discussed in terms of the fish catch. Furthermore, the only suitable method which can be 

 thought of for considering the density of their distribution is based on the fishing conditions. 



Hitherto tuna fishing conditions have been expressed in terms of the catch per voyage 

 or the catch within a certain area within a definite period, however, these modes of expression 

 have the defect that fishing effort is not accurately expressed. In order to correct this defect, 

 I have, as previously set forth, taken the catch per hundred hooks fished, Ccilled it the catch rate, 

 and used it to show fishing conditions. 



In the case of the skipjack live bait fishery, the so-called " biting qualities" of the 

 schools greatly influence the fishing conditions, and even though the catch rate is calculated on 

 the basis of the number of poles fished, it cannot be said to be always necessarily correlated with 

 the density of distribution. 



It is of course conceivable that we have the factor of biting qualities in the tunas aJid 

 spearfishes too, but in the case of the longline fishery we can think of absolutely no means for 

 evaluating this factor. This means that there are no grounds on which to refute the hypothesis 

 that the catch rate is always in proportion to the density of occurrence of the fishes. In other 

 words, at present we can think of no other method than to assume that the catch rate is propor- 

 tional to the density of distribution of the fish. 



Now if we do nnake this assumption, the chief problems which arise are differences 

 in the construction of the fishing gear, differences in the bait used, errors arising from differ- 

 ences in the number of times fished aind the number of units of gear ennployed, and fluctuations 

 in the fishing conditions from year to year. 



If all of our data represented the use of identical fishing gear, there would be no 

 problem at all, however, the fishing gear has changed with the passage of time, different re- 

 search vessels sometimes used different types of gear, and gear of differing design is used de- 

 pending upon the fishing ground and the type of fish sought. 



The effect of fishing gear construction on the catch rate has as yet not been 

 sufficiently studied, but there are data which indicate that it is not to be talcen lightly. For ex- 

 ample, the data collected by the Hakuyo Maru in various southern areas show, as may be seen in 

 the following table, that differences in the length of the branch lines greatly influence the catch 

 rate. 



Table 1 12. --Fishing results with branch lines of different 

 lengths (Hakuyo Maru) 



137 



