adult tuna by .f , and the young tuna include fish cf the year, all of the second- 

 year class, and. 1 - .78 s ,22 of the third-year class ^lle the adult tuna group 

 includea .78 of the third-year class and all of tho older year classes, then the 

 ratio between adult tuna and young tuna is 



f X . 73x.4.8x.3^f.4-8x.3/^l-.75 • .»62 f s .61 f 

 Since^ this gives .064 for the ^ole area, J'. 9.5. If we use 6 to represent an 

 identical natural mortality rate for all fish above the second-year class, then 

 (l-i)(l-fO=3A,(l-s)(l-f)=.75. Accordingly f=.060,f' «.57, and5s.20. 



On the Stock of the Yellowfin Tuna Weothunnus macropterus 



(Temmlnck & Schlegel) 



Slynopsis [ixi English^ 



Based on the catch records given for each body- length and body-weight 

 classes, the stock of Meothunnas macropterus (Temmlnck et Schlegel) was studied. 

 If the natural mortality rate is assumed to be .20, the survival rate is known 

 to be .75 for young fish but .57 for the adults irtille the fishing rate to be 

 ,06 for the youngs but .29 for the adults, [end of English synopsis) 



The ages deduced by Kimura^^' frora.the length distribution in the catch 

 and those determined by Aikawa and Kato^^) on the basis of the circuli appear- 

 ing on the vertebrae are not in agreement. In this paper I have followed the 

 conclusions of the latter, and have studied the stock by deducing the ages of 

 the fish from their length and weight. 



According to Kimura and Ishii^^' among approximately 2,980 small yellowfin 

 tuna and about 630 large yellowfin tuna taken on the Shigedera fishing grounds 

 during the nine-year period from 192A to 1932 fourth-year fish predominated 

 among those weighing 12 kg or more followed by seventh and eighth-year fish in 

 that order, (Table 1, calculated from Kimura' s^^) graph of weight distribution). 

 According to Aikawa and Kato'av2) table of the weights of yellowfin landed at 

 the Nuimzu market in 1937, which includes 1,214 fish under 12 kg and 1,292 fish 

 over 12 kg, fourth-year fish predominated among those weighing more than 12 

 kg (Table 2). However, among fish taken east of FormosaU) on longllnes sixth- 

 year fish were most numerous follo77ed by seventh-year fish (Table 3). Wear 

 the South Sea islands quite a few small yellowfin are taken nixed in with skip- 

 jack (5) (6) (7)^ but among those taken farther off shore sixth-year fish pre- 

 dominate (Table 4), In both cases the number of young yellowfin taken is small. 



Collating these facts it appears that third-year fish leave the islands 

 and bays and take up a migratory life, that fourth-year fish are comparatively 



