142 R. M. LAWS 



catching season, must all influence the catch per C.D.W. but are difficult 

 to allow for. In spite of this the C.D.W. , while far from ideal, is the most 

 acceptable measure of effort, and if the catch per C.D.W. is stable or decreas- 

 ing, while at the same time the average catcher efficiency is increasing, 

 it is safe to conclude that the stocks are decreasing. This has long been 

 the case as regards the blue whale, even when allowance is made for the 

 later season, and can now be shown to apply to the fm whale also. The data 

 presented in this discussion have been supplied by the Bureau of International 

 Whaling Statistics. 



It has been the practice in some quarters to think of the catch per unit 

 effort mainly in terms of B.W.U. per C.D.W. While this is convenient from 

 the point of view of the whaling industry, since a stable or increasing value 

 in terms of B.W.U. means a stable or increasing value in terms of oil produc- 

 tion, there is little to recommend it to biologists. In post-war years wide 

 variations in species composition of the catch have been associated with only 

 slight changes in the catch of B.W.U./C.D.W. (Fig. 2). 



In an industry based on searching, which concurrently takes several species 

 in varying proportions, changes in the catch per unit effort of one species 

 are influenced by the amount of effort being expended on searching for and 

 catching other species. For practical purposes the catches of humpback and 

 sei whales have been small (Fig. i), and sperm whale catches during the 

 baleen whaling season have also been small; they may safely be ignored in 

 the present discussion. This leaves two species, the blue whale and the fm 

 whale, individuals of the former being in general twice as valuable to the 

 industry as the latter. 



Fig. I shows how closely the average catch of fm whales per C.D.W. is 

 related to the percentage of fm whales in the total catch of fm and blue 

 whales, and to the size of the catch of fm whales. In post-war years with a 

 fixed quota of B.W.U. the increased size of the catch of fm whales is neces- 

 sarily complementary to the reduced catch of blue whales. 



In Fig. 2 the annual values of fm whales per C.D.W. are plotted against 

 blue whales per C.D.W., and pre-war and post-war seasons are distinguished. 

 It shows that, in pre-war years, as the number of blue whales per C.D.W. 

 declined there was a rise in the number of fm whales per C.D.W., the 

 relationship being more or less linear. In all these pre-war seasons the catch 

 of blue whales was more valuable than the catch of fm whales. In other 

 words fm whales never constituted two-thirds or more of the catch of blue 

 and fm whales combined (Fig. i). In post-war years the emphasis shifted to 

 fm whales, because of the depletion of the blue whale stocks, and in only 

 one post-war season (1946-7) was the fm whale component less than two- 

 thirds of the combined catch of these two species. The slope of the linear 



