41 



Abstract.— Porbeagle sharks, Lamna 

 nasus. are caught in large numbers as 

 bycatch in tuna longline fisheries in the 

 southwest Pacific Ocean. Information on 

 reproduction, embryonic development, 

 and size and sex composition was col- 

 lected by scientific obser\'ers fi-om New 

 Zealand and Australian waters, and sup- 

 plemented with data fi-om other sources. 

 Most sharks were juveniles less than 

 150 cm fork length (FL), and length-fre- 

 quency distributions showed 3-5 modal 

 peaks that we interpret as age classes. 

 Juveniles grow linearly and rapidly ( 16- 

 20 cm per year), reaching 110-125 cm FL 

 in three years. Females mature at around 

 165-180 cm. Litter size is usually four 

 embryos and parturition probably peaks 

 in June-July (winter). This finding con- 

 trasts with data for North Atlantic 

 porbeagles which give birth in spring- 

 summer Embryos grow about 7 cm per 

 month, and are bom at 58-67 cm FL. The 

 gestation period appears to be about 8-9 

 months, but there is considerable vari- 

 ability in embryo length at any one time, 

 suggesting an extended mating period. 

 Embryos are nourished by oophagy, and 

 develop a grossly distended abdomen as 

 their "yolk stomach" fills with ova. Small 

 embryos have fang-like functional teeth 

 that tear open egg capsules to release 

 the contained ova. The fangs are shed 

 at 34-38 cm FL. The weight of yolk in 

 the stomach peaks at 30-42 cm FL, and 

 accounts for up to Sl'i of total body 

 weight. Waste products of yolk diges- 

 tion accumulate steadily in the spiral 

 valve throughout gestation, and the liver 

 reaches its maximum size in near-term 

 embryos as excess energy from yolk 

 digestion is stored for postnatal use. 



Reproduction, embryonic development, 

 and growth of the porbeagle shark, 

 Lamna nasus, in the southwest 

 Pacific Ocean 



Malcolm P. Francis 



National Institute of Water and Atmosphenc Research 



PO Box 14-901 



Wellington, New Zealand 



E mail address m franciSiS'niwa.ai.nz 



John D. Stevens 



CSIRO Manne Research 

 G PO Box 1538 

 Hobart, Tasmania, Australia 



Manuscript accepted 26 June 1999. 

 Fish. Bull. 98:41-63 (2000). 



The porbeagle, Lamna nasus (Bon- 

 naterre, 1788) is a pelagic mackerel 

 shark (family Lamnidae) that inhab- 

 its cool, temperate oceans. It occurs 

 in the North Atlantic Ocean and in 

 a circumglobal band in the southern 

 Pacific, Atlantic and Indian Oceans 

 (Compagno, 1984; Last and Stevens, 

 1994; Yatsu, 1995). It is absent from 

 the North Pacific, where it is re- 

 placed by its closest relative, the 

 salmon shark (Lamna ditropis). 



Lamnid sharks produce a small 

 number of large, live young that 

 are nourished by oophagy (Gilmore, 

 1993), In this unusual form of em- 

 bryonic development, the pregnant 

 female ovulates an enormous num- 

 ber of ova which are consumed by the 

 embryos in the uteri. The embryos 

 develop grossly swollen abdomens as 

 they store large quantities of yolk 

 for later growth. Oophagy was first 

 described in porbeagles (Swenander, 

 1906, 1907; Shann, 1911, 1923) and 

 salmon sharks (Lohberger, 1910), 

 but has only recently been confirmed 

 in shortfin and longfin makos (Isu- 

 rus oxyrinchus and /, paucus) and 

 white sharks iCarcharodon carchar- 

 ias) (Gilmore, 1983; Stevens, 1983; 

 Francis, 1996; Uchida et al., 1996), 



The unusual bloated appearance of 

 porbeagle embryos has led to a num- 

 ber of reports in the literature (Big- 

 elow and Schroeder, 1948; Graham, 

 1956; Templeman, 1963), but the ab- 

 sence of a series of embryos at dif- 

 ferent stages of gestation has ham- 

 pered attempts to understand their 

 development. Litters usually consist 

 of four embryos (Templeman, 1963; 

 Gauld, 1989), which are thought to be 

 bom at about 60-80 cm total length 

 (TL) (Shann, 1923; Compagno, 1984; 

 Last and Stevens, 1994). Female size 

 at maturity is often cited as 152 cm 

 TL (Bigelow and Schroeder, 1948; 

 Compagno, 1984; Last and Stevens, 

 1994), apparently based on two preg- 

 nant females reported to have been 

 "about five feet long" (Shann, 1911). 

 However, no other mature females 

 under 2 m TL have been reported, 

 leading some authors to regard the 

 length at maturity as 2-2.5 m TL 

 (Aasen, 1963; Pratt and Casey 1990). 

 The length of the gestation period is 

 unknown; estimates, however, range 

 from eight months to two years 

 (Shann, 1923; Aasen, 1963; Gauld, 

 1989 ). The timing of parturition is var- 

 iously stated as spring, summer, or 

 autumn in the North Atlantic (Bige- 



