156 



R. M. LAWS 



but they are comparable with each other. Subject to these quahfications and 

 reservations let us now compare them. 



For area II comparison of the 1939-41 and 1955-7 curves shows that the 

 section of the survival curve referring to the older age-groups has steepened, 

 and there are now far fewer females older than 15-20 years (Fig. 8). The 

 maximum age in samples of similar size is now about 28 years (sample size 

 302) compared with about 42 years in the earlier period (sample size 336). 

 Another feature of these curves is that in recent years the slope in the early 

 mature groups is less steep than formerly. If the stock were not subject to a 

 greatly increased fishing mortality in the early years this might perhaps be 



1000 



100: 



o 10 20 



Fig. 9. — 'Survival curves' for female fin whales, area I. 



30 O 



Age in years 



attributed to reduced natural mortality, but it will be remembered that the 

 early age groups (e.g. baleen groups O-III) now comprise about 45 per cent 

 or more of the total catch compared with less than 25 per cent in the earlier 

 period (Fig. 5). Also the total catches are now much higher (Fig. i) so the 

 discrepancy is even greater. This relative change in the first section of the 

 survival curves must therefore be a result of reduced recruitment. 



The steepening of the later part of the survival curve is not entirely due to 

 the effect of an increased rate of mortality working through the age groups. 

 If mortality were uniform with respect to age over most age groups, as the 

 1939-41 curve implies, it would take over thirty years for this effect to work 

 through the population. Yet the period between the samples is only sixteen 



