(b) They are determined at a definite time 

 in some certain stage. It is impossible to 

 carry out any consideration based directly on 

 data with regard to (a), but it is thought that 

 there is very little possibility of it unless it 

 can be anticipated that the environmental con- 

 ditions of this population may vary nnarkedly 

 from year to year during the course of growth 

 of age group a. As for b, probably it is most 

 correct to ascribe it to the problem of 

 recruitment under hypothesis 1. 



If we postulate the simplest possible case 

 with regard to the problem of (b), we can 

 assume two cases: (a) either the amount of 

 spawning differs conspicuously every other 

 year, or (b) even though the annount of spawning 

 does not differ greatly there is a conspicuous 

 difference every other year in the amount of 

 attrition of the newly hatched or larval stages. 



Before considering this problem we will 

 first summarize what is known about the dis- 

 tribution and spawning habits of the bigeye. 



close to spawning, and he has postulated that 

 the spawning of the bigeye takes place in this 

 sea area. The matters set forth below have not 

 yet been formally reported, but they are the out- 

 line of what has become known as a result of the 

 investigations of bigeye spawning at the Nankai 

 Regional Fisheries Research Laboratory. 



i) Spawning appears to be carried on 

 principally in the main part of the North 

 Equatorial Current or in the sea areas 

 south of that current. 



ii) The minimum size of maturity appears 

 to be about 100 cm. 



iii) The spawning season is very long and on 

 the whole appears to extend alnnost 

 throughout the year. 



On the other hand, the results of the examination 

 of gonads of bigeye taken from September to 

 April in the sea areas of the North Pacific has 

 revealed that they are all immature. 



1. Distribution ; 



According to Nakamura (1949, 1951), in 

 the northern hemisphere in the Pacific Ocean 

 the bigeye is distributed almost everywhere 

 from to 40 N. latitude, and it is further 

 presumed to be broadly distributed in the 

 southern hemisphere waters of the Pacific 

 Ocean and in the Indian Ocean. According to 

 recent records of operations of Japanese long- 

 line vessels, the outstanding grounds where 

 large catches of bigeye are made are first of 

 all in the low latitudes, the main current area 

 of the North Equatorial Current and the Equa- 

 torial Countercurrent, the aforementioned sea 

 areas of the North Pacific, the Banda, and 

 Flores seas, and the northeastern parts of the 

 Indian Ocean. In the southern hemisphere 

 waters of the Pacific Ocean at present the oper- 

 ating area is generally limited to N. of 20 S. 

 latitude, but it has been ascertained that the 

 bigeye is apparently rather densely distributed 

 in the southern hemisphere also. 



2. Spawning habits: 



Nakamura (1949, 1951) has stated as 

 common characteristics of the tunas and spear- 

 fishes that the spawning of these fishes takes 

 place in broad areas of the low latitudes over 

 long periods of time. From June to August of 

 1951 Kikawa (1953) observed in the area of 2 N, 

 to 9°N. latitude, 161°E. to 174°E. longitude 

 large numbers of bigeye having completely ripe, 

 transparent eggs, which were thought to be 



From the Itenns given under 1 and 2 above 

 we can deduce two things: that the North Pacific 

 sea area is only a part of the total range of oc- 

 currence of the bigeye, and that the North Paci- 

 fic sea area has no direct relationship to the 

 bigeye spawning grounds, and consequently in 

 the northern hemisphere waters of the Pacific 

 Ocean there must be some direct mixing of the 

 bigeye occurring in the high-latitude waters of 

 the North Pacific and in the low-latitude sea 

 areas through their spawning activity. Setting 

 up these provisos, we will attempt to examine 

 cases (a) and (b). 



Withregard to the case of (a), if we assume 

 that the minimum size at maturity for bigeye is 

 about 100 cm., it is clear that the age groups 

 contributing to spawning must b e at least 2 

 years or older. Consequently, insofar as no 

 other special consitions exist, it may be thought 

 highly unlikely that the amount of spawning will 

 differ markedly every other year. 



As for case (b), judging from the fact that 

 the spawning of the bigeye takes place in low- 

 latitude sea areas with stable environmental 

 conditions and that the spawning is carried out 

 over a long period of time, it may be thought 

 highly unlikely that the rate of attrition will 

 differ markedly every other year. Looking at 

 the matter in this way, it is deduced that the 

 mechanism which produces the alternate-year 

 cycle is not something which appears in a 

 simple fornn. If we assume that it does not 

 arise in a simple form, it is thought that 



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