Natanson et a\- Age and growth of Lamnus nasus in the western North Atlantic 



277 



Aasen (1963) relied extensively on length-frequency 

 modes to estimate the growth of porbeagle. Although his 

 modes were similar to ours, his interpretation of the age-1 

 mode differed. Our data indicated that age-0 porbeagle 

 average 85 cm FL in July. Aasen (1963) interpreted this 

 same mode (91 cm FL) as age 1+, thus shifting his ages 

 by one year. Neither Aasen's ( 1963) modal distribution (his 

 Fig. 4) nor ours, supports the contention that this first 

 mode is age 1+. His classification of these fish as age 

 1+ was based on size at birth and his opinion that the 

 smallest measured fish were from the age-0 group. There 

 is, however, no mode at this small size. Any age-0 fish 

 born in April and caught between July and September 

 I his sampling period) would certainly be larger than the 

 birth size. Therefore, we feel that these fish represented 

 the faster growing age-0 fish that were large enough to 

 be caught with commercial longline gear. Francis and Ste- 

 vens (2000) also used length-frequency analysis to esti- 

 mate the growth rate of porbeagles in the South Pacific. 

 Although their modes were once again comparable to ours, 

 their age l-i- fish were similar in size to our age-O-i- fish 

 (Fig. 8). In their view, this first mode represented slow 

 growing age-1 individuals rather than fast growing age-0 

 individuals. As an alternative explanation, we suggest 

 that the first mode in both the southwest and northeast 

 New Zealand samples represents YOY that have grown 

 during the 4-month sampling period, thus accounting for 

 the apparent absence of individuals close to a birth size. 

 This alternative explanation would also explain why the 

 modal analysis of the Australian sample shows a strong 

 peak at birth and subsequent modes that are similar to 

 ours. If correct, our interpretation of the Francis and Ste- 

 vens (2000) data would bring their estimates of size at age 

 and gi-owth rate in line with ours. Of course, the compari- 

 son of growth rates from such widely separated stocks is 

 difficult, and its value questionable. 



Longevity estimates for the porbeagle indicate that they 

 may live for more than 40 years. The maximum time at lib- 

 erty for any tagged porbeagle is 13 years (Stevens, 1990). 

 The length of this shark when tagged was approximately 

 120 cm TL (107 cm FL; l-i- years); it was recaptured at an 

 estimated 225 cm FL (age 14-1-) which would correspond to 

 an age of 16-i- years according to our vertebral growth cui've. 

 This is substantially less than the oldest observed age from 

 vertebrae (25) and the estimates from the Hoenig ( 1983 ) and 

 Campana et al." methods of 45 and 46 years, respectively. 



The growth rate and longevity of the porbeagle are simi- 

 lar to those of other lamnids. Wintner and Cliff ( 1999) cal- 

 culated a K value of 0.065 for the white shark from the 

 east coast of South Africa, and Cailliet et al. (1985) esti- 

 mated a if value of 0.058 for the same species off the coast 

 of California. Both estimates are very similar to the K 

 value of 0.066 calculated for porbeagle in our study. Short- 

 fin mako K values have been estimated at 0.072 (Cailliet 

 et al, 1983) and 0.266 (Pratt and Casey 1983); however 

 the Pratt and Casey value was based on the assumption 

 that two band pairs were deposited annually. Longevity 

 estimates have ranged between 27 years for the Califor- 

 nia white shark (Cailliet et al., 1985) and 45 years for the 

 shortfin mako in the Pacific (Cailliet et al., 1983). 



Comprehensive age and growth studies of pelagic sharks 

 are difficult to implement because many species are highly 

 migi-atory and are caught sporadically as part of seasonal 

 fisheries. Thus, aging studies of pelagic sharks have usu- 

 ally been less rigorous than desired, despite the oft-repeat- 

 ed call for age validation (Beamish and McFarlane, 1983; 

 Cailliet et al., 1986, Cailliet, 1990). Previous studies on pe- 

 lagic species such as the blue shark (Stevens, 1975; Cail- 

 liet etal., 1983), white shark (Cailliet etal., 1985), thresher 

 (Cailliet et al., 1983), .shortfin mako (Cailliet et al, 1983), 

 pelagic thresher (Liu et al., 1999), oceanic whitetip shark 

 (Lessa et al., 1999), and porbeagle (Aasen, 1963; Francis 

 and Stevens, 2000), have included analyses of ages deter- 

 mined by vertebral or length-frequency analyses (or by 

 both methods), but none of the age interpretations were 

 validated. Wintner and Cliff (1999) used vertebral counts 

 and had one OTC-injected recapture but were unable to 

 provide validation or consistency with other methods. Pratt 

 and Casey (1983) aged the shortfin mako by using four 

 methods (temporal analysis of length-month information, 

 tag-recapture data, length-frequency data, and vertebral 

 band counts) but could not validate their age interpreta- 

 tions. The conclusion that band pairs were deposited bian- 

 nually was based on vertebrae from four tag-recaptures 

 and consistency between methods. Skomal ( 1986) aged the 

 blue shark in the western North Atlantic using a combina- 

 tion of vertebral, length frequency and tag-recapture meth- 

 ods for verification. Although Skomal ( 1986) had two OTC- 

 injected recaptures, they provided conflicting results for 

 validation. The present study is the first that has used 

 validated vertebral band-pair counts in conjunction with 

 length-frequency and tag-recapture analyses to provide 

 consistent and accurate age estimates for a pelagic shark 

 species. We suggest that a similar approach would be use- 

 ful in studies of other pelagic shark species. 



Acknowledgments 



We thank Clearwater Fine Foods, Karlsen Shipping, the 

 Atlantic Shark Association, and Stephanie Jane, Inc. for 

 providing access to their fishing vessels. We also thank 

 Andy Kingman, Christopher Jensen, and Warren Joyce for 

 collecting samples. Malcolm Francis kindly provided verte- 

 brae from porbeagle embryos as well as much appreciated 

 knowledge on the procedures associated with MULTIFAN. 

 Colin Simpfendorfer's assistance and spreadsheet were 

 invaluable during the use of GROTAG. Nancy Kohler 

 and Sabine Wintner provided invaluable comments on the 

 manuscript. We are indebted to the thousands of fisher- 

 men who voluntarily tag and return tags to us and thus 

 make tagging programs possible. 



Literature cited 



Aasen, O. 



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

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

 Havund. 13(6):20-37. 



