area, and Uda has postulated the ex- 

 istence of an inshore population and 

 an offshore population and further has 

 hypothesized that the offshore popula- 

 tion includes two migratory strains. 

 He also reported that as one moves 

 from the coastal waters offshore 

 groups of small, medium, and large 

 fish are found in that order, each of 

 them forming a separate migratory 

 population, and that these groups 

 mingle in the vicinity of 40 N., 165 E. 

 If we follow these hypotheses of Uda, 

 the catch of the longline fishery clear- 

 ly includes fish from both the inshore 

 and offshore groups, while the catch 

 of the pole-and-line fishery can be 

 thought to be made up principally of 

 fish from the inshore population. 

 Consequently, if we think of the dif- 

 ference in size composition seen in 

 figures 1 and 2 as a difference in the 

 size composition of the inshore and 

 offshore populations, we can consider 

 this to tend to agree with the hypothe- 

 sis of Uda and Tokunaga. However, 

 there is still roonn left for examina- 

 tion of the question of whether or not 

 this sort of difference in the size 

 composition is due to a population 

 difference. In investigating problems 

 of this sort it will be extremely im- 

 portant to analyze the makeup of the 

 fishing grounds and the seasonal and 

 regional changes in size composition, 

 but these are questions which must be 

 left to the future. 



When we attempt, using figure 1, 

 to compare the size composition for 

 the two periods 1948-49 and 1949-50, 

 we can see a nnar k e d difference 

 between the two. For 1948-49 it can 

 be seen that the modes are centered 

 at 65-66 cm. (A), 79-80 cm. (C), 

 99-100 cm. (E), and 109-110 cm. 

 (F). In 1949-50, on the other hand, 

 we can see an obscure mode corres- 

 ponding to (A), clear modes centered 

 at 73-74 cm. (B), between (A) and 

 (C), and at 87-88 cm. (D), between 

 (C) and (E), and another extremely 

 obscure mode corresponding to (E) 

 and (F). In other words, the outstanding 

 apparent differences between the two 

 are the complete discrepancy between 

 the positions of (C) in the fornner and 

 (B) and (D) in the latter and the fact 

 that (E) and (F), which were clearly 

 apparent in the fornner, have beconne 



extremely obscure in the latter. As 

 causes for differences of this sort 

 we can think of shortcomings in the 

 method of collecting the data and of 

 basic natural phenomena. Since there 

 was no great difference in the method 

 of collecting data throughout the two 

 periods, this can hardly be thought to 

 be a cause of the above described dis- 

 crepancy. If they are considered to be 

 based on natural phenomena, then the 

 following three hypotheses can be 

 advanced. 



The first hypothesis is that of annual 

 differences in growth. We will assunne 

 that a certain age group migrates into 

 the same area at the same season. 

 Modes (E) and (F) are certainly obscure 

 but since they do appear in both years 

 there is no problem. Now if we assume 

 that the groups making up modes (A), 

 (C), and (E) are respectively X age, 

 X + 1 age, and X + Z age, the group 

 making up mode (B) can be thought of 

 either as fish of X age which have 

 grown remarkably well in comparison 

 with those of the previous year or as 

 fish of X + 1 age which have grown re- 

 markably poorly. In the same nnanner 

 the group making up mode (D) nnust be 

 thought of as either well grown fish of 

 X + 1 age or as poorly grown fish of 

 X + 2 age. 



In fishes of ponds and lakes, marked 

 differences in growth from year to 

 year because of environmental factors 

 and because of the amount of reproduc- 

 tion of the fish can be seen, and it is 

 known that in the case of the pond smelt 

 there is good growth and bad growth in 

 alternate years. Assuming that we can 

 detect in the albacore distributed broad- 

 ly throughout the open sea exactly the 

 sanne sort of phenomenon we find in 

 such limited water areas, it is thought 

 that this phenomenon may give an im- 

 portant indication for the investigation 

 of the ecology and stocks of this fish. 



The second hypothesis is that of 

 annual differences in recruitment. On 

 the basis of this hypothesis, the groups 

 making up the modes (A), (B), (C), (D), 

 (E), and (F) in the figure are thought to 

 belong respectively to ages X, X + 1, 

 X + 2, and so forth, and the fact that 

 fish of the same age group may form a 

 mode in one year and not form one in 



