260 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1946 
inhabit the North Pacific and undertake regular migrations along the 
coasts of North America and in Japanese waters. There is less infor- 
mation on the distribution of the sei, lesser rorqual, pigmy right, and 
Bryde’s whale. (8) Certain planktonic Crustacea form the principal 
food of the whalebone whales. In the Antarctic they feed virtually 
exclusively on the shoals of Euphausia superba. In the northern seas 
the diet seems to be more varied. Meganyctiphanes norvegica is prob- 
ably the most important food organism in the North Atlantic, but 
further investigations are needed. Little food is taken in winter, 
though fish and small quantities of other Crustacea are sometimes 
eaten. (4) Examination of the reproductive organs and measure- 
ments of foetuses at different times of year show that breeding mainly 
takes place in winter and that the period of gestation is about a year. 
Normally one young is born at a time, and the usual interval between 
successive pregnancies is probably 2 years. This applies to blue and 
fin whales, but other species are probably similar. Blue and fin whales 
are believed to become sexually mature in about 2 or 3 years. The old 
corpora lutea of the ovaries persist and accumulate, and constitute the 
best indication so far found of the age of an adult whale. There is 
some evidence that the rate of increment is about 1 per year, but this 
again needs confirmation. The largest recorded number is 54. Indi- 
cations of periodic growth in the baleen plates constitute a new method 
of determining the ages of young whales. The rate of growth is faster 
in the anterior than in the posterior part of the body. (5) Whalebone 
whales are more plentiful in the Southern than in the Northern Hemi- 
sphere. The sexes are nearly equal. The existing ratio of blue, fin, 
and humpback whales is estimated to be of the order of 15, 75, and 10 
respectively in the Southern Ocean, but no estimate has yet been made 
of the absolute numbers in the populations. Most whaling is carried 
out by the Antarctic pelagic factories, and little is done now in the 
Northern Hemisphere. The stocks of blue and humpback whales have 
been depleted by the modern industry, but fin whales have been less 
affected, and progress has been made in the international regulation 
of whaling. (6) In the future it will be necessary to continue research 
to some extent on the same lines as before, but new or modified methods 
could be developed, and aircraft and modern technical devices might 
be used with advantage. 
REFERENCES 
ALLEN, G. M. 
1916. The whalebone whales of New England. Mem. Boston Soe. Nat. Hist., 
vol. 8, No. 2, p. 105. 
ALLEN, J. A. 
1908. The North Atlantic right whale and its near allies. Bull. Amer. Mus, 
Nat. Hist,, vol. 24, p. 277. 
