Date Length Location 



early June 153 mm 



July 1923 105 mm 



July 1923 125 mm 



August 1923 210 mm 



This list indicates that the growth of the skipjack is extraordinarily fast, 

 however, it does not differ too much from the rates of growth of the black tuna 

 f Thunnus orientalis ] and the Cybiidae. It appears that the fish grow more than 

 40 mm per month, and figuring at that rate it is thought that the first skipjack 

 were hatched in March. Since fish with enlarged ovaries are also seen around July 

 and August, it appears that the spawning season of the skipjack continues over 

 quite a long period of time. 



The juvenile skipjack listed above are blue-black on the dorsal surfaces and 

 their bellies are silvery without any visible markings. On specimens 200 mm or 

 more in length the stripes begin to be faintly visible. The shape of the fish 

 varies considerably as between individuals, but generally the juvenile fish are 

 slender. The characteristics of the skeletal parts and the viscera (particularly 

 the liver and the pyloric caeca) in these juveniles are the same as those seen in 

 mature fish, so if these points are examined, the species can be clearly distin- 

 guished without any danger of confusion. 



In small juvenile skipjack the boundary between the rectum and the rest of 

 the intestine is not clear, and the walls are thin in all parts of the intestine. 

 The juveniles of Euthynnus yaito and the various species of Auxis . on the other 

 hand, have a thick-walled rectum, while the other parts of the intestine are so 

 thin- walled that the contents can be seen through them. The three lobes of the 

 liver are all slender and thin and roughly of equal length. 



The food of the juvenile fish consists mainly of copepods, amphipods, and 

 other larval and small-sized crustaceans. These animals float close to the surface 

 of the ocean between dusk and dawn, and it seems likely that the juvenile skip- 

 jack follow the movements of these crustaceans. As the juveniles grow they gradu- 

 ally come to eat larger and larger crustaceans. Juveniles 110 mm long feed on 

 Typhis (an amphipod) about 5 mm in length, and cases have been seen in which 

 juveniles 130 ram long fed on young scyllarids about 30 mm in length. 



Euthynnus yaito 



The smallest pleco3tean juvenile which I have been able to obtain is an in- 

 dividual of this species 36 nfm long. It was recovered by Mr. Umemoto from the 

 stomach of a skipjack taken at Gonsone on April U of this year. The body of this 

 specimen is rather broad and the mouth is rather large, characteristics which dis- 

 tinguish it from juvenile skipjack, however, without examining the viscera and the 

 skeleton it would be easy to confuse the two. The markings are not visible and 

 the color of the back is pale. 



Surprisingly enough this little fish had eaten one unidentified juvenile fish 

 10 mm long and two others about 6 mm long. Furthermore some black material was 

 seen in the intestine, but since it had not retained any particular shape it was 

 difficult to tell what it was. The walls of the rectum were thicker than those of 

 the rest of the intestine. The right lobe of the liver was slender and long, 

 reaching to the rectum, and the middle lobe was broad. 



13 



