SPAWNING OF YELLOWFIN TUNA 



259 



Table 5. — Fractions of samples of seitialty active yetlowfin tuna (maturing and ripe) at various longitudes, by , 



IPcrcontage of se.vually mature fish in sample in parentheses) 



lonlhs 



with tlie fact that larvae below 10 mm. have been 

 found in all of these sections (Matsumoto') 

 indicates that yellowfin spawning occurs through- 

 out the central equatorial Pacific. The fact that 

 spawning probably occurs throughout the entire 

 equatorial Pacific is indicated by additional records 

 of spawning yellowfin in the western area by Wade 

 (1950 and 1951), Marr (1948), and Shimada 

 (1951), and in the eastern area by Schaefer and 

 Marr (1948) and Mead (1951). 



TIME OF SPAWNING 



The percentage of sexually active fish of 120 cm. 

 and longer was calculated for each month of the 

 year and was plotted on a graph (fig. 2). Yellow- 

 fin that had almost reached the spawning state 

 were found in each month except December, and 

 the greatest percentages of active fish occurred 

 from March (94.2%) through July (83.3%). 

 It was only during November, December, and 

 January that the occurrence of maturing and ripe 

 fish dropped below 40 percent. This, however, 

 does not prove that spawning is a year-round 

 activity, inasmuch as the length of time that the 



I .Miit.suniuto. Walter M.: T">escriptions of four species of tuna larvae and 

 their distribution in central Pacific waters. POFI. (Unpublished MS.) 



. . cry. 1/11 IV Ar-TiurlBASED ON OBSERVATIONS FROM 



• •btxuALLT "l-l l''tj-^^|_ SOURCES 



^ < EARLY STAGE OF RESORPTION"! BASED ON OVARIES 



^ COLLECTED AFTER 

 . -• LATE STAGE OF RESORPTION J 1951 ONLY 



JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC 



Figure 2.— Monthly percentages of yellowfin tuna -■^exnally 

 active or with rcsitUial eggs. 



fish are in these stages before spawning is not 

 known. 



To define tiie spawning season further, the 

 occurrence of residual eggs in these larger fish was 

 invc^stigated with respect to time. Tlie results, 

 plotted in figure 2, show that ovaries with early- 



