Fisheries Research 84 (2007) 282–300 Application of the sex-specific age-structured assessment method for swordfish, Xiphias gladius, in the North Pacific Ocean Sheng-Ping Wang a,1 , Chi-Lu Sun a,∗ , Andr´ e E. Punt b , Su-Zan Yeh a a Institute of Oceanography, National Taiwan University, Taipei 106, Taiwan b School of Aquatic and Fishery Sciences, University of Washington, Seattle, WA 98195, United States Received 29 May 2006; received in revised form 28 October 2006; accepted 3 November 2006 Abstract Swordfish are known to be sexually dimorphic. However, previous assessments of the status of swordfish in the North Pacific Ocean have ignored this. A sex-specific age-structured assessment model was therefore constructed and fitted to catch, catch-rate and length–frequency data for the swordfish fisheries that operate in the North Pacific Ocean. Except if natural mortality is lower than its “best” estimate, the results indicate that the spawning stock biomass in 2002 was at a high fraction of its unfished level and that the fishing intensity in 2002 was less than F MSY . Therefore, the swordfish stock in the North Pacific Ocean appears to be relatively stable at the current level of exploitation. However, the results of the assessment model are sensitive to the values for natural mortality and the steepness of the stock-recruitment relationship. Forward projections based on samples from a Bayesian posterior distribution indicate that there is negligible risk of the stock dropping below 40% of the unfished spawning stock biomass if fleet-aggregated fishing intensity remains at the current level. However, the risk of population depletion is higher if natural mortality is lower than the “best” estimate. © 2006 Elsevier B.V. All rights reserved. Keywords: Sex-specific age-structured assessment method; Stock assessment; Swordfish; North Pacific 1. Introduction Swordfish (Xiphias gladius, Linnaeus 1758) is a cos- mopolitan species found in tropical, subtropical, temperate, and sometimes cold waters of all oceans and adjacent seas (Nakamura, 1985). In the North Pacific Ocean (defined, for the purposes of this paper, to be the area of north of the equator; Fig. 1), the bulk of the swordfish catch has been taken by Japan, the United States, Taiwan and Mexico, with very small catches by Korea and China, whose swordfish catch is estimated to be less than 4% of the total swordfish catch in the North Pacific (Anon., 2002, 2004). The biomass of swordfish in the North Atlantic Ocean is esti- mated to be close to that corresponding to Maximum Sustainable Yield (MSY) while the fishing mortality and catch are estimated to be slightly less than those at MSY (Anon., 2005a). Catches of ∗ Corresponding author. Tel.: +886 2 23629842; fax: +886 2 23629842. E-mail address: chilu@ntu.edu.tw (C.-L. Sun). 1 Present address: Department of Environmental Biology and Fisheries Sci- ence, National Taiwan Ocean University, Keelung 202, Taiwan. swordfish in the South Atlantic Ocean increased rapidly between 1988 and 1995. However, the population status of swordfish in the South Atlantic remains unknown because of uncertainty about fishing effort (Anon., 2005a). The results of assessments of swordfish in the Indian Ocean based on surplus production models indicate that exploitation rates are presently likely above those at which MSY is achieved and hence that overfishing is likely to be occurring (Anon., 2006). Most previous assessments of swordfish in the North Pacific Ocean have been based on trends in catch-rates (i.e. catch-per- unit-effort, CPUE) (e.g. Bartoo and Coan, 1989; Di Nardo and Kwok, 1998; Kleiber and Bartoo, 1998; Nakano, 1998; Uosaki, 1998; Anon., 2002, 2004) and the results of production model assessments (Anon., 1999), although attempts have been made to apply MULTIFAN-CL (Fournier et al., 1998) to catch, effort and length–frequency data collected from Japanese and Hawaiian vessels. All of these analyses indicate that the swordfish stock in the North Pacific Ocean is not over-exploited, and that it has been relatively stable at current levels of exploitation. Swordfish are known to be sexually dimorphic (females grow faster and to a larger size than males (Sun et al., 2002), females mature later than males and the sex-ratio varies with length 0165-7836/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2006.11.027