AGE-DETERMINATION BY MEANS OF THE OVARIAN CORPORA 465 



±0-67 years, assuming that growth is at the rate of just over 3 ft. a year at this part of the growth 



curve. 



It appears then that about 157 corpora accumulate in from 12-63 to J 3*97 Y ears (i3"3±° ,6 7 years), 

 that is, at a rate of about 1 -24 to 1 • 1 2 per year. So far it has been assumed that the ten mark recoveries 

 with ovary data are a random sample from the population. This is probable, but the variance for such 

 a small sample is large, and the true mean rate of accumulation may be higher or lower than this 

 figure. There is also another source of error which must be mentioned. 



In the confusion of work on the deck of a whale factory ship there is a possibility that ovaries may 

 sometimes be collected by mistake from the wrong whale. It seems likely that this was the case with 

 mark number 6137. The female from which this came was 22 years from marking to recapture, but 

 the ovaries which accompanied this mark contained only one corpus luteum and 10 corpora albicantia. 

 The baleen plates had the appearance of those of a young whale. It seems probable that this was such 

 a case of mistaken identity, because by a remarkable coincidence two marks were recovered that same 

 day, on the same factory ship, by the same man, both from female fin whales of identical lengths. 

 For one (mark no. 12673) tne mar ^ is notecl as having been 'found in the dorsal muscle'; for the 

 other (mark no. 6137) there is no note of the location of the mark. Now, if a mark was found in the 

 dorsal muscle it is probable that the ovaries would be obtainable as well. It is, therefore, possible that 

 the ovaries were actually from the whale which bore mark no. 12673, which was only 3-22 years from 

 marking to recovery. This would raise the mean annual rate of accumulation calculated in this way 

 to 1-30-1-46 corpora. In view of this uncertainty it will probably be best to ignore the doubtful 

 record and to repeat the calculations for the 9 mark recoveries about which no doubts have been 

 raised. Then it is estimated that in about 12-39^0-67 years on average some 16-2 corpora have 

 accumulated, that is about 1-24-1-38 per year (or 1-31 ±0-07). 



Two other females shown in this table (nos. 1199/1203/1300, and no. 7972) are at first sight 

 incompatible with a rate of ovulation of i-43±o-i per year, but it should be emphasized that this 

 is a mean value. Even if ovulations in these two were at the rate of 1 -4 a year, then the apparent 

 discrepancies in the mark data can be explained by assuming that these females matured late (at 1 1 years 

 and 8 years respectively). If the rate of ovulation were lower then puberty could have been attained 



earlier. 



We can have little confidence in this evidence from the recovery of marked females, which is 

 inconclusive (owing to the small size of the sample), but does not necessarily disagree with the esti- 

 mated average increment of 1-43 ±o-i corpora per year, obtained from studies of the ovaries and the 

 reproductive cycle. This is the best we can do with the limited data. In view of the difficulty of 

 recovering whale marks together with other data, it seems unlikely that a precise confirmation of this 

 method of age-determination for the fin whale (or of any method) will be obtained for many years, 

 (but see footnote p. 470). 



Age-determination 

 For average growth curves or for population studies the corpora counts may be used to determine 

 the post-pubertal age using the estimated figure of 1-43 ±o-i for the average annual rate of accumula- 

 tion. For animals which have not attained puberty the method cannot of course be used. 



The estimated age at puberty averages 5 years (p. 407), varying in individuals at least from 3 to 

 8 years in a small sample, and the extreme range is probably greater. There will, therefore, be a con- 

 siderable variation in individual ages estimated in this way, according to whether the female became 

 mature at an early or a late age. A simple calculation shows that this variation alone would result in 

 a variation of ±4 corpora at a given absolute age. This is in reasonably close agreement with the 



18-2 



