150 



R. M. LAWS 



Then I will attempt to show that in spite of this reaction the recruitment is 

 insufficient to maintain the stocks at the present rate of exploitation. 



RESPONSES TO EXPLOITATION 



(a) Pregnancy rate 



The most immediately apparent response to exploitation is an increase in the 

 pregnancy rate. Mackintosh (1942, p. 233), referring to blue and fm whales, 

 remarked that 'the percentage of adult females which are pregnant has been 

 increasing in a remarkable degree year by year, as if the actual rate of breeding 

 were becoming faster'. He suggested that this was conceivably a reaction to 

 whaling. Additional material from post-war seasons supports this suggestion. 



Q. 



100 



50 



100 



50 



1940 



1945 



1950 



1955 



I960 



Fig. 6.— Variations in the relative fertility of blue whales (all areas) and fm whales 

 (white, area II; black, areas III and IV). 



In order to make use of the material collected by non-biologists the 

 presence of one or more corpora lutea in the ovaries is taken to indicate a 

 pregnancy. In fact, this gives an estimate of the number of pregnant females 

 in the catch which is too high by about 8-10 per cent. The reason for this 

 is that in the female fm whale (and probably in the blue whale) a feature of 

 the annual cycle is a post-lactation oestrus. This means that a proportion of 

 non-pregnant, non-lactating females in antarctic waters have a corpus luteum 

 of ovulation in the ovaries (Laws, 1959^ and in press). Pregnancy can be 

 confirmed only by a search of the uterus for an embryo. Another point, 

 which I should emphasize is that the term 'percentage pregnant' refers to 

 the catch of adult females, and applies to the total population only in a 

 relative way. This is due to the fact that a large proportion of lactating adult 

 females delay the southward migration into antarctic waters (Laws, in press) ; 

 because of this and also because of the prohibition on the taking of lactating 

 females, this group is under-represented in the catches. The term 'percentage 

 pregnant' as used here applies therefore to the proportion of non-lactating 



