140 R. M. LAWS 



are not entirely valid for this species. In particular the boundary between 

 areas II and III would appear to be without meaning in relation to the fin 

 whale stocks. 



This view is confirmed by a study of the length distribution of the catches. 

 It is not yet complete but it is apparent that there is a size segregation accord- 

 ing to longitude, and there appear to be four main groups roughly corres- 

 ponding to the South Atlantic, South Indian, West Pacific and East Pacific 

 Oceans. The size distribution is fairly closely correlated with the distribution 

 of catches. 



The significance of this geographical segregation in relation to population 

 studies will be obvious and further work is planned and in progress. The 

 results may necessitate a reappraisal of our data, but I do not think this will 

 affect the conclusions drawn in this paper because I have been careful to 

 choose examples and to make comparisons in such a way as to discount the 

 effect of this segregation. However, because the size segregation may reflect 

 a segregation by age (though not necessarily), I feel that it would be unwise 

 to attempt to determine with precision such parameters as mortality or 

 survival rates. Fortunately most of the points I wish to make are not affected 

 by this. 



(d) Segregation in time 



In their long migrations between the breeding and feeding areas, the various 

 species of baleen whales migrate at different times. Blue and humpback 

 whales appear on the feeding grounds before fin whales, and sei whales 

 arrive much later. 



Within a species there is a similar segregation. In the fm whale the older 

 whales appear on the feeding grounds first, and the immature whales last 

 (Wheeler, 1934; Mackintosh, 1942). The period when samples are taken 

 therefore has an important influence on the age composition of the sample. 

 For example if separate survival curves are constructed for samples taken in 

 November, December, January, February and March, the slope is found to 

 be steeper with the progression of the season. 



This changing age composition month by month does not permit absolute 

 mortahty rates to be calculated and it is only possible to compare changes 

 in the shape of survival curves in a relative way. We must make the best 

 use of our inadequate material. 



THE CATCH PER UNIT EFFORT 



Hjort, Lie & Ruud (1933) appear to have first introduced the concept of the 

 catch per boat per day as a measure of abundance, using the expression 



