152 R. M. LAWS 



values for area II and areas III and IV separately. (Areas III and IV have a 

 similar history of exploitation, beginning long after area II.) There is not 

 much material relating to pre-war years but the 'percentage pregnant' in 

 area II catches was about 80 per cent just at the beginning of the war years 

 and had fallen to a very low value in the first post-war season. There are, 

 however, reasons for supposing that this value is too low, and that the 

 sample was less representative of the stock than in other seasons. By 1950-1 

 or earlier the 'percentage pregnant' had reached the upper limit of about 

 80 per cent. Similarly in areas III and IV, in the earlier post-war seasons the 

 'percentage pregnant' was relatively low and in later seasons stabilized at 

 about 80 per cent. The apparent drop in the percentage pregnant in area II 

 in 1958-9 is disturbing, but it is too early to assess its full significance. If this 

 trend should continue then it implies a fall in fertihty which may be a 

 consequence of the continued reduction in the stock. 



In area I (part of the former sanctuary) the 'percentage pregnant' in the 

 first seasons catch was estimated as 68-5. However, marking returns show 

 that there may be a significant interchange between areas I and II (Brown, 

 1954), and this value is probably to be considered as a compound of the 

 area II value of 80 per cent and a lower value obtaining for an indigenous 

 area I stock. In the two subsequent seasons the values were 73 • o per cent 

 and 71 • 3 per cent. The fertility rate in area I had not reached the level of 

 other areas up to the 1957-8 season. Unfortunately no later data are available. 



I have calculated, on the basis of a detailed analysis of the reproductive 

 cycle that a figure of 80 per cent pregnant is equivalent to a real annual 

 fertihty rate of about 60 per cent (Laws, in press). This appears to be the 

 upper limit for the fm whale. 



(b) Earlier sexual maturity 



The data presented in Fig. 5 suggest that one response to exploitation is a 

 reduction of the age at which breeding begins. The baleen plate method is 

 the only one which can be used to show this because the ear-plug method 

 was not available until 1955-6, by which time, as I shall show, this response 

 was complete and the average age at sexual maturity had been stable for 

 some years. For an obvious reason the ovary method is inapphcable. 



Hylen et al (1955, fig. i) illustrated the percentage age composition of fin 

 whale catches, and differentiated between sexually immature and mature 

 animals. In both sexes there is seen to be an increase in the proportion of 

 mature animals in baleen groups II, III and IV from 1945-56 to 1952-3- 

 Professor Ruud has provided additional material, classified in this way, for 

 post-war seasons up to and including 1955-6. I am indebted to him for 

 permission to publish it. 



