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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 2. — Sexual maturity of female yellowjin tuna caught 

 in the western equatorial Pacific, June-August 1950 



Note.— Y. Yabuta of the Nankai Fisheries Experiment Station, Tokyo, 

 Japan, assisted in making part of these observations. 



July with active spawning commencing in June 

 and extending into August. Further hypothe- 

 sizing that a common yellowfin population had 

 been fished during the season — and there appears 

 to be no evidence to the contrary — it does not seem 

 unreasonable to believe that spawning had oc- 



curred coiiuidentally throughout the area fished. 

 The spawning season is most likely a long one and 

 may not necessarih' be limited to the summer 

 months, but the peak of spawning probably is at- 

 tained during that period. 



Yellowfin tuna found elsewhere in the tropical 

 western Pacific Ocean are generally believed to 

 spawn most actively during the summer months. 

 Preliminaiy studies by biologists of the Pacific 

 Oceanic Fishery Investigations indicate that dur- 

 ing 1950 this species spawned in the vicinity of 

 the Hawaiian Islands from early June to Septem- 

 ber. Ill the eastern Pacific, however, the spawn- 

 ing season is considered to be during the late 

 winter and early spring months (Schaefer and 

 Marr 1948a). This variation in time of spawn- 

 ing may be connected to some extent with latitude, 

 or it may be a race-connected characteristic. Dif- 

 ferences in spawning times of different races of 

 the same species in the same or similar places have 

 been observed in other species of fish, such as the 

 Pacific surf smelt (Schaefer 1936) and European 

 herring (Lissner 1934). 



Big-eyed tuna (Parathunnus sibi) 



Since big-eyed tuna were usually eviscerated at 

 sea, as previously mentioned, I was not able to 

 examine many reproductive organs of this species. 

 No check was made of the maturity of male fish, 

 but some females that were brought in whole were 

 opened and examined throughout the fishing sea- 

 son from late June to early September. These 

 females possessed either ripening or ripe ovaries, 

 with a few having what could be considered ad- 

 vanced-ripe ovaries. No running-ripe or fully 

 spent ovaries were found. Ovaries classified as 

 ripening may have been in a spawning state, be- 

 cause the big-eyed tuna, like the yellowfin, prob- 

 ably spawns over an extended period with succes- 

 sive batches of eggs being ripened and extiTided. 



Ovaries that appeared ripe were gi'eatly en- 

 larged, round in cross section, and light pink in 

 color. Those approacliing the running-ripe stage 

 had translucent ova whicli were ready to emerge 

 from the follicles. A sample of 1,000 eggs from 

 such an ovary removed from a 1,102-mm. female 

 showed a modal group of large eggs centering 

 around 1.06 mm. in diameter (fig. 2) . The largest 

 eggs measured approximately 1.22 mm. The eggs 

 probably increase a little more in size as water is 

 absorbed after emission into the sea. 



