406 DISCOVERY REPORTS 



season, and that annual variations in the mean date of pairing have been within the extreme range of 

 ±2^ weeks, during the period for which data are available. It is possible that variations in the spatial 

 distribution of the catches are partly responsible for these fluctuations, that is to say, the timing of the 

 pairing season or the foetal growth-rate may be slightly different in the different oceans, but it seems 

 more likely that climatic factors are at work. A year when pack-ice was late in retreating might retard 

 the southward and northward migrations and delay the pairing season. Dawbin (1956) has shown that 

 at Cook Strait, New Zealand, the mid-point of the northward migration of humpback whales is 5 July. 

 In 36 seasons the maximum variation has been from 21 June to 29 July, that is about 5 weeks. This 

 is in quite close agreement with the conclusions about the annual variations in the monthly mean 

 sizes of fin-whale foetuses. It is a problem which might well repay further study. 



Sexual maturity 

 Strictly speaking the term sexual maturity should be applied to the peak period of reproductive 

 performance when fertility and reproductive rate are at a maximum. In the literature on whales the 

 attainment of sexual maturity is understood to refer to that part of the life-cycle when females first 

 ovulate and males first produce sperm. Although it thus corresponds more accurately to puberty, 

 because of its wide use the term sexual maturity is retained here, although it is not strictly correct. 



The mean length at sexual maturity 



Various estimates of the length at the attainment of maturity of female southern hemisphere fin whales 

 have been made, all of which correspond rather closely. Mackintosh and Wheeler (1929) estimated 

 the mean length of the female at sexual maturity to be 65 ft. 7 in. This was later amended by Mackin- 

 tosh (1942) to 65 ft. 3 in., using the same data as well as additional records accumulated up to 1941. 

 Brinkmann (1948) concluded that the mean length at sexual maturity is 65-66 ft. 



Estimates of the mean length of the female at sexual maturity made by Japanese workers range 

 from 67 to 68 ft. (Nishiwaki, 1950; Nishiwaki and Hayashi, 1950; Nishiwaki and Oye, 1951), but the 

 figure of 64 ft. given by Nishiwaki (1957, p. 30) does not agree with these earlier estimates. The 

 apparent discrepancy between the results of Japanese and European workers is accounted for by the 

 fact that the former take the average length at sexual maturity as the length when 75 % of females are 

 sexually mature. (Their reasons for this procedure are not clear.) When recalculated by taking the 

 length at which 50% are mature and 50% immature, the mean length is seen to be about 65 ft. 

 (Nishiwaki, 1950, fig. 226) and conforms closely with the other estimates obtained in this way. The 

 European material is mainly from antarctic areas II and III, whereas the Japanese material is mainly 



Table 12. Length frequencies of sexually immature and mature fin whale females 

 Length (ft.) Immature Mature 



