Age and growth of the blue shark Prionace glauca (Linnaeus, 1758) off northeastern Brazil

Abstract

Between 1992 and 2000, 681 blue sharks, Prionace glauca (173.8–310 cm total length, TL) were collected off northeastern Brazil (Brazilian EEZ). Vertebral sections of 156 males (173.8–310 cm TL) and 80 females (185.5–283 cm TL) were analysed. The index of average percentage error (IAPE) ranged from 0 to 3.1% for 3–12 growth rings (GRs). Mean marginal increment (MI) decreased from November to January without, however, conclusively confirm the annual pattern of deposition. Growth parameters were derived using the von Bertalanffy growth function (VBGF), the Richards function and the Schnute function. VBGF was that which best fit the data. Parameters derived from back-calculated lengths, K=0.157, L ∞=352.1 cm and t 0=−1.01 year, were considered to best describe growth. First maturity sizes of 225 cm for males and 228 cm for females corresponded to 5-GRs individuals. A 310-cm male had 12 GRs which was the highest number in the entire sample. Growth rates of 33.9 cm per GR were estimated from birth to the first band corresponding to 60.2% of size at birth, which is 56.4 cm. Adult males were 90.5% of the entire male sample and, adult females corresponded to 79.6% of the sample of females. The male sample was composed of individuals from 3 to >11 GRs and females were 3 to 10 GRs. Males were larger (with higher number of GRs) and outnumbered females in the overall sample, as commonly occurs in catches worldwide. Despite the hypothesis of differential growth by sex, postulated by several authors, the present study demonstrated that growth curves did not differ significantly, even considering significant vertebral radius (VR)–TL regressions for males and females. Higher growth rates were found in the southern Hemisphere, leading to significant differences in VBGF when compared to other areas. Discards and unreported catches are of great concern for conservation of this important apex predator.