Skomal and Natanson: Age and growth of Prionace glauca 



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studies on large highly migratory elasmobranchs. 

 Wintner and Dudley (2000) used two OTC-injected 

 recaptured individuals to conclude that growth 

 band deposition is annual in the tiger shark {Ga- 

 leocerdo ciivier). Moreover, Natanson et al. (2002) 

 validated annuli in the porbeagle shark up to 11 

 years of age by using only two OTC-injected and six 

 YOY recaptured individuals, although the species 

 was aged to 25 years. 



The processes that govern vertebral growth 

 have yet to be described in elasmobranchs. The 

 pattern varies from one ring per year in most 

 carcharhinids (Cailliet, 1990), and two rings per 

 year in some lamnids (Parker and Stott, 1965; 

 Pratt and Casey, 1983) to the complete absence 

 of periodicity (Natanson, 1984). Some research- 

 ers feel that temperature plays a major role in 

 this process (Stevens, 1975; Ferreira and Vooren, 

 1991). The blue shark, however, remains within a 

 discrete temperature range year-round (Stevens, 

 1975; Sciarrota and Nelson, 1977; Casey, 1982). 

 Moreover, acoustic tracking has shown that blue 

 sharks experience large changes in body tempera- 

 ture (up to 7°C) as they routinely pass through 

 the thermocline in their daily periodic dives from 

 the surface to depths of 200-600 m (Carey and 

 Scharold, 1990). 



The ecology of this species may provide a more likely 

 explanation of annulus formation. Kohler (1987) found 

 a seasonal cycle for energy storage that correlated with 

 the migratory patterns of the blue shark. In general, blue 

 shark condition was found to be at an annual low in the 

 winter and spring. Blue sharks use energy stores during 

 this time for extensive north-south and trans-Atlantic 

 migrations (Casey, 1985; Kohler, 1987) and periodic deep 

 dives (Carey and Scharold, 1990). It is logical that growth 

 may be depressed during these months, thereby causing a 

 check or annulus in the vertebrae. 



Tag-recapture data provide verification of the growth 

 curves derived from vertebral banding. Francis (1988b) 

 suggested that growth curves generated from age-length 

 and length-increment (tagging) data are not directly 

 comparable and that the comparison of growth rates at 

 length was more appropriate. Although VBGF parameters 

 derived from tagging data are noticeably higher, growth 

 rates were similar for both methods (Fig. 7). The higher 

 L^ and K can be attributed to the different derivation of 

 the VBGF parameters and the absence of older recaptured 

 sharks in the sample. 



Pratt (1979) proposed that maturity in the male blue 

 shark occurs at 183 cm FL and this would coincide with an 

 age of 4-5 years based on the results of the present study. 

 Females enter a distinct subadult phase (Pratt, 1979) at 

 145 cm FL and 2* years of age. Full maturity in females is 

 attained at 185 cm FL (Pratt, 1979), which corresponds to 

 about 5 years of age. 



Previous estimates of age and growth of the blue shark in 

 the Atlantic have been determined from vertebral banding 

 patterns, and verification has been made from the interpre- 

 tation of length-frequency and tagging data (Stevens, 1975; 



Silva et al., 1996; Henderson et al., 2001) (Table 5, Fig. 8). 

 The eastern Atlantic vertebral sample of Stevens (1975) 

 comprised largely females (89%), ranging from 34 cm to 

 227 cm FL. The resulting growth curve, therefore, largely 

 reflects female growth (Fig. 8C). His use of whole silver- 

 stained centra coupled with the lack of maximum-size fish 

 allowed for the interpretation of only six annuli. From only 

 mean back-calculated lengths at ages two through five, Ste- 

 vens extrapolated growth of the species with a VBGF to an 

 age of 20 years. Similarly, Silva et al. (1996) and Henderson 

 et al. (2001) investigated age and growth in this species 

 with whole vertebrae from sharks sampled in the eastern 

 North Atlantic. In the former study, vertebral SEunples from 



