Schaefer Spawning time, frequency, and batch fecundity of Thunnus albacares 



103 



at similar times over several different days and dur- 

 ing the two different years. Postovulatory follicles 

 estimated to be 12 to 24 hours old occurred in ova- 

 ries which also contain migratory-nucleus or hy- 

 drated oocytes (Table 1). 



The incidence of both postovulatory follicles and 

 late-stage oocytes indicated daily spawning in yel- 

 lowfin tuna (Fig. 5). Five of the six fish collected be- 

 tween 1300 and 1700 h in 1986 and all seven fish 

 collected between 1500 and 1900 h in 1987 that had 

 oocytes in the migratory-nucleus or hydrated stages 

 also had postovulatory follicles present (Table 1). 

 Assuming that the postovulatory follicles are not 

 detectable for longer than 24 hours, this observation 

 indicates that these fish had spawned the previous 

 night and would spawn again the next night. For the 

 combined samples of 1986 and 1987, the fraction of 

 mature females with postovulatory follicles was 0.79, 

 equivalent to a mean spawning interval of about 

 1.27 d. If only females that were classified as repro- 

 ductively active are considered, the fraction of fish 

 spawning (those with postovulatory follicles) was 



0.88, a mean interval of 1.14 d. The mean spawning 

 intervals for each of the years were similar, 1.27 d in 

 1986 and 1.25 d in 1987. 



