EFFECTS OF WHALING ON BALEEN WHALES 157 



years. It therefore appears that there has been selective fishing mortahty with 

 respect to age. 



The survival curves for two successive seasons in area I (Fig. 9) strongly 

 support this view. In the space of one year the curve has steepened very 

 markedly and this can only be explained by selective mortality of older 

 whales. Both of these samples were collected by biologists working on the 

 same factory ship, with the same catchers and men, working in the same 

 relatively small area. Moreover, Japanese biologists in area VI (also formerly 

 a sanctuary like area I) got almost identical results in these seasons (Inter- 

 national Whaling Commission, Ninth Report, 1958, p. 21). 



I believe that the explanation of this effect lies in the relative segregation 

 of different age-groups, the oldest animals arriving on the feeding grounds 

 first and the youngest last, as mentioned at the beginning of this paper. 

 With a Umited whaling season the older whales are therefore vulnerable for 

 a longer period than the younger animals. The magnitude of this effect in 

 area I is probably related to intensive fishing on a relatively small, geo- 

 graphically concentrated stock. The value for fin whales per C.D.W. fell 

 from the very high figure of 2-40 in 1955-6 to i-6i in 1956-7, and to the 

 low value of i- 13 in 1957-8. In 1958-9 there was no whaling in this area, 

 possibly because it had become unremunerative. 



ACKNOWLEDGEMENTS 

 I should like to express my thanks to the British and Norwegian whaling 

 companies who have co-operated in this work, and to the many biologists 

 and whaling Inspectors who have undertaken the collection of material. 

 Unfortunately space does not permit more than a collective acknowledge- 

 ment. I am indebted to Professor J. T. Ruud for permission to refer to some 

 of his unpubhshed data, and to Dr N. A. Mackintosh for helpful advice. 



SUMMARY 

 (i) This paper is mainly concerned with the fm whale and with antarctic 

 pelagic whaling over the last three decades. 



(2) For a number of reasons really adequate sampling of the stocks has not 

 been achieved, but the samples available can be used to demonstrate the more 

 important effects of exploitation. 



(3) The catcher's day's work is the most acceptable measure of effort. In 

 terms of whales per C.D.W. the southern stocks of both blue and fin whales 

 are decreasing. 



(4) Decreasing mean lengths and the rising percentage of young animals in 

 the catches of blue and fin whales are interpreted as signs of declining stocks. 



