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



1 I 1 



ration estimates of about 6.2% and 5.5% of body 

 weight for females and males, respectively, is ob- 

 tained for total energy expenditures. 



There are two implicit assumptions in these cal- 

 culations of reproductive costs: 1 ) the energy content 

 of oocytes and milt is equivalent on a weight basis 

 and 2 ) dry-weight-wet-weight relationships are simi- 

 lar for somatic and gonadal tissues. Furthermore, 

 these estimates should be considered conservative 

 because there are other physiological and behavioral 

 costs associated with spawning that are not included 

 in these calculations. 



For skipjack tuna. Hunter et al. ( 1986), estimated 

 the cost of a single spawning to be about 2% of the 

 body weight. Based on an estimated spawning fre- 

 quency of 1.18 days, the daily cost of spawning a 

 single batch of eggs would be 1.7% of body weight 

 per day. However, reproductive costs were probably 

 overestimated because a value of 100 eggs per gram 

 of body weight (Matsumoto et al., 1984) was used. 

 This relative fecundity estimate was not based upon 

 counts of hydrated oocytes. In addition, the calcula- 

 tion included the weight of a spawned egg, rather 

 than the weight of a late-migratory-nucleus-stage 

 oocyte immediately before hydration. An estimate of 

 the cost of a single spawning in black skipjack tuna 

 (Schaefer, 1987) was reported to be 1.77% of the body 

 weight. Based on an estimated spawning frequency 

 of 2.1 to 5.7 days, the daily cost of spawning a single 

 batch of eggs would be 0.31 to 0.84% of body weight 

 per day. These estimates may also be biased because 

 of the inherent problems in an estimation of spawn- 

 ing frequency by the occurrence offish with hydrated 

 ovaries versus by the postovulatory follicle method 

 (Hunter and Macewicz, 1985). 



Estimates of the average daily cost of spawning 

 for female and male yellowfin tuna from the present 

 study appear to be reasonable in terms of energy al- 

 location. The annual investment of energy in repro- 

 duction as a proportion of total energy is estimated 

 to be about 16% for females and 5% for males. These 

 values are within the range of those reported for other 

 fishes (Wootton, 1990). Given that yellowfin tuna 

 probably spawn year round in the tropical regions of 

 the eastern Pacific Ocean (Orange, 1961) and that 

 the mean interval between spawnings is around 1.2 

 days, a female (90 to 100 cm FL) would spawn the 

 equivalent of about 3.5 times its body weight per year, 

 whereas a male (90 to 100 cm FL) would spawn about 

 the equivalent of its body weight per year. For a fe- 

 male, this is almost 2.5 times the energy allocated 

 for somatic growth, whereas for a male it is only about 

 0.75 times (Olson and Boggs, 1986). 



These preliminary results provide the necessary 

 histological criteria for further assessment of the 



spawning dynamics of yellowfin tuna. Previous 

 batch-fecundity estimates for yellowfin tuna and 

 other scombrids not based on counts of migratory- 

 nucleus or hydrated oocytes are probably overesti- 

 mates. A more comprehensive investigation of the 

 reproductive biology of yellowfin in the eastern Pa- 

 cific (Bayliff, 1991) will provide size-specific estimates 

 of batch fecundity and spawning frequency. 



Acknowledgments 



I wish to acknowledge the owners and captains T. 

 Dunn, S. Loomis, and F LoPreste, and the crews and 

 passengers of the MV Royal Polaris, without whom 

 this research would not have been possible. I thank 

 W. Bayliff, R. Deriso, and D. Margulies of the IATTC 

 staff, as well as R. Hardy and two anonymous re- 

 viewers for useful suggestions for improvement of 

 the manuscript. I am grateful for the reviews pro- 

 vided by two of my mentors, H. Grier and J. Hunter. 



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