other cases juvenile carangids / akamuro 7« mackerel^, and sphyraenids, 

 as well as mature skipjack (38o3 — 4-0.5 cm in length) are discovered in 

 the stomachs, 



(C) Age determinations on tuna (by the vertebrae) 



Age determinations have been made using the first to the sixth 

 cervical vertebrae from yellowf in tuna living in equatorial waters, but 

 the method is extremely inaccurate and various values are obtained depend- 

 ing on the observer. Ages of seven to eight years are determined, but 

 judging by the length of the fish and the state of development of their 

 gonads these values are thought to be somewhat unreasonable, 



Dr, Masamitsu Oshima has proposed that in the future a study be made 

 of the scales from the anterior part of the belly, 



(D) Gonads of the tuna 



In general the stage of ripeness of the reproductive organs in both 

 male and female yellowf in tuna is about the same throughout the year, and 

 at all times of the year the schools which are fished are found to contain 

 a mixture of fish in markedly high and low stages of developmento From 

 a superficial examination it appears that from January to May the schools 

 are riper than they are around September. 



(VII) Conclusion 



Boats which operate in the southern tuna grounds should fully utilize 

 the results of scientific studies of the South Seas tuna fishery in order 

 to proceed to the opening up of new fishing groundSc The number of persons 

 in Japan who advocate the development of the South Seas fisheries and the 

 number of technicians who have something to say on the subject of investi- 

 gations of the southern tuna fishery has greatly increased of late. One 

 cannot, however, help being amazed at the paucity of factual knowledge 

 about the scientific study of the tuna fishing grounds of the South Seas, 



It is regrettable but unavoidable that in the foregoing discussion 

 of "oceanographic factors of good fishing grounds" it has been impossible 

 fully to explain the matter so that it might be easily understood by 

 persons operating boats in southern waters. 



To summarize, those who pursue the yellowfin tuna on the equatorial 

 fishing grounds should first of all find the Equatorial Counter-current 

 and then seek the portions of its current area having a particular form. 

 Within these areas they should look for waters where the velocity of the 

 current is 0,5 to 1.5 knots, the transparency is 25 to 35 meters, and the 

 water temperature at the 100-meter level is over 20°, The determination 

 of the temperature at this level is difficult for the average fishing 

 craft, but because the surface water temperature will also vary by about 

 1°, suitable waters can be found if the surface temperature is accurately 

 measured, and consequently this is a point which requires careful attention„ 

 If the vessel proceeds into the southern latitudes^ some variations will 

 be found in these oceanographic factors. For example, this is true of the 



