EFFECTS OF WHALING ON BALEEN WHALES 149 



the post-war catches (derived from the length statistics) for comparison with 

 some data bearing on this question which Professor Ruud has kindly allowed 

 me to pubhsh. Some of this material is presented in the paper by Hylen, 

 Jonsgard, Pike & Ruud (1955) who discuss its validity and give reasons for 

 believing that the material for the first two seasons is unrepresentative of the 

 catches. The figures for the proportion of sexually immature females in the 

 Norwegian samples are based on inspection of the ovaries, the presence of 

 one or more corpora lutea or corpora albicantia indicating maturity. As 

 expected, these figures are slightly higher than those pubhshed in the 

 International Whaling Statistics (see above). They show a similar trend, 

 increasing by about 10 per cent from 1947-8 to 1954-5, and confirm that 

 the proportion of immature and therefore younger animals in the catch has 

 increased. 



(c) Age composition 



In Fig. 5 are plotted some data on the age composition of the catches, also 

 provided by Professor Ruud. These show that the proportion of the younger 

 age groups in the catches has increased markedly, the criterion of age being 

 the pattern of ridges on the baleen plates (Ruud, 1945). The proportion of 

 females in baleen groups O-III has increased from about 25 per cent in 1947-8 

 to about 45 per cent in 1954-5 (smoothed curve). These data from samples 

 of the catches made by Norwegian expeditions confirm the general conclu- 

 sions drawn from the length statistics of the total pelagic catches, as to the 

 increasing representation of younger animals in the catches. The baleen plate 

 data show that the shift towards younger age-groups in the catch is greater 

 than is implied by the length statistics or the proportion of immatures. 



It has been suggested that an increasing number of younger animals in the 

 catches could well be an indication of increased recruitment and even of 

 increasing stocks. Evidence for the view that the stocks of blue and fin whales 

 are decreasing has been presented above, and in whales, which bear a single 

 young each pregnancy, recruitment is within narrow limits related to the 

 size of the adult female component of the stock. If this is progressively 

 decreasing (as in the blue whale) then recruitment can hardly be increasing. 



The changes in the composition of the fin whale catches, presented and 

 discussed above, suggest that the average age at sexual maturity has decreased, 

 for there has been a greater increase in the proportion of the catch which is 

 composed of the youngest age groups than in the percentage of immature 

 females. 



I intend to present some other evidence for precocious sexual maturity in 

 the fm whale, and for an increase in the reproduction rate of mature females 

 linked with and presumably caused by the increased rate of exploitation. 



