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



57 



gles (Stevens, 1976. 1990). Their lengths at tagging 

 and recapture were only estimated; therefore growth 

 increments were approximate. The sharks were rela- 

 tively small when released (84-105 cm), and were at 

 liberty for 0.75-13 years. Annual growth increments 

 were about 9-32 cm, with a mean of 20.4 cm. Despite 

 good agreement among all sources of porbeagle growth 

 data, a vertebral-based growth curve from adequate 

 sample sizes and covering the full size range is still 

 required for both hemispheres. 



Longevity is unknown, but Aasen (1963) aged his 

 226 cm female as 19-i- years and suggested that they 

 may live around 30 years. This conclusion needs con- 

 firmation because longevity is often used to estimate 

 the natural mortality rate, which is an important pa- 

 rameter in population models. 



Length at maturity and reproductive development 



The lengths of pregnant females (167-199 cm, mean 

 185 cm ) suggest that females mature at about 165-180 

 cm. This estimate is consistent with reports of a 

 191 -cm mature female from the South Atlantic (Svet- 

 lov, 1978), and immature females of 138, 139, and 150 

 cm from around New Zealand (Stevens et al., 1983; 

 Dufiyi°). The length at maturity of North Atlantic 

 females is controversial. Shann (1911) reported two 

 pregnant females of "about five feet long" (152 cm 

 TL, or 133 cm FL). In both cases, the length estimate 

 was probably third-hand, and it is unlikely that the 

 females were measured. That estimate of length at 

 maturity, which we believe to be unreliable and too 

 low, has permeated the literature ( Bigelow and Schro- 

 eder, 1948; Compagno, 1984; Last and Stevens, 1994). 

 Other authors have reported a wide maturity range of 

 175-220 cm FL (Aasen, 1961, 1963; Pratt and Casey, 

 1990). Templeman ( 1963) and Moss^ reported females 

 of 191 and 203 cm to be immature. The smallest ma- 

 ture females reported by Templeman ( 1963 ) and Gauld 

 (1989) were 203 and 196 cm respectively, and Pratt 

 (1993) examined one of 227 cm. Aasen (1961) showed 

 that uterine width increased rapidly in females longer 

 than 197 cm. These results suggest that North Atlan- 

 tic females mature at about 195-205 cm (218-229 cm 

 TL ), which is higher than the range determined for the 

 Southern Hemisphere. A similar between-hemisphere 

 difference in length at maturity has been found for 

 shortfin makos (MoUet et al.^^). 



1° Duffy, C. 1998. Department ofConservation, Private Bag 3072. 

 Hamilton, New Zealand. Personal commun. 



" MoUet, H. R, G. Cliff, H. L. Pratt, and J. D. Stevens. 1998. 

 Reproductive biology of female shortfin mako Isurus oxyrinchus 

 Rafinesque 1809. H. F. Mollet, Monterey Bay Aquarium, Mon- 

 terey, California 93940, Unpubl. manuscript. 



There was no information on length at maturity in 

 males in our data. On the basis of changes in clasper 

 length and calcification. North Atlantic males appar- 

 ently mature at a smaller size than do females, in the 

 range 131-175 cm (150-200 cm TL) (Aasen, 1961; El- 

 lis and Shackley, 1995). 



The "internal" type ovary from our Macquarie Is- 

 land pregnant female conformed with the morphol- 

 ogy found in all lamnid and alopiid sharks examined 

 so far (Pratt, 1988). It weighed 2.75 kg (2.35% of 

 total weight). The ovary of Swenander's (1906, 1907) 

 246-cm North Atlantic pregnant female measured 41 

 by 28 cm, and weighed 6.3 kg, or 3.6% of estimated 

 total weight. Mean embryo lengths in the two females 

 were 22.1 cm and about 25 cm respectively. The em- 

 bryo stomach contents peak at 30-42 cm, suggesting 

 that ovarian size and ovulation may peak when em- 

 bryos are about 25-30 cm long. In the shortfin mako, 

 the ovary of an actively ovulating female with early- 

 term embryos weighed about 5% of her total weight, 

 whereas the ovaries of females carrying near-term 

 embryos were spent and weighed as little as 0.2-0.3% 

 of total weight (Mollet et al.''). In sandtiger sharks 

 (Carcharias taurus), relative ovarian weight peaks at 

 6—7% of body weight, and then declines during the 

 second half of gestation (Gilmore et al., 1983; Gilmore, 

 1993; Mollet et al.''). A low relative ovary weight was 

 also reported in a longfin mako with near-term em- 

 bryos (Gilmore, 1983). 



The size-frequency distribution of ova from the Mac- 

 quarie Island female indicated that they are ovulated 

 at around 4—5 mm. Ova diameters measured in two 

 pregnant North Atlantic females after preservation 

 in 10% formalin were mainly in the range 2.3-4.3 

 mm, with the largest measuring 6.0 mm (Moss^). 

 Swenander (1906, 1907) reported ova diameters be- 

 tween 1 and 5-6 mm in a North Atlantic pregnant fe- 

 male, and encapsulated 4—5 mm ova in the uteri of an- 

 other female. Maximum ova diameters in other ooph- 

 agous sharks range between 4 and 10 mm (Springer, 

 1948; Bass et al., 1975; Otake and Mizue, 1981; Gilm- 

 ore, 1983; Gilmore et al, 1983; Stevens, 1983; Uchida 

 et al., 1996; Chen et al., 1997). 



Empty and near-empty egg capsules were found in 

 the uteri of the Macquarie Island female, and in the 

 mouth of one of its embryos. Moss'^ also found empty 

 egg capsules in the mouths and gill slits of embryos 

 measuring 33.8 and 36.2 cm. Apparently, embryos 

 are capable of rupturing egg capsules and swallowing 

 the contents, although the occasional presence of ge- 

 latinous material resembling egg capsules in embryo 

 stomachs (Swenander, 1907; this study) suggests that 

 whole or empty capsules are sometimes swallowed. 

 Swenander (1906, 1907) found over 40 egg capsules, 

 each measuring about 80 by 15 mm and containing 



