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



109 



was 1.09%, which yields an estimate for the cost of 

 spawning of 0.34% of body weight. Based on the es- 

 timated mean interval between spawnings of 1.14 

 days for females and 1.22 days for males, estimates 

 of the average daily costs of spawning are 1.18% and 

 0.28% of the body weight per day for females and 

 males, respectively. 



Discussion 



Spawning time 



The estimated time of spawning for yellowfin tuna 

 at Clipperton Atoll was between 2230 and 0330 h. 

 This is similar to the spawning time of 2100 to 2400 h 

 by a group of 3- and 6-year-old yellowfin tuna re- 

 ported by Masuma et al. (1993). The spawning of 

 yellowfin tuna at night was reported earlier by 

 Harada et al. ( 1980) on the basis of running-ripe and 

 fully hydrated oocytes. Based on oocyte developmen- 

 tal stages and new postovulatory follicles, the as- 

 sumption was made that yellowfin tuna spawn after 

 2200 h in the Coral Sea (McPherson, 1991) and big- 

 eye tuna from about 1900 to 2400 h off Java and 

 southwest of Hawaii (Nikaido et al., 1991). Captive 

 skipjack tuna typically spawn at 2400 h, which ap- 

 pears to be close to the time of natural spawning on 

 the basis of histological examinations of skipjack 

 ovaries ( Hunter et al. , 1986 ). Although there are some 

 reported observations of courting behavior in 

 scombrids during daylight hours (Iverson et al., 

 1970), they are primarily anecdotal and appear to 



be exceptions to the normal behavior of night spawn- 

 ing. Nocturnal spawning also appears to be a com- 

 mon pattern among clupeoids (Blaxter and Hunter, 

 1982), coral reef species that have pelagic larvae 

 (Helfman, 1986), and other groups of fishes (Ferraro, 

 1980). Spawning at night may be an adaptation to 

 minimize predation by planktivores or damage by 

 ultraviolet light, or both. 



Spawning frequency 



The 1986 and 1987 yellowfin tuna samples from 

 Clipperton Atoll indicated that the average interval 

 between spawnings was 1.27 d. Based on the fre- 

 quency of ovaries containing postovulatory follicles 

 of known age, the average interval between 

 spawnings was 1.18 d for skipjack tuna from the 

 South Pacific (Hunter etal., 1986), 1.54 d for yellow- 

 fin tuna in the Coral Sea (McPherson, 1991), and 

 1.11 d for bigeye tuna off Java and southwest of Ha- 

 waii (Nikaido et al., 1991). Spawning occurred dur- 

 ing 127 of 155 days for a group of 3- and 6-year-old 

 yellowfin held in a sea pen, but the spawning fre- 

 quency of individuals was not determined (Masuma 

 et al., 1993). This implies that reproductively active 

 yellowfin tuna, as well as skipjack and bigeye tunas, 

 in tropical waters, spawn almost daily. This high fre- 

 quency of spawning implies that in a reproductively 

 active female there is continuous maturation of oo- 

 cytes, which are recruited from the reservoir of pri- 

 mary oocytes. Deviations from the apparently nor- 

 mal daily spawning pattern could be related to re- 

 cent feeding success (Wootton, 1990) or physiologi- 



