3D 

 5 



IJ4«sedioa 

 IW * 



1950 * 



1951 ' 



1952 ♦ 



«?55nGI 



Mill 

 8 5Z^e04- 



Body Length 



woa 



Figure 3. --Aggregated length frequency by fishing season 



to reflect the actual composition of the schools 

 that occur in the various sea areas. It can 

 probably be concluded, if there are no errors 

 in this hypothesis, that there is a strong pos- 

 sibility that the population of bigeye inhabiting 

 the North Pacific area is composed of schools 

 belonging to a single stock. At least, it seems 

 that there is very little possibility that they are 

 made up of two or more sharply differentiated 

 racial strains. However, there are examples 

 where small discrepancies can be detected in 

 the positions at which the modes appear in the 

 different sea areas, and it is thought that there 

 is room for further study of various points 

 concerned with the finer internal organization. 

 These points the author would like to take up 

 when the data have been amplified. 



II. Observations on the Shifting of the Modes 

 from Year to Year 



Nakamura, Kannimura, and Yabuta (1953) 

 have already advanced the three hypotheses 

 mentioned above in connection with the phenom- 

 enon of the conspicuous discrepancies from 

 year to year in the positions at which the modes 

 appear and in the degree of their appearance. 

 These hypotheses are: 



1. Differences from year to year in the 

 growth rate 



2. Differences from year to y e a r in 

 recruitment 



3. Differences from year to y e a r in 

 migrational phenomena 



We will continue our examination of this 

 subject by taking up these hypotheses one at a 

 time in the order 2, 1, and 3. 



Hypothesis I - Differences from year to year 

 in the amount of recruitment 



If it is postulated that a difference from 

 year to year in the amount of recruitment is the 

 main factor in the shifting of the modes, it 

 should be possible to detect through the varia- 

 tions in the modes phenomena connected with 

 growth. Contrariwise, if it is possible to as- 

 certain the existence of phenomena related to 

 growth from the group of phenomena related to 

 the shifting of the modes from year to year, it 

 can be concluded that the validity of the above- 

 mentioned hypothesis will be strengthened. At 

 least it can probably be said that it will be im- 

 possible to discard this hypothesis. We will 

 attempt to examine the data directly from this 

 point of view. 



Figure 4 shows the body length composition 

 split up by years. Assuming first of all that the 

 schools which made up the modes ajAj in 1949 

 have returned to the fishing grounds year after 

 year, if we try to relate the discrepancies in the 

 modes to growth which has taken place during 

 this period, it is thought most natural to try to 

 establish the following sort of postulate from 

 the inter -relationships among the positions at 



which the modes appear: ajAj (1948) ^A'b' 



(1949) ^B2 (1950) — *- C" (1951) ». D3 



(1952) (A) 



The main difficulty with this assumption is 

 in the form of the modes or in other words in 

 the degree to which they appear, for incompat- 

 ible relationships can be seen between aiAj and 

 A'b' and between A'b' and B2. 



If we try for modes Bj and azAj the same 

 sort of postulation as for (A), 



27 



