Age determination and growth of the blue shark (Prionace glauca) in the western North Pacific Ocean

Accurate estimation of growth parameters is vital for stock assessments and management of exploited species. To determine if changes in sex-specific growth parameters of the blue shark (Prionace glauca) have occurred in the North Pacific Ocean following population declines in the 1980s and 1990s, we analyzed data obtained from the vertebrae of 659 male and 620 female sharks that had precaudal lengths (PCLs) of 33.4-258.3 cm and were captured over a wide geographic area between 2010 and 2016. Maximum counts of growth bands were 18 for males and 17 for females. Significant (P<0.001) between-sex differences were detected in growth parameters. We estimated parameters of the von Bertalanffy growth function: for males, the theoretical asymptotic length (L-infinity) was 284.9 cm PCL, the growth coefficient (k) was 0.117/year, and the theoretical age at zero length (t(0)) was -1.35 years, and, for females, L-infinity was 257.2 cm PCL, k was 0.146/year, and t(0) was -0.97 years. Sexual discrepancies in growth rates are likely a function of differences in energy allocation relating to reproduction between sexes. Given that no remarkable change in growth parameters was observed over 3 decades, life history parameters of this population do not appear to have been affected by shifts in stock abundance or environmental fluctuation.