Skomal and Natanson: Age and growth of Prionace glauca 



635 



vertebrae generally underestimate ages in larger indi- 

 viduals. The counts obtained in the three eastern Atlantic 

 studies may be accurate because they are from juvenile 

 sharks where vertebral bands are not compressed. In fact, 

 juvenile growth from our size-at-age data overlaps the 

 growth cui-ves from these studies. However, the VBGF 

 growth curves and resulting estimates of growth rate 

 and age at maturity from the eastern Atlantic studies are 

 suspect because of the lack of fish at the lower and upper 

 end of the curve. The general lack of maximum-size fish in 

 these studies resulted in the estimation of an artificially 

 inflated L^ and, therefore, a lower growth rate iK) for this 

 species (Table 5). Vertebral band deposition was assumed 

 to be annual in these studies, but low sample sizes, sample 

 bias, and lack of validation limits the utility of this previ- 

 ous work. In the current study, the use of sections and the 

 adequate representation of the entire size range for both 

 sexes yielded more accurate age estimates of 16 and 15 

 years for males and females, respectively. 



Age and growth estimates of the blue shark in the North 

 Pacific have been determined by using vertebral bands and 

 length-frequency data (Cailliet et al., 1983; Tanaka et al., 

 1990; Nakano, 1994 ). Although the VBGF was used to mod- 

 el growth based on vertebral interpretation, the resulting 

 parameters differed greatly among studies (Table 5). In 

 general, Cailliet et al. (1983) reported a male growth rate 

 similar to that in our present study, but a much smaller 

 L^ (Table 5). For females, the latter holds true, but the 

 growth coefficient is much higher (0.25) than reported in 

 our study. Tanaka et al. ( 1990 ) found a similar growth trend 

 in the western North Pacific with females growing faster 

 than males, but the VBGF parameters were very differ- 

 ent with higher L^ and lower K values. When compared 

 to our study, the VBGF parameters of Tanaka et al. ( 1990) 

 yield slower growth and a greater maximum size for males 

 and a similar growth rate and smaller maximum size for 

 females. Tanaka et al. ( 1990 ) attributed these intra- and in- 

 ter-oceanic differences to the different methods used. More 

 recently, Nakano (1994) sampled blue sharks across the 

 North Pacific and derived VBGF growth parameters that 



were similar to those of Tanaka et al. ( 1990), but estimated 

 growth rate to be slower than that of our present study. It 

 is difficult to ascertain whether interoceanic differences 

 in growth are real or are an artifact of method. Although 

 Tanaka et al. (1990) presented data to support the latter 

 within the North Pacific, the much larger maximum size 

 attained by this species in the North Atlantic (Strasburg, 

 1958; Tanaka, 1984) cannot be overlooked in relation to 

 interoceanic growth differences. 



