Francis and Stevens: Reproduction, embryonic development, and growth of Lamna nasus 



61 



the gross abdominal distension during midgestation, a 

 phenomenon that must create problems for the mother 

 in accommodating them. It would seem energetically 

 and hydrodynamically more efficient for a pregnant 

 female to match her ovulation rate to the immediate 

 growth and energy needs of the embryo, rather than 

 to provide an over-supply of food during a short time 

 period. We speculate that the answer lies in the avail- 

 ability of food resources. Porbeagles feed mainly on 

 small to medium pelagic fishes and cephalopods, but 

 also eat larger demersal teleosts and elasmobranchs 

 (Bigelow and Schroeder, 1948; Graham, 1956; Aasen, 

 1961; Templeman, 1963; Stevens et al., 1983; Gauld, 

 1989; Yatsu, 1995). Oophagy may be an adaptation 

 that allows pregnant porbeagles ( and other oophagous 

 species ) to maximize their use of food resources that 

 are abundant only during a short season. 



The lack of a six-month phase shift between the 

 reproductive cycles of Northern and Southern Hemi- 

 sphere porbeagles is surprising, and suggests that 

 water temperature and day length have little influ- 

 ence on reproduction. This may be due to porbeagles 

 having a highly developed endothermic ability (Carey 

 et al., 1985) which buffers body temperature against 

 seasonal fluctuations in temperature. But why should 

 the timing of reproduction be so similar in the two 

 hemispheres? The shortfin mako is also endothermic, 

 although not to such a high degree as porbeagles 

 (Carey et al., 1985), and its reproductive cycles are 

 six months out of phase in the two hemispheres (Mol- 

 let et al.i^). In the northwest Atlantic, porbeagle par- 

 turition coincides with the aiTival of migratory stocks 

 of Atlantic mackerel, capelin and 0+ Atlantic herring 

 (Moss^). Linking parturition with the period of peak 

 abundance of the common prey species in each hemi- 

 sphere would provide new-bom young with their best 

 chance of rapid growth and survival. Unfortunately, 

 neither of these hypotheses can be tested, because 

 there is no information on the existence or timing of 

 abundance cycles of porbeagle prey in the Southern 

 Hemisphere. 



This study has clarified several important aspects 

 of the reproductive biology of porbeagles, including 

 the length of the gestation period, mean fecundity, 

 length at birth, and the timing of parturition. Growth 

 rates have been estimated for embryos and juveniles 

 and are consistent with other studies. However con- 

 siderable imprecision and uncertainty remain in all 

 of these estimates, especially the lengths of the ges- 

 tation period and reproductive cycle, and therefore 

 the annual fecundity. Such information is crucial to 

 the determination of stock productivity in porbea- 

 gles; therefore better estimates are required before 

 effective stock assessment and management can be 

 achieved. 



Acknowledgments 



We are indebted to the scientific observers who fre- 

 quently operated under arduous conditions to collect 

 data and specimens. Their enthusiasm and dedication 

 made this study possible. Stephanie Kalish and Lynda 

 Griggs provided database support, and arranged the 

 delivery of specimens. The New Zealand Ministry of 

 Fisheries allowed us access to the scientific observer 

 database. Data, photographs and embryos were kindly 

 provided by Guy Duhamel, Sandy Moss, Andrew New- 

 ton, Stuart Hanchet, Rachel Berquist, Dick Williams, 

 Jeff Williams, and Clinton Duffy. Sabine Wintner, 

 Henry MoUet, and Markus Leppa translated the im- 

 portant papers by Lohberger and Swenander. Henry 

 also stimulated us with much discussion of lamnoid 

 embryonic development. Three anonymous referees 

 made helpful comments on the manuscript. 



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