Spawning per recruit analysis of the pelagic thresher shark, Alopias pelagicus, in the eastern Taiwan waters

Abstract

The pelagic thresher shark, Alopias pelagicus, is a cosmopolitan species and abundant in Taiwan waters. Some of its biological information has been documented yet its population dynamics are poorly known. The purpose of this study is to assess the pelagic thresher shark stock status in the eastern Taiwan waters. The whole weights ( W) of 51,748 individuals of the pelagic thresher shark landed at Nanfanao and Chengkung fish markets, eastern Taiwan from 1990 to 2004 were converted to precaudal length (PCL) based on the W–PCL relationship ( W = 2.25 × 10 −4 × PCL 2.533, n = 2165). The sexes combined VBGE L t = 189.5 × (1 − e −0.10( t+6.47) ) was used to estimate the age for each length group. Total mortality rates ( Z) obtained with length-converted catch curves ranged from 0.208 to 0.277 year −1. Natural mortality rate ( M) estimated from Hoenig method was 0.132 year −1, and exploitation rate ( E) ranged from 0.069 to 0.127 for 1990–2004. Annual abundance was estimated to range from 97,551 in 2000 to 153,331 in 2003 from virtual population analysis, and the highest fishing mortality occurred in ages 8–18 years. There were four different scenarios being simulated in this study. Scenario 1 indicated that spawning per recruit (SPR) ranged from 23.07% in 2001 to 47.71% in 1990 with a mean of 36.41% for the period of 1990–2004. The mean SPR of pelagic thresher for 1990–2004 was below the BRP of SPR = 35% in scenarios 2–4 suggesting that this stock was slightly overexploited. Therefore, to ensure sustainable utilization of this stock, reduction of fishing effort and close monitoring on A. pelagicus are needed.