Papers in Marine Biology and Oceanography. Suppl. to vol. 3 of Deep-Sea Research, pp. 257-2W). 



The mortality rates of Antarctic fin whale stocks 



By JoHAN T. RuuD 

 Universitetets Biologiske Laboratorium and Statens Institult for Hvalforskning, Oslo 



In 1820, William Scoresby, Jr., advanced the theory that the transverse striations 

 occurring at regular intervals on the surface of the baleen plates of a whalebone 

 whale afford an intimation of the age of the whale. Eschricht and Reinhardt 

 (1866) did not agree with this proposal, although they did agree that the striations 

 were due to a periodicity in the growth of the plate. 



Stimulated by the need for a method of making individual and direct age deter- 

 minations of whales, we have studied the surface structure of the baleen plates and 

 their possible application to age analyses since 1939. At the same time and quite 

 independently, A. G. Tomilin (1945) examined baleen plates in the collections of 

 Russian museums. 



In a series of papers (Ruud, 1940, 1945; Ruud and Jonsgard, 1950). we have 

 shown that the transverse striations of the baleen plates are due to variations in the 

 thickness of the cortical layer or enamel. We have also described how records made 

 of these variations reveal growth periods which we have assumed to be annual. 

 On this assumption we have developed a method for age analyses. The method has 

 limited apphcabiUty because the baleen plates are subject to wear at their distal ends 

 and therefore growth periods are gradually worn away. 



We believe, however, that the age of fin whales in their first, second, third and 

 fourth years (age groups O, I, II and III) can be determined with great exactitude, 

 because of the presence of structures on the distal portion of their baleen plates, 

 which indicate that this part was formed during the suckling period or during the 

 first krill-feeding season. We assume further that our analyses of records from baleen 

 plates of whales in age groups IV and V give fairly accurate results. Admittedly we 

 can give only minimum estimates of the ages of older animals, because at some 

 undetermined age the annual growth increment of the baleen plates is balanced by 

 the annual wear at the tip; from then on we find a maximum number of 4 to 7 growth 

 periods in the plates. Hence, our age groups above V should more correctly be 

 VI + , VII + , and VIII + , meaning that older animals are telescoped into our three 

 highest baleen-record groups. 



For some years we have engaged in age studies of Antarctic fin whales. The 

 material consists of baleen samples from catches of Norwegian pelagic expeditions 

 in post-war seasons. Details concerning the material, analyses and results will be 

 published shortly in Hvalradets Skrifter by Hylen, Pike, Jonsgard and Ruud. 

 The histogram (Fig. 1) shows the percentage age distribution of the Antarctic fin 

 whales taken during this period. In 1945/46 and 1946/47 our samples were small. 

 It is therefore questionable whether they were representative of the catches, but 

 since 1947/48 we have determined the ages of 850 to 1250 fin whales each season. 

 Chi-square tests on length distributions show that our samples from the last six 

 seasons are fairly representative of the catches from which they are drawn. 



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