the ovaries of maturing tunas. All of the ova in 

 the most advanced modal group (about 0.60 mm 

 or larger in these specimens) appeared to under- 

 go final maturation and ovulation during this 

 response but the second largest modal group 

 seemed not to be affected. Ovaries from "control" 

 specimens killed and refrigerated on capture 

 and from those that died within 5 h contained an 

 advanced modal group of maturing ova, as previ- 

 ously described, and a second, smaller modal 

 group in which the ova averaged between 0.39 

 and 0.44 mm in diameter (Table 2). Ovaries from 

 fully ovulated, ripe females and from recently 

 spent females contained a residual modal group 

 of similar, unovulated ova that averaged 0.39 to 

 0.49 mm in diameter (Table 3). These latter 

 observations support the common assumption 

 that in species with multimodal size distribu- 

 tions of developing ova, only the most advanced 

 modal group will mature and be ovulated' for a 

 given spawning. 



Table 3.— Mean sizes (mm) 1 of ova in largest modal group of 

 unovulated ova in ripe or recently spent skipjack tuna. 



'Standard deviations 02-0.04 



2 Found dead in holding tanks, time interval since last seen alive. 



Discussion 



This rapid ovarian maturation, ovulation, and 

 spawning appears to be a unique response to cap- 

 ture not previously reported. The trigger to this 

 response is not known but appears related to 

 stresses associated with capture and confine- 

 ment. Witschi and Chang (1959) earlier con- 

 cluded that ovulation of vertebrates could be 

 facilitated by stress, but there has been a lack of 

 direct evidence to support this conclusion. In- 

 direct evidence for such a relationship within 

 teleosts is suggested by ovulatory responses of 

 certain species to treatment with corticosteroids 

 (Hirose 1976; Sundararaj and Goswami 1977) 

 and with epinephrine (Jalabert 1976), both of 

 which have been reported to increase rapidly in 

 serum concentrations following such stresses as 

 handling and increased temperature (Mazeaud 



et al. 1977; Strange et al. 1977; Cook et al. 1980). 

 The handling associated with being hooked, 

 transported in crowded baitwells, transferred to 

 shore tanks, and confined is obviously stressful 

 and often fatal to newly captured skipjack tuna. 

 Thermal stress may occur when they are con- 

 fined in warm surface waters and prevented 

 from returning to cooler depths after feeding. 



Many additional aspects of this postcapture 

 ovulatory response are not yet understood. Sev- 

 eral aspects would be of particular interest: 1) 

 the state of ovarian maturation that would be 

 prerequisite for rapid egg development in fe- 

 males; 2) the seasonal availability of responsive 

 females; 3) whether the time to complete ovula- 

 tion, about 7 to 8 h in this study, will vary depend- 

 ing on such factors as water temperature, ovar- 

 ian maturation, or time of day the fish are 

 caught; and 4) whether this apparent response to 

 acute stress is entirely an artificially produced 

 anomaly, or whether it does have some relation to 

 their natural spawning biology. 



Past efforts to rear tunas in captivity (briefly 

 reviewed by Kaya et al. 1981) had not heretofore 

 resulted in dependable spawning procedures for 

 any species. However, the occurrence and pre- 

 dictability of the ovulatory response to capture 

 have now been applied to establish a routine pro- 

 cedure for spawning skipjack tuna at the Kewalo 

 Research Facility. Additional spawnings have 

 thus been accomplished during the summer of 

 1981, the second season of trials, and the response 

 has been observed also in a second species of 

 tuna — kawakawa, Euthynnus affinis. It would 

 be of interest to determine whether other species 

 will undergo a similar response to stresses of cap- 

 ture and confinement. 



Acknowledgments 



These observations were made possible 

 through a research contract from the National 

 Marine Fisheries Service to the senior author 

 and through the administrative support and en- 

 couragement provided by Richard S. Shomura, 

 Director of the Honolulu Laboratory of the Na- 

 tional Marine Fisheries Service Southwest Fish- 

 eries Center. 



Literature Cited 



Brock, V. E. 



1954. Some aspects of the biology of the aku, Katsuwonns 

 pelamis, in the Hawaiian Islands. Pac. Sci. 8:94-104. 



395 



