Stock assessment of the shortfin mako shark (Isurus oxyrinchus) in the Northwest Pacific Ocean using per recruit and virtual population analyses

Abstract

The shortfin mako shark (Isurus oxyrinchus) is a cosmopolitan species abundant in the Northwest Pacific. Some aspects of its biological information have been well documented yet its population dynamics is poorly known. The objective of this study is to assess the population status of the shortfin mako in the Northwest Pacific. The whole weights of 68,943 female and 64,338 male shortfin mako landed at Nanfangao and Chengkung fish markets, eastern Taiwan from 1990 to 2004 were converted to total length and the age for each individual was estimated based on the sex-specific von Bertalanffy growth equation. Total mortality obtained with length-converted catch curves ranged from 0.175 yr−1 to 0.272 yr−1 for females and from 0.196 yr−1 to 0.286 yr−1 for males. Natural mortality estimated from Peterson and Wroblewski's equation were 0.077–0.244 yr−1 for females and 0.091–0.203 yr−1 for males. Based on virtual population analysis, the highest fishing mortality occurred in ages 6–10 years for females and 7–12 years for males. Increases of fishing mortality in ages 3–5 years for females and 3–6 years for males since 1996 indicated that the young shortfin mako experienced higher fishing pressure in recent years. Both deterministic and stochastic simulations showed that annual spawning potential ratio (SPR) of shortfin mako was lower than the biological reference point (BRP) of SPR 35% and had a decreasing trend since 2000. Current fishing mortality (0.066 yr−1) was greater than the BRP of F30% (0.052 yr−1), F35% (0.045 yr−1), F40% (0.04 yr−1) and F0.1 (0.063 yr−1) suggesting that this stock might have been overexploited. Therefore, to ensure sustainable utilization, a management measure of 32% reduction of current fishing effort was suggested for the shortfin mako stock in the Northwest Pacific.