The season at which these changes in the catch rates occur 

 may be earlier or later depending on the spGcies, but in all 

 cases the period betrwreen the minimum and the maximum is about 

 a half a yearo It is thought that this fact is worthy of 

 attentiono It may be considered that even in the seas of the 

 low latitudesj where conditions are always favorable for the 

 tunas J, there are great annual movements of the school So 



When it comes to such questions as whether these migra= 

 tions cross the Equator into the southern hemisphere^, whether 

 they extend far eastward to the coasts of the American continent, 

 or whether they pass Malaysia and form a link with the Indian 

 Ocean region, there are no data at all which might clarify 

 these problems and there is under present conditions no way of 

 tackling them. Furthermore^ it is not possible to explain the 

 relations of these migrations to the movements of the schools 

 in the fishing grounds of high latitudes such as those of 

 Japanese waters because what data we have only cover certain 

 seasons and certain areaso 



Figure 26 shows the changes in the fishing situation 

 throughout the year in the waters of the Izu and Ogasawara 

 island groups. According to the graph, the yellowfin catch 

 rate gradually increases from April to July, drops in August 

 and September, and rises shai'ply in Novembere The accepted 

 explanation is that the July peak consists of ascending fish, 

 while the November peak represents returning (or descending) 

 fisho If this is compared with figure 25A, the peak which 

 appears in the season of ascending fish is about 1 month 

 later than the maximum in the low latitudesc The peak rep- 

 resenting the descending fish is not distinguishable in the 

 southo As the following table shows, the degree of reliability 

 of these data is much inferior to that of the data presented 

 in table 11 o 



Table 12 

 Number of Times Fished and Number of Hooks Fished in Each Month 



Jano 



Month 



Trials 

 Hooks 



15 



15„954 



Febc 



50 

 37,896 



Mar_c 

 32 

 29„234 



Apr„ 



1«448 



May 



21 



15£,486 13„959 



June 



22 



jJur^ 



7 

 3,632 



Note? Area surveyed 



25°--30° No, 

 140° -150° E„ 



91 



