47° DISCOVERY REPORTS 



animals of known age. If the validity of any one of the methods could be confirmed by reference to 

 known-age animals, then, in view of the agreement between the results obtained separately by the 

 other methods, it could be taken that all three methods are valid for studies of the age-composition 

 of the catches. In the 25 years since effective whale-marking began, only 10 marked fin whales have 

 been recovered together with material enabling age-determinations to be made. It seems unlikely that 

 there will be any great increase in such recoveries in the near future, and for the time being the sample 

 now available is the only independent check on age 1 . The estimated annual increment of corpora 

 and the figure suggested by these marked females are very similar, but owing to the small size of 

 the sample the apparently quite close agreement is not conclusive. There can, however, be no reason- 

 able doubt that the ovaries provide reliable estimates of age, and that the ear-plugs and baleen plates 

 (with some reservations) are also reliable. 



One important qualification is called for. The estimated ages are greatly dependent on the method 

 of examination. In this respect the counts of ovarian corpora are perhaps least liable to error. The 

 interpretation of baleen plates is difficult, and the same applies to the ear-plug. In this connexion 

 reference must be made to the recently published papers of Ohsumi, Nishiwaki and Hibiya (1958) 

 and Nishiwaki, Ichihara and Osumi (1958). These authors, by plotting lamination number against 

 corpora number, and assuming two laminations are laid down each year, obtain a figure of 0-8-0-9 

 for the average annual increment of corpora in northern and southern hemisphere fin whales. Their 

 results differ in this and other respects from Nishiwaki (1957), Purves and Mountford (1959) and the 

 present paper. It seems likely that interpretations of the ear-plug laminations differ (and/or counts 

 of corpora), and a standardization of methods is desirable. 



n . , Applications 



Survival curves 



Having shown that the corpora albicantia persist throughout life and accumulate at a regular rate, 

 it is possible to use corpora counts, made on samples of the antarctic catches since 1925, to investigate 

 the changing age structure. 



A full detailed treatment of this subject would be out of place here, but to indicate the possibilities 

 let us take as an example the data for the two seasons 1939/40 and 1940/41, for Antarctic area n 

 (o° to 70 W.). For this period there are corpora counts relating to 389 adult females, 170 from the 

 season 1939/40 and 219 from 1940/41. Brown (1954) has shown that there is little dispersal between 

 the different whaling areas, but suggests that there may be a significant interchange between areas 1 

 and 11. Virtually no whaling had been carried out in area 1, prior to 1955/56 and it may be assumed 

 for present purposes that the whales in area 11 constitute a stock which is separate from the other areas. 



This sample from area 11 (1939-41) has been chosen because it is fairly large and of the samples 

 available is probably least influenced by changes in the size of the catches. The age composition of 

 post-war samples is affected by the low level of catching between 1940 and 1945. From Text-fig. 54 

 it would appear that for the effect of fluctuating catches to be minimal the most suitable samples 

 should come from area 1 in 1955/56, area 111 before 1934, area iv before 1937, and area v before 1930. 

 If the populations of fin whales in the different areas are distinct and isolated (though not in a genetical 

 sense) such samples might enable estimates of the natural mortality rates of adult females to be made. 

 Unfortunately adequate samples meeting these conditions are not forthcoming. 



Three possible methods of constructing survival curves have been examined and although, owing 

 to sampling difficulties none of them is really satisfactory, they can be used to compare the changes 

 in the stocks from year to year in a relative way. 



1 Since this was written there has in fact been an increase in the number of such recoveries in Japanese factory ships. 



