98 



Abstract.— Spawning of yellowfin 

 tuna, Thunnus albacares, around 

 Clipperton Atoll, in the eastern Pacific 

 Ocean, occurred between 2230 and 

 0330 h on the basis of the presence of 

 migratory-nucleus and hydrated-stage 

 oocytes and new postovulatory follicles 

 in ovaries of females sampled at differ- 

 ent times of the day. Histological crite- 

 ria were developed to estimate the ages 

 of the postovulatory follicles and used 

 to estimate spawning frequency. His- 

 tological examinations of testicular tis- 

 sues provided criteria on the structural 

 characteristics of the sperm duct use- 

 ful for estimation of spawning fre- 

 quency. The mean interval between 

 spawnings was 1.14 days for females 

 and 1.22 days for males. The average 

 batch fecundity was 1.57 million oo- 

 cytes, or 68 oocytes per gram of body 

 weight. The average daily cost of 

 spawning, excluding behavioral activi- 

 ties, is estimated to be 0.97% and 0.28% 

 of the body weight per day for females 

 and males, respectively. 



Spawning time, frequency, and batch 

 fecundity of yellowfin tuna, 

 Thunnus albacares, near Clipperton 

 Atoll in the eastern Pacific Ocean 



Kurt M. Schaefer 



Inter-American Tropical Tuna Commission, Scripps Institution of Oceanography 

 8604 La Jolla Shores Drive, La Jolla. California 92037-1508 



Manuscript accepted 14 July 1995. 

 Fishery Bulletin 94:98-112 (1996). 



Knowledge of the reproductive bi- 

 ology of yellowfin tuna, Thunnus 

 albacares, is essential for a compre- 

 hensive understanding of the popu- 

 lation dynamics of this species. Pre- 

 vious investigations on the repro- 

 ductive biology of yellowfin tuna 

 have addressed the topics of spawn- 

 ing distribution, sex ratios, length 

 at maturity, and fecundity (Wild, 

 1994). However, fundamental infor- 

 mation on maturation and spawn- 

 ing has not previously been eluci- 

 dated for either male or female yel- 

 lowfin tuna. 



Knowledge of the appearance and 

 longevity of postovulatory follicles 

 in ovaries after spawning is neces- 

 sary for estimating spawning fre- 

 quency. The frequency of ovaries 

 containing postovulatory follicles 

 has been used to estimate spawn- 

 ing frequency in some scombroid 

 fishes, including skipjack tuna, 

 Ka tsu won us pela mi : s ( Hunter et al . , 

 1986); yellowfin tuna (McPherson, 

 1991 ); bigeye tuna, Thunnus obesus 

 (Nikaido et al., 1991); and chub 

 mackerel, Scomber japonicus (Dick- 

 erson et al., 1992). However, the age 

 and longevity of postovulatory fol- 

 licles have been determined only for 

 skipjack tuna and chub mackerel 

 held in captivity (Hunter et al., 

 1986; Dickerson et al., 1992). The 

 use of histological methods gener- 

 ally provides more precise criteria 

 than do gonosomatic indices or oo- 

 cyte diameter measurements for 

 assessing reproductive status of in- 



dividuals (Hunter and Macewicz, 

 1985; West, 1990). 



Although structural changes over 

 seasonal cycles have been described 

 from histological examinations of 

 teleost testes (Grier, 1981) and used 

 to estimate length at maturity and 

 define spawning seasons, there 

 have been no investigations of diel 

 testicular changes in relation to 

 rhythmic spawning activity. Infor- 

 mation on testicular histology in 

 tunas is scarce because most re- 

 searchers have been satisfied with 

 gross morphological or gonosomatic 

 indices to measure the reproductive 

 activity of males. Although histo- 

 logical examinations of the testes 

 from albacore tuna, Thunnus ala- 

 lunga (Ratty et al., 1990), and big- 

 eye tuna, Thunnus obesus (Nikaido 

 et al., 1991 ), have provided descrip- 

 tions of some aspects of spermato- 

 genesis and characteristics of 

 sexual maturity in male tunas, 

 there have been no descriptions of 

 histological characteristics of tuna 

 testes which can be used to deter- 

 mine when a male has spawned and 

 which would thus be useful for esti- 

 mating the spawning frequency and 

 reproductive effort of males. 



Yellowfin tuna, like many sub- 

 tropical and tropical pelagic fishes, 

 continuously produce batches of 

 hydrated oocytes (Hunter et al., 

 1985). Their annual fecundity is 

 indeterminate and exceeds the 

 standing stock of oocytes within the 

 ovaries at any given time. Annual 



