another is explained as being based on 

 the fact that recruitment fluctuates 

 markedly from .year to year. Starting 

 out from this line of thought, it is pos- 

 sible to consider in the case of modes 

 (B), (C), (D), and (E) that from the 

 remarkable differences apparent be- 

 tween the two periods it is to be as- 

 sumed that the alternation of a year 

 of successful reproduction and a year 

 of unsuccessful reproduction was 

 continued through two cycles. Fur- 

 thermore, if we consider the modes 

 (A). (B), (C). (D). (E), and (F) each 

 as a separate age group and get their 

 growth rate, we come out in approxi- 

 mate agreement with the estimates 

 of Aikawa and Kato (1938). If this 

 second hypothesis is to stand, we 

 nnust in the future place the emphasis 

 in our studies of the stocks on finding 

 the period of cyclical changes in re- 

 production and in investigating their 

 causes. Furthermore, length meas- 

 urements of the catch may provide a 

 direct clue to finding the growth rate. 



The third hypothesis is that there 

 is no great difference in recruitment 

 from year to year, but that the year 

 groups X, X + 1, X + 2, and so forth 

 change their pattern of migration 

 markedly from year to year, some- 

 times appearing in the North Pacific 

 fishing grounds and sometimes not. 

 However, there is no way of getting 

 evidence to support this theory with- 

 out carrying on investigations over a 

 wider area of the ocean. It cannot 

 be said at present whether the sudden 

 decline of the large fish above the 

 group making up mode (F) is due to 

 natural attrition or to their making 



a complete change in their range of 

 occurrence and naoving from this sea 

 area into another. 



Bigeye Tuna, Parathunnus mebachi 

 (Kishinouye) 



Figure 3 shows phenomena very 

 similar to those seen in the case of 

 the albacore. For the period from 

 November 1948 to March 1949 the com- 

 position shows clear modes centered 

 at 85-88 cm., 121-124 cm., and 149- 

 152 cm. Another obscure mode can be 

 seen centered at 73-76 cm. In contrast 

 to this, in the period from October 

 1949 to March 1950 the composition 

 shows highly predominant modes cen- 

 tered at 93-96 cm. and 137-140 cm., 

 with an obscure mode centered at 105- 

 108 cm. The size compositions in the 

 two periods show marked discrepancies 

 in the position at which the modes ap- 

 pear and the form in which they appear. 

 Accordingly, as causes for this sort of 

 difference, we can consider the estab- 

 lishment of exactly the same sort of 

 hypotheses that were brought forward 

 in the case of the albacore. 



At the present time we do not know 

 whether the above-described phenome- 

 na, which are shared by the albacore 

 and the bigeye, are due to any one of 

 the three hypotheses just set forth, or 

 to some completely different reason, 

 or to a complex mixture of these. We 

 believe that these questions will be 

 gradually clarified in the future, and 

 assume that they will have an extremely 

 important significance for the investi- 

 gation of tuna resources. 



n r 



1948,12-1949,3 

 1949,10 - 1950,3 



% 

 20 



IS 



10 



5 



61 



41 



I 



44 



164 



181 



I 



184 



LENGTH (CM.) 



Figure 3. --Length composition of the bigeye tuna. 



201 



I 



204 



