AGE-DETERMINATION BY MEANS OF THE OVARIAN CORPORA 463 



ovulation of 1-446. Assuming the incidence of post-partum ovulations to be nearer to 80%, we may 

 adopt a figure of about 1-43 ovulations per year. Even if the incidence of post-partum ovulations is as 

 low as 60%, which is improbable, the average annual increment (for 18% pregnant/lactating) is 1-282. 



If the fin whale is not strictly monoestrous, but only shows a strong tendency to this condition, 

 then the average annual rate of ovulation will be slightly higher than this estimate. An average of 

 i-i ovulations per ovulatory period (excluding multiple ovulations) would raise the estimate from 

 1-43 to 1-57, but in the absence of supporting evidence this possibility has not been allowed for in 

 adopting an estimate of 1-43. 



There are now two independent estimates based on the evidence presented in the present paper, 

 which are in fairly close agreement, namely about 1-4-1-5 (p. 385) and about 1-43. Allowing for 

 possible causes of variation or sources of error it is considered that the average annual rate of ovulation 

 in the southern hemisphere fin whale is i-43±o-i. In calculating ages from the numbers of corpora 

 albicantia in the ovaries the annual rate of accumulation is taken to be 1-43 corpora albicantia. In 

 general estimates of relative individual ages, post-puberty, are likely to be accurate to within ±7% 

 (namely ±1-4 corpora, that is, 1 year, in 14 years) 1 . 



Recovered whale marks 



A long-term programme of whale-marking was initiated in 1934-35 by ' Discovery ' Investigations, and 

 continued after the war by the National Institute of Oceanography and other interested bodies in 

 Norway, Japan, Russia, Australia, and New Zealand, with the support of the International Whaling 

 Commission and the Whaling Companies (Brown, 1954; Chittleborough and Godfrey, 1957; Clarke 

 and Brown, 1957; Dawbin, 1956a; Mackintosh, 1942; Rayner, 1940). 



One of the objects of this marking programme was to provide a check on methods of age-determina- 

 tion. Unfortunately, however, there are difficulties which are peculiar to whale research. It is not 

 possible to capture whales, mark them and then release them as in marking fish, seals, and other 

 animals. Whale-marking is effected by firing a numbered metal tube into the dorsal muscle of the 

 free-swimming animal — a costly operation. One consequence of this is that it has not so far been 

 possible to confine marking to one particular known age-class (e.g. the calves), nor is it possible to 

 determine the age or size of individual whales at the time of marking. In any case measurements of 

 body length are very variable, even within a single age-class, and estimates of size in the water are 

 usually inaccurate. 



Up to the season 1958/59 about 5000 fin whales had been marked in antarctic waters and 373 marked 

 whales had been recovered up to 1 958/59, one of which had a minimum age (from marking to recapture) 

 of 24 years. 



Unfortunately only 10 of these recoveries were also accompanied by material (the ovaries) enabling 

 us to make an attempt to check this method of age-determination. The reason for this very low figure 

 (only 4% of the females from which marks were recovered), is that usually the processing of the carcass 

 is well advanced before the mark is found. A fair proportion of the marks are not found until the 

 cookers are cleaned some hours, or days, after processing. 



1 A rate of ovulation of 0-7-0-75, suggested as a possible alternative (p. 385) appears to be incompatible with the 

 evidence of the sexual cycle. Such a low rate is only possible if the cycle is markedly different from that described here. 

 Thus, it would be necessary to accept much lower ovulation rates at the post-partum and post-lactation periods (of the 

 order of only 10-30 %) in order to bring the ovulation rate into line with a value of 0-7-0-75. It is possible that the 

 incidence of post-partum ovulations is lower than has been assumed ; the crucial evidence for this relates to the relative 

 sizes and rates of regression of corpora albicantia (pp. 430-4, and especially p. 433). The assumption of a high incidence 

 of post-lactation ovulations is thought to be well-founded (see pp. 436-44). The evidence considered as a whole favours 

 the higher rate of ovulation. 



18 DI - 



