Age, growth, and reproduction of the pelagic thresher shark, Alopias pelagicus in the northwestern Pacific

Abstract

Age, growth, and reproduction of Alopias pelagicus are described based on 831 specimens (508 females and 323 males) collected from September 1993 to August 1995 in waters off northeastern Taiwan. Alopias pelagicus embryos are oophagous. Five developmental stages (two encapsulated and three posthatching) are recognized. The number of embryos per litter is two, and size at birth is between 158 cm and 190 cm total length (TL). The sex ratio of embryos is 1:1. Total length at maturity is 282-292 cm for females and 267-276 cm for males, which corresponds to age of 8.0-9.2 yr and 7.0-8.0 yr, respectively. Gestation period could not be determined from present data because females were in various stages of pregnancy throughout the year. Based on marginal increment analysis and verified by length-frequency analysis, annuli in precaudal vertebrae form once a year and numbered up to 16 and 14 for females and males, respectively. However, extrapolating the growth rates for the largest sharks would yield maximum ages of 28.5 yr and 17.5 yr for females and males, respectively. The von Bertalanffy growth equations in PCL were estimated as L t=197.2(1- e-0.085( t+7.67)) for females, and L t=182.2(1- e-0.118( t+5.48)) for males.