FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



APPROX. WEIGHT(LBS) 



Figure 1. — Percentages of sexually active fish at different 

 fork lengths. Figures in parentheses indicate the num- 

 bers of individuals on which the percentages are based. 



percent were in the maturing or ripe stages. The 

 percentage of sexually active fish increased gradu- 

 ally and irregularly from this length class through 

 the llO-to-119-cm. class (about 57 to 72 lbs.), of 

 which 17.4 percent were active. In the next 

 10-cm. class (about 74 to 92 lbs.), the percentage 

 of reproductively active fish jumped sharply to 

 47.0 percent. Above this class the percentage of 

 active fish increased steadily with length. Of 

 the fish measuring 150 to 159 cm. (about 145 to 

 172 lbs.), 66.7 percent were active. 



These data suggest that, although yellowfin as 

 small as 70 cm. are capable of reproducing, the 

 greater part of the population reaches sexual 

 maturity at about 120 cm. Schaefer and Marr 

 (1948), however, noted that in Costa Rican waters 

 yellowfin ranging from 70 to 100 cm. spawn later 

 in the year than the larger fish. This presents 

 the possibility that larger fish have longer spawn- 

 ing periods than smaller fish, which in turn sug- 

 gests that the smaller percentage of sexually 

 active fish from 70 to 120 cm. in our samples may 



have resulted from a shorter spawning season 

 rather than from a diff'erence between the propor- 

 tions of sexually mature fish above 120 cm. and 

 those below 120 cm. Although the representation 

 of fish from 70 to 120 cm. for each month is spotty 

 in our samples, an examination of the monthly 

 percent maturing and ripe (table 4) shows the 

 peak percentage to be far less than that reached 

 by the larger fish. This supports our interpre- 

 tation of the results, that is, that the greater part 

 of the population reaches sexual maturity at 

 about 120 cm. 



Table 4. — Monthly percentages of sexually active yellowfin 

 tuna below 120 cm. fork length 



LOCALITY OF SPAWNING 



The data were grouped by months and by 

 10-degree longitudinal sections. The data for 

 those ovaries collected between 115°00' W. and 

 124°59' W. are shown in table 5 in the 120° W. 

 longitudinal section (the midpoint of that section), 

 those collected between 125°00' and i;34°59' W. 

 in the 130° W. section, and so on, with the excep- 

 tion of the 180° section, which includes 175°00' 

 W. to 180°. Because the percentage of sexually 

 active fish below 120 cm. was so much smaller, only 

 fish above this size were considered in order to get 

 results that could be used for comparison. The 

 percentage of active fish for each month for each 

 10-degree section was calculated. The percent- 

 ages for June and July along 180° were calculated 

 from summarized Japanese data, which did not 

 separate the catch of those 2 months. To arrive 

 at the monthly totals for these months (in the 

 extreme right-hand column of table 5), the 193 

 fish caught along 180° were separated into 154 

 fish for June and 39 fish for July, because 80 per- 

 cent of the catch was made in June. 



The results (table 5) show that all the sections 

 had at least one month in which 85 percent or more 

 of the fish were sexuallv active. This, coupled 



