EFFECTS OF WHALING ON BALEEN WHALES 147 



The more important changes in the species ratio have been discussed, and 

 there have been no significant changes in the sex ratio, which is about unity. 

 This suggests that the differential effect of the minimum length regulation 

 on male and female, expected because of the sexual disparity in size, is 

 balanced by the protection afforded to lactating females. 



There have been conspicuous changes in the size and age composition. 



(a) Mean lengths 



The mean length of the catches is dependent on growth rate and age composi- 

 tion, and on the minimum lengths at which whales may be taken. 



In Fig. 4 the yearly mean lengths of the catches of fin and blue whales 

 from 193 1-2 to 1958-9 are plotted. In both species there was a rapid decrease 

 in pre-war years and a more gradual decrease in post-war years. Interpreta- 

 tion of these figures is a little complicated because of changes in the minimum 

 lengths above which whales were allowed to be taken. An increase in the 

 permissible minimum length should be associated with a potential increase 

 in the mean length of the catch. This appears to be the explanation of the 

 rise in the mean length of the blue whale catch in 1937-8 when the minimum 

 length was raised from 65 ft to 70 ft. It also accounts for the abrupt rise in 

 the mean length of the fm whale catch of 195 1-2 when the minimum length 

 was raised from 55 ft to 60 ft for this species. 



Two other factors which affect the interpretation of these length statistics 

 should be mentioned. There can be little doubt that the very high mean 

 lengths of the blue whale catches in the early pre-war years were the result of 

 selection by the gunners. Survival curves of blue whales for this period 

 (based on corpora albicantia counts) confirm that catches were biased towards 

 older and larger animals, though this bias later disappeared. In the fin whale 

 there has probably been less selection of this kind. With the increased 

 competition of post-war years active size selection by the gunners can be 

 ruled out, but another possible type of selection must be mentioned. This is 

 the result of the later opening date of the whaling season which means that 

 the catches will contain a larger proportion of younger and therefore smaller 

 animals (see above). Nevertheless the later season is not in itself sufficient to 

 account for the observed decline in the mean lengths of the catches. The 

 timing of the season was similar in 1949-50 and 1950-1 and in 1951-2 and 

 1952-3, yet each of these pairs of mean lengths for blue whales show a 

 decline. Between 1952-3 and 1953-4 the opening date for blue whales was 

 postponed by two weeks and the closing date was almost unchanged, yet 

 the mean length of the blue whale catch rose by half a foot. For fm whales 

 the season has been virtually unchanged since 1952-3, but the mean length of 

 the catches has decreased by over i| ft. 



II 



