnancy rate was obtained from the dataset of the 1950s when the Japanese fleet operated in the northern area during the intervals between periods of fin whaling, and another pregnancy rate was obtained from the dataset of the late 1970s when the Japanese fleet took sperm whales in the southern area after the ban of fin whaling. It is a matter of course that the pregnancy rates of two datasets were quite different. However, although anti-whaling scientists knew the ecology of sperm whales, they did not withdraw their insistence on the decline of the pregnancy rate on the basis that the two values were obtained from the same stock.

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