(from "ISANA" No. 10, 1994)

Hidehiro Kato, Ph. D.
National Research Institute of Far Seas Fisheries

The blue whale is a member of sub-order of Mysticeti (baleen whale; 10 species), and belongs to the genus of Balaenoptera of the family of Balaenopteridae. Its English name, "blue whales", was derived from its grayish body appearing bluish when it is submerged in the water. It is called "shironagasu kujira" in Japanese, but in ancient times (as long as we gather from old whaling history scrolls) it had been called simply as "nagasu kujira." The fin whale which is now called "nagasu kujira" had been called "noso kujira."

Its academic name is Balaenoptera musculus. Balaenoptera which designates genus is a combination of two Latin words, balaena (meaning whale) and "pteron" (meaning fin). It thus means a whale having a dorsal fin so as to distinguish from balaena (meaning whale without dorsal fin). "Musculus" designating species is a Latin word originally meaning a "mouse." It is said that Carolus Linnaeus mistakenly gave the blue whale the name of a fish which enticed this whale⁽¹⁾. Giving the name of a mouse to the largest creature on earth seems to be a tremendous mistake. But it seems that a name, once we get accustomed to it, comes to sound right, whether it is a mouse or if it is a tiger.

Among morphological features of the blue whale, one can point to its unique flat skull (upper jaw) like a thick traditional Japanese straw sandal, dorsal fin slightly projecting on the back of the body, and inner curve of the outer margin of the flipper. These three features constitute the unique characteristics of a blue whale. The National Science Museum in Tokyo is now planning to replace the real-size model of a humpback whale with that of a blue whale, and I joined the adviser's panel for the designing of the model with other cetologists. The first thing we agreed in making the prototype - one thirtieth of the real size - was to make sure that these three points accurately represented. Interested readers can see the real model when it is completed early this spring.

The longest body size on record based on a reliable source is said to be about 31 meters, and on the basis of this figure, the total weight is calculated at around 200 tons. But not all the individuals grow up to this size. The average size at which growth ceases for Southern Hemisphere blue whales (which are normally larger than their Northern Hemisphere counterparts as well as for other Balaenopterids) can be calculated to be 25 meters for male and 26 meters for female.

A blue whale is born at a body length of some seven meters after 11 months of gestation. This length is almost equivalent to that of a smaller minke whale - but this is truly remarkable as the size of a baby at the time of birth. After about seven months of lactation by mother, it grows to the size of an average Brydes whale (13 meters), at which the calf weans from the mother. The net of increase in size of growth during the 180-day period is six meters, or about 3 centimeter a day based on a simple calculation.

The blue whale becomes sexually mature at the size of 22.6 meters for male and 24 meters for female around the age of 5. As the age at sexual maturity changes by environmental factors, we may not need to have a fixed concept of the age at sexual maturity. This estimate was taken from the later period in the history of the blue whale hunting. Considering that the age at sexual maturity generally declines in line with the decrease of stock level (or in line with the improvement of feeding environment), it may be reasonable to consider that the original age of sexual maturity of blue whales should have been higher.

The longest life span of a blue whale is believed to be as long as 110 years, but this is slightly shorter than that of the fin whale. This estimate was made when the age assessment methods then had not been established and the number of ear plug specimens collected - which determined the age - was small relative to the catch amount. Therefore, the age of individuals to be caught could have been set at a slightly higher level.

One thing we should not forget when talking about the blue whale is the presence of the pygmy blue whale (Balaenoptera musculus brevicauda). As this name shows, this whale is smaller than the true blue whale, with an average body length of sexually mature male and female at 20.6 meters and 21.8 meters, respectively, with the maximum size probably reaching 24 meters. It is distributed in the Southern Hemisphere and northern Indian Ocean. In summer they are common in the area off Kerguelen or Chile. Segregation from ordinary blue whales is observed throughout the Southern Hemisphere with the Antarctic convergence serving as a demarcation line⁽²⁾. The species was made target of catch by Japanese and Soviet whaling fleets from late 1950s to early 1960s. There has been some confusion about the year in which it was listed as a sub-species. However, Japanese scientist Dr. T. Ichihara proposed the need to distinguish the pygmy blue whale as a sub-species from ordinary blue whale based on the findings of relatively small-sized body⁽³⁾, tail part and baleen. In this sense, therefore, the pygmy blue whale remains an impressive name for Japanese cetologists. The blue whale species often observed in sighting surveys in the Southern Hemisphere is pygmy blue whale. Although experienced whalers may have accumulated special skills in identifying species one from another, identification of whale species on the sea is not an easy job. It is hoped that appropriate steps will be taken to facilitate the process of this work.

References
1) Seiji Ohsumi, 1986 Geiken-tsushin (Newsletter of the WRI) 336;41-44
2) Hidehiro Kato et a1. 1993, Abstract, The 16th Symposium on Polar Biology, P.76
3) Tadayoshi Ichihara, 1966, pp. 79-113 in "Whales, Dolphins and Porpoises". Uuiv. of Calif. Press p. 789

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