problems of the following sort will have an 

 extremely important significance judging from 

 the pattern of distribution of the bigeye. This 

 is the problem of in what form and to what ex- 

 tent the groups of bigeye in the North Pacific 

 and the South Pacific and the Indian Ocean are 

 connected through their spawning activity. If, 

 as stated earlier, the principal spawning 

 grounds of the group occurring in the northern 

 hemisphere lie in the loW-latitude sea areas, 

 including the equatorial area, the principal 

 spawning grounds of the groups occurring in 

 the southern hemisphere waters of the Pacific 

 and in the Indian Ocean will probably be of the 

 same character as those for the northern 

 hemisphere. In this case it nnay be thought 

 that the spawning grounds of these three groups 

 will at least in part overlap. If we start off 

 from this idea, it will not be enough in dealing 

 with the amount of recruitment to consider it a 

 problem simply of the total amount, but it will 

 be necessary to give consideration at the same 

 time at least to the mechanism of replenish- 

 ment in the northern hemisphere and southern 

 hemisphere waters of the Pacific Ocean. 



It is not possible at present to go any 

 further into this problem than we have here. 

 At any rate it is hard to believe that the prob- 

 lem of the alternate-year cycle is of such a 

 character that it can be clarified by taking only 

 the sea areas of the North Pacific into consid- 

 eration. If we think of all the bigeye so broadly 

 distributed throughout the Pacific and Indian 

 Oceans as one great population, it is naturally 

 to be anticipated to be in organic connection at 

 various points with problems related to the in- 

 ternal organization of this mass, and it is 

 thought that there is a necessity to push ahead 

 the study of the problem from this standpoint. 

 To express this in another way, it can also be 

 said that for future studies the alternate-year 

 cyclical phenomena have an important signifi- 

 cance as a powerful means for the investigation 

 of the structure of this population. 



Summary 



1. Length measurement data of bigeye taken in 

 the North Pacific fishing grounds have been 

 exannined in various ways. Data were ob- 

 tained for five periods extending from au- 

 tumn to winter of the years 1948 to 1953. 



2. As a result of comparisons of the length 

 composition in the different sea areas, 



a. It has been observed that the positions 

 at which the modes appear are as a rule 



in agreennent for different sea areas in 

 the same year. 



b. Although the relative heights of the 

 individual modes differ conspicuously 

 from sea area to sea area, as a char- 

 acteristic which extends to all the years, 

 in t h e more westerly sea areas the 

 modes formed by the smaller size 

 groups are predominant while in the 

 easterly sea areas the modes formed by 

 the larger size groups predominate. In 

 other words, in the more westerly 

 areas the groups of smaller fish are 

 more abundant, while in the easterly 

 areas the groups of larger fish are 

 more abundant. 



Judging from (a) and (b) it is thought that 

 there is very little possibility that the com- 

 position of the groups which come into the 

 fishing grounds results from two or more 

 clearly different racial stocks. 



c. There are conspicuous discrepancies 

 from year to year, particularly in the 

 positions of the modes and in their 

 height, indicating alternate-y e a r 

 cyclical phenomena. 



With regard to the causes of the annual 

 displacements of the modes we have con- 

 sidered the validity of three hypotheses 

 earlier advanced by Nakamura, Kamimura, 

 and Yabuta (1953): 



(1) Variation from year to year in the 

 amount of recruitment 



(2) Variation from year to year in the 

 growth rate 



(3) Variation from year to year in the 

 migrational phenomena 



As a result, with regard to hypothesis (1), 

 we concluded that its validity is strengthened 

 by the fact that the assumptions A, B, and C 

 that have been made in relation to the pro- 

 cesses accompanying growth have a rather 

 fairly rational basis. As for hypothesis (2), 

 since it is difficult to get a rational grasp of 

 the phenomena insofar as this hypothesis is 

 used as a basis, it has been decided that it 

 cannot at least be the controlling factor. In 

 the case of hypothesis (3), it has been shown 

 that there is room to apply it by postulating 

 nnigratory phenomena on an alternate-year 

 cycle. 



35 



