4. The fact that the survey period fell on 

 the months of June to July when the fish would 

 be migrating northward {Fig. 3) seems to 

 suggest that the easterly increase in the mean 

 length may be attributable to different ten- 

 dencies of the migrating behavior between the 

 fish in the eastern area and those in the west. 

 In other words, the fish of both groups A and 

 B in the eastern area are supposed generally 

 larger in size than those of the corresponding 

 groups in the west, while all migrating 

 northward. 



/end of English summary/ 



It has been known in the past that in the 

 tuna pole-and-line fishery some young bigeye 

 tunaj.' are taken incidental to the capture of 

 skipjack, albacore, and small yellowfin, but 

 up to the present time there have been no de- 

 tailed reports concerning them. In general the 

 fishes on which the pole-and-line fishery oper- 

 ates have a strongly schooling character, and 

 this character is clearly evident in the young 

 bigeye. In taking up problems related to the 

 structure of the population, with fishes of a 

 strong schooling character, it is thought that 

 the weight given the schools, which are the 

 structural units of the populations, is of very 

 great significance. In the present study the 

 size composition of the separate schools is 

 considered from this point of view, note is 

 taken of the discrepancies among schools ap- 

 pearing in the results, and some consideration 

 is given to linking these discrepancies with the 

 provenance of the schools. 



The data are mainly fish measurements and 

 information on fishing conditions collected at 

 the Yaizu fish market in June and July 1953. A/ 

 Some data collected at the Makurazaki fish 

 market are also included; they were supplied 

 by the Oceanic Resources Section of the Tohoku 

 Regional Fisheries Research Laboratory. 



Appreciation is here expressed to Director 

 Hiroshi Nakamura of the Nankai Laboratory and 

 to Dr. Yabe, Chief of its High-seas Resources 

 Section, for their advice in connection with this 



study, and also to the Oceanic Resources 

 Section of the Tohoku Laboratory for its un- 

 failing cooperation and for the valuable data 

 supplied. 



Size Composition by Separate Schools 



In panels 1 to 17 of figure 1 we have taken 

 those sannples which, on the basis of the fish- 

 ermen's reports, were judged to have clearly 

 been caught from a single school, have chosen 

 only those from which 30 or more fish were 

 measured, and have arranged them in order of 

 average size; the bottom panel shows the aver- 

 age composition of 1 to 17. Figure 2 is a com- 

 parison of the average composition of the 

 schools represented in figure 1 (panel a) and of 

 the composition based on all of the measure- 

 ments taken during approximately the same 

 period (b). 



As is clear from figure 2, the positions of 

 the modes in the size compositions represented 

 by (a) and (b) are in very good agreement, and 

 in both of them the presence of two modes, A 

 and B, is clearly apparent. In addition, in the 

 average size composition represented in panel 

 (a) there is a faintly discernible mode C cen- 

 tered at 100. 1 - 102. cm. , but in the compo- 

 sition based on all the fish measured, repre- 

 sented in panel (b), this mode is not clear. As 

 there are very few data on the group making up 

 mode C, it is omittedfrom further consideration. 

 For convenience in discussion, the groups 

 making up modes A and B will be referred to 

 hereafter as groups A and B. 



The presence of groups A and B can also be 

 pointed out in the several schools represented 

 in figure 1. For example, schools 4, 14, and 

 17 can be cited as having a simple composition, 

 and each of them is composed either of group A 

 or of group B. In the other schools^/, also, 

 modes appear at locations corresponding to A 

 and B, and we do not, at any rate, see anything 

 that would positively deny the presence of modes 

 A and B. As has been indicated above, groups 

 A and B are interpreted as forming individual 

 units4/ even when they are in different schools, 



— These snaall bigeye are generally 

 called bachimeji or daruma, whereas the 

 adults are called mebachi. 



2/ 



— The length used was the so-called 



"fork length", measured from the tip of the 

 snout to t h e deepest part of the fork of the 

 caudal fin. 



3/ 



If we look at the relation of mixed 



schools to the objects with which they were as- 

 sociated (see notes to fig. 1), we find that they 

 were all with whales, sharks, drift logs, etc. 

 Taking this point into consideration, it may be 

 wondered whether the intervention of such ob- 

 jects of association may not play a significant 

 role in the formation of mixed schools. 



37 



