OCEANIC MAMMALS 517 



season for the blue and finback whales, and at a time when the 

 sea temperature is highest. Small numbers have been taken 

 in the last decade off the coast of California. 



Matthews (1938b, p. 277) has summarized the study made 

 of these whales in southern waters, from which it appears that 

 during several seasons from 1923 to 1927 the catch at Natal 

 and in the Cape Province of South Africa has varied from some 

 50 or 75 to in some years as high as 350. The external charac- 

 ters of the southern whales do not seem to differ from those of 

 the North Atlantic or Pacific animals. "The migrations of 

 the Sei whale consist of a feeding migration to the south in the 

 southern summer and a breeding migration towards the north 

 in the winter. Parturition and pairing occur mainly in tropical 

 and sub-tropical waters. Lactation is mainly finished by the 

 time the whales arrive on their southern feeding grounds 

 . . . The neighbourhood of South Georgia represents the 

 approximate southern range of the migration." The main 

 part of the pairing season in the southern oceans is from May 

 to August, with the peak in July. The period of gestation is 

 about 12 months. According to Matthews the evidence of the 

 internal genitalia shows that the species "breeds once every 

 two years, with a period of anoestrus of about 6 to 7 months 

 between the end of lactation and the beginning of the next 

 pregnancy." "Sexual maturity is reached at an age of about 

 18 months, and breeding first occurs at the end of the second 

 year after the birth of the whale. " It is clear from these facts 

 that with only a single young at a birth, the rate of increase is 

 bound to be relatively slow. 



Though this whale is not as yet in any obvious danger of 

 serious depletion, and on most of the world's whaling grounds 

 is of minor importance, nevertheless, Matthews (1938b) points 

 out that "when the serious diminution in numbers of the larger 

 and more profitable species, which appears to be imminent, 

 arrives, this species will suffer considerably and will be in 

 danger of being reduced to a very small remnant in a short 

 time. " Among other factors that might conduce to this result 

 is the sex ratio, in which it appears from a study of embryos 

 and adults that males occur in a ratio of about 60 percent, so 

 that the number of young would be less than in species in 

 which the ratio were more nearly equal. Moreover, says 

 Matthews, this ratio is often obscured in a study of whales 



