Margulies et al : Spawning and early development of captive Thunnus albacares 



263 



cesses to lower water temperature during oogenesis 

 and embryonic stages (Chambers, 1997). At lower water 

 temperatures, the durations of developmental stages are 

 longer, and eggs and hatched larvae are larger. 



An interesting aspect of egg development in yellowfin 

 tuna is the change in buoyancy as the eggs develop. 

 Fertilized eggs are positively buoyant throughout de- 

 velopment until a few hours before hatching, when they 

 become negatively buoyant. We assume that the onset 

 of negative buoyancy occurs as the chorion of the egg 

 begins to break down and more water diffuses into the 

 egg. Just before hatching, the chorion of fish eggs is 

 slowly liquefied by proteolytic enzymes (Blaxter, 1969), 

 and it is likely that this process occurs also in late- 

 stage yellowfin tuna eggs . The change in buoyancy of 

 eggs has not been studied extensively in tunas. Mayo 

 (1973) reported a similar change in buoyancy in the 

 eggs of seven taxa of scombrids from Florida waters, 

 and we have observed a similar pattern in fertilized 

 eggs of black skipjack (Euthynnus lineatus, first author, 

 personal commun. ). In nature, the adaptive significance 

 of negative buoyancy in late-stage eggs is not clear, but 

 presumably the process of sinking just before hatching 

 would remove the late-stage eggs and newly hatched 

 yolksac larvae from the neuston layer and reduce mor- 

 tality caused by wave action, wind, and damage from 

 ultraviolet radiation. 



Egg size increased with female yellowfin tuna size. 

 This is a common trait in many fish species, although 

 a direct relationship is apparent in some stocks but not 

 in others (Hempel and Blaxter, 1967; Marteinsdottir 

 and Able, 1988; Chambers and Leggett, 1996). The re- 

 lationship between egg size and female size is relatively 

 unstudied in tunas and is probably less important, in 

 terms of reproductive potential, than the relationship 

 between fecundity and female age and size (Chambers, 

 1997). 



For aquaculture purposes, the relationship of egg size 

 to female size in our study could be used to determine 

 the optimum size of broodstock females required to pro- 

 duce maximum egg sizes. For our original broodstock 

 group, the largest females produced the largest eggs 

 and larvae. However, larger egg size or hatching size 

 had no relationship with hatching success in our study. 

 Whether larger sizes at hatching confer some greater 

 fitness for yellowfin tuna during prerecruit stages in na- 

 ture is unclear, and may depend on processes of growth, 

 feeding, and development that occur later during devel- 

 opment. The ecological implications of the effects of size 

 during early life stages of yellowfin tuna or other tuna 

 species remain unknown and would require detailed 

 investigations of spawning and early life history traits 

 in wild tuna populations. 



Acknowledgments 



We thank the talented and dedicated staff of the 

 Achotines Laboratory for their hard work to ensure the 

 success of this project. The Overseas Fishery Coopera- 



tion Foundation of Japan provided the majority of the 

 funding for the expansion of the seawater and tank sys- 

 tems, and they were also our collaborative partners in 

 the development of the yellowfin tuna spawning project. 

 S. Masuma (Fisheries Research Agency of Japan ), C. 

 Farwell and D. Thomasberg (Monterey Bay Aquarium), 

 and H. Yoneshima (Kinki University) provided tech- 

 nical advice on the culture of tunas in captivity. In 

 Panama, R. Martans, A. Franco, E. Diaz, and A. Cano 

 (Direccion General de Recursos Marinos y Costeros) 

 provided administrative support. R. Jope and K. Bentler 

 (formerly of the lATTC) assisted with project logistics 

 and purchasing. C. Montez (lATTC) skillfully prepared 

 the figures. P. Tomlinson (lATTC) provided invaluable 

 advice on growth curve fitting and bootstrap procedures. 

 W. Bayliff provided helpful review comments. We wish 

 to thank A. Suda for his assistance in the early devel- 

 opment of the project. We would also like to thank the 

 former and present Directors of the lATTC, J. Joseph 

 and R. Allen, and the Chief Scientist of the Tuna-Billfish 

 program, R. Deriso, for their support of this study. 



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