Fisheries Research 98 (2009) 92–101 Contents lists available at ScienceDirect Fisheries Research journal homepage: www.elsevier.com/locate/fishres Stock assessment of the shortfin mako shark (Isurus oxyrinchus) in the Northwest Pacific Ocean using per recruit and virtual population analyses Jui-Han Chang 1 , Kwang-Ming Liu ∗ Institute of Marine Affairs and Resource Management, National Taiwan Ocean University, 2 Pei-Ning Road, Keelung 20224, Taiwan article info Article history: Received 3 July 2008 Received in revised form 10 April 2009 Accepted 14 April 2009 Keywords: Shortfin mako shark Virtual population analysis Spawning per recruit analysis Yield per recruit analysis Biological reference point 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 Nanfan- gao 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 0.175yr -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 fish- ing mortality (0.066 yr -1 ) was greater than the BRP of F 30% (0.052 yr -1 ), F 35% (0.045 yr -1 ), F 40% (0.04 yr -1 ) and F 0.1 (0.063 yr -1 ) suggesting that this stock might have been overexploited. Therefore, to ensure sus- tainable utilization, a management measure of 32% reduction of current fishing effort was suggested for the shortfin mako stock in the Northwest Pacific. © 2009 Elsevier B.V. All rights reserved. 1. Introduction In general, large sharks are extremely susceptible to overfish- ing because of their low reproductive rate and direct relationship between stock and recruitment (Holden, 1974, 1977). An increase in shark catches may necessitate the implementation of a man- agement plan for the sustainable use of the resource. However, development of fishery management plans can be difficult, when there is a lack of adequate fisheries and biological data of sharks. The shortfin mako shark, Isurus oxyrinchus, is widely distributed in tropical and subtropical waters (Compagno, 2001). It is not only caught by offshore fisheries but also is one of the major by-catch shark species for the tuna longline fishery in the open seas. The shortfin mako comprised 2.7% (ranked fourth) of the shark-fin trade in Hong Kong (Clarke et al., 2006), which suggested an average of ∗ Corresponding author. Tel.: +886 2 2462 2192x5018; fax: +886 2 2462 0291. E-mail address: kmliu@mail.ntou.edu.tw (K.-M. Liu). 1 Present address: School of Marine Sciences, University of Maine, Orono, ME 04469, USA. 500,000 individuals were killed annually in the world. This species is commonly found over the continental slope in the Northwest Pacific with highest abundances in eastern Taiwan waters (Fig. 1) (Joung and Hsu, 2005). It is common and commercially important to Taiwan fisheries since the 1930’s (Liu et al., 2001). Nanfangao and Chengkung are the two major offshore longline fishery land- ing ports in Taiwan (Fig. 1). Annual landings (whole weight) of the shortfin mako at Nanfangao and Chengkung fish markets in eastern Taiwan increased from 362t in 1990 to 912t in 2004 with a mean of 525 t, 8886 individuals, which was 23.6% (ranked 1st) of the total annual shark landings (except blue shark, Prionace glauca, which was not included in sale records) in the markets between 1990 and 2004 (Fig. 2). The shortfin mako is a large species (L max = 375 cm TL for females, L max = 289 cm TL for males) that grows slowly (k = 0.05 yr -1 for females, k = 0.056 yr -1 for males), matures late (271 cm TL, 18 years for females and 210 cm TL, 12 years for males), and generates few offspring (4–15 embryos/litter) (Joung and Hsu, 2005; Chang unpublished data). Some aspects of fishery biology including age and growth (Pratt and Casey, 1983; Cailliet et al., 1983; Natanson, 2001; Chan, 2001; Campana et al., 2002a; Ribot-Carballal et al., 2005; Ardizzone et al., 2006; Bishop et al., 2006; Natanson et al., 0165-7836/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2009.04.005