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R. M. LAWS 



This progressive decline in the mean lengths of the catches of both species 

 suggests that there has been a real change in the size and age composition of 

 the stocks. 



(b) Percentage sexually immature 



In Fig. 4 the percentage of immature animals in the catches is presented. This 

 is calculated each year by the Bureau of International Whaling Statistics on 

 the assumption (based on earlier biological studies) that on average male 

 and female blue whales respectively 73 ft or less and 76 ft or less, are im- 

 mature. For fm whales the corresponding lengths are 62 ft and 64 ft. This 

 should in fact give slightly low values for the percentage immature, because 

 the mean lengths at maturity are actually slightly higher than those used in 

 the calculations. The resulting figures show an inverse picture to that given 

 by the mean lengths (since they are based on length statistics), and suggest 

 that the percentage of immature animals in the catches increased in pre-war 

 seasons, then fell slightly during the war years, and again increased in post- 

 war seasons. The proportion of immature blue whales estimated in this way, 

 rose from about 25 per cent in 1945-6 to nearly 40 per cent in 1952-3 ; for 

 several years it remained fairly stable but in 1958-9 there was a marked 

 decrease in the immature component of the catch. The significance of this 

 recent decline is difficult to assess, though the stabilization and fall in the 

 proportion of immatures suggests declining recruitment. In the fm whale the 

 proportion of immatures has increased fairly steadily from about 14 per cent 

 in 1945-6 to 30 per cent in 1958-9. Changes in the minimum length regula- 

 tions have affected these values as in the case of mean lengths. 



In Fig. 5 I have plotted the percentage of immature female fm whales in 



60 



50 



o 

 o 40 



30- 



20 



Baleen groups 



o-m 



Sexually immature 

 by inspection 



Length <64tt 



1957 



^ I \ 1 1 1 1 1 1 1 



1945 1947 1949 1951 1953 1955 



Years 



Fig. 5. — Changes in the composition of the antarctic pelagic catches of female 



fm whales. 



1959 



