Jensen et a\ The reproductive biology of Lamna nasus in the western North Atlantic Ocean 



737 



lantic. Bigelow and Schroeder (1948) described gravid 

 porbeagles in the (iulf of Maine during November, Janu- 

 ary, and August, althougii the latter month eonfiicts with 

 l)oth the present findings and those of'Aasen ( 1963). Aasen 

 (1963) reported that pregnant females carrying large 

 embryos were observed in late May at the Flemish Cap. 

 Templeman ( 1963) reported three gravid females from the 

 SW Grand Banks during January and February 1953-56. 

 Gauld ( 1989) reported the presence of gravid females from 

 December to June in the NE Atlantic ocean, whereas 

 Francis and Stevens (2000) reported gravid females from 

 March to July in the SW Pacific Ocean. 



Length variability within and between litters has been 

 found in most porbeagle studies (Fig. 10). We found a dif- 

 ference of up to 14.6 cm in the lengths of individuals in one 

 litter. Gauld (1989) found length differences of up to 11 cm 

 within individual litters, and Shann (1923) found a 12-15 

 cm difference in embryo length within individual litters. 

 Runts were observed in pregnant porbeagles in our study 

 as well as by Francis and Stevens (2000). 



As with other lamnids, female porbeagle sharks nurture 

 their young through oophagy. Upon hatching from the sin- 

 gle-ovum capsules and after absorption of the external yolk 

 sac, embryos begin orally feeding on yolk-filled nutritive 

 ova capsules. As a result, the internal yolk stomach of the 

 embryos expands to the large size characteristic of lamnid 

 embryos at this stage in their development. Evidence from 

 this study suggests that adelphophagy does not occur in 

 the porbeagle. The ova capsules observed in our study were 

 similar to those found in the sandtiger and other lamnid 

 sharks described by Gubanov (1972), Fujita (1981), Gruber 

 and Compagno (1981), Gilmore et al. (1983), Francis and 

 Stevens (2000), and Mollet et al. (2000). 



Embryonic growth was estimated at 8.15 cm per month 

 in our study (Fig. 10). Francis and Stevens (2000) esti- 

 mated embryonic growth of approximately 7.48 cm per 

 month for embryos in the South Pacific. New data from 

 the South Pacific (Francis'') refined this estimate to 8.47 

 cm per month. Examination of more late-term embryos 

 will further refine this estimate for the NW Atlantic. 



Our results indicated a one year reproductive cycle, with 

 gestation lasting 8-9 months. Mating occurs between Sep- 

 tember and December Based on the one late-term female 

 and several postpartum females it appears that parturi- 

 tion occurs from early April though June. Aasen (1963) 

 estimated that parturition occurs from late May to early 

 June in the NW Atlantic and suggested about an 8-month 

 gestation period based on the lack of gravid female por- 

 beagles from June to September. All females we examined 

 in December were gravid. Although a nongravid mature 

 portion of the female population could reside elsewhere, 

 we have no data to support this and thus assume that 

 porbeagle females reproduce annually. Parturition time 

 and location and late embryonic development and gnwth 

 rates need further investigation. 



Francis, M. 2000. Personal commun. National Institute of 

 Water and Atmospheric Research P.O. Box 14-901. Kilbirnie, 

 Wellington, New Zealand. 



Mean litter size in the NW Atlantic was found to be 4.0 — 

 usually two embryos per uteri. Mean litter size from the 

 SW Pacific Ocean was 3.8 from 138 embryos representing 

 43 litters (Francis and Stevens, 2000). Mean litter size 

 from the NE Atlantic Ocean was 3.7 based on 12 litters 

 examined by Gauld (1989). The sex ratio of the embryos 

 examined in our study was 1:1, which agrees with the 

 findings of Francis and Stevens (2000) for the SW Pa- 

 cific Ocean. The smallest free-swimming porbeagles in 

 the NMFS historical tagging database range in size from 

 55 to 79 cm FL (mean 71 cm FL)^ from April to June; this 

 length range, along with that of the one late-term litter 

 (50-59 cm FL), suggests a birth size similar to Aasen's"* 

 ( 1963) prediction of 67 cm FL and to Francis and Steven's 

 (2000) estimate of 58-67 cm FL for the SW Pacific. How- 

 ever, length at birth needs further analysis because few 

 neonates were observed during the parturition period. 



Acknowledgments 



We thank Clearwater Fine Foods, Karlsen Shipping, the 

 Atlantic Shark Association, and Stephanie Jane, Inc. for 

 providing access to their fishing vessels. We thank Cap- 

 tain Steve James of the Boston Big Game Fishing Club for 

 access to tournament-landed porbeagles from Stellwagan 

 Bank. We also thank Warren Joyce and Andy Kingman 

 for sample collection. Gregg Skomal of the Massachusetts 

 Division of Marine Fisheries coordinated our participation 

 in the porbeagle tournament. Malcolm Francis kindly 

 provided data on embryo growth from the Southern Hemi- 

 sphere, and corresponding data that he compiled from the 

 Northern Hemisphere, and reviewed an early draft. We 

 also thank Jose Castro for reviewing an early draft. 



Literature cited 



Aasen, O. 



1963. Length and growth of the porbeagle {Lamna nasus, 

 Bonnaterre) in the North West Atlantic. Fisk. Skrift. Ser. 

 Havund. 13(6):20-37. 

 Bigelow, H. B., and W. C. Schroeder. 



1948. Fishes of the Western North Atlantic. Part 1. Lance- 

 lets, cyclostomes, sharks. Mem. Sears Found. Mar. Res., 

 576 p. 

 Castro, J. I. 



1993. The biology of the finetooth shark, Carcharhinus 



isodon. Environ. Biol. Fish. 36:219-232. 

 1996. The biology of the blacktip shark, Carcharhinus lim- 

 batus, off the Southeastern United States. Bull. Mar. Sci. 

 59(31:508-522. 

 2000. The biology of the nurse shark, Ginglymostoina cir- 

 ratum. off the Florida east coast and the Bahama Islands. 

 Environ. Biol. Fish. 58:1-28. 

 Compagno, L. J. V. 



1984. FAO species catalogues. Vol. 4. Sharks of the world. 

 An annotated and illustrated catalogue of the shark spe- 

 cies known to date, parts 1 and 2. FAO Fish. Synopsis 

 125, vol. 4, parts 1 and 2, 655 p. FAO, Rome. 

 Dunlop. J. 



1897. The habits and anatomical structure of the porbeagle 

 shark {Lamna cornubica,L, Cuv. i. Proceedings and Trans- 



