DeMartini et al.; Age and growth of Xiphias g/odius 



365 



200 



E 

 o 



T 150 



r 100 



50 



crement (35 cm per yr) that we observed for 

 the three tagged-recaptured fish, yearlings 

 in size when first caught and at liberty for 

 1, 2, and 4 years, is reasonable, as well as 

 consistent, with our modeled growth trajec- 

 tories for fish of this size range. We none- 

 theless caution, however, that these few 

 consistencies by themselves do not verify 

 our growth curves. 



Sexually dimorphic growth 



By now there can be no doubt that female 

 swordfish grow faster and attain larger 

 maximum body sizes than do males in 

 the Pacific (Castro-Longoria, 1995; Sun 

 et al., 2002; Cerna^; this study), as many 

 others (e.g., Berkeley and Houde, 1983) 

 have observed for swordfish in the Atlantic. 

 That females on average live longer than 

 males is less certain, and this uncertainty 

 will likely persist because of the rarity of 

 the largest, oldest fish captured by fisheries 

 — especially in developed fisheries — and 

 associated difficulties in validating esti- 

 mates of their ages. Even though sexually 

 dimorphic growth does not become promi- 

 nent in either body length or fin rays until 

 after age 1, it is obvious that the sexes 

 should be evaluated separately in stock assessments 

 (Wang et al., 2005) for retained swordfish (generally 

 greater than 1 yr old in most fisheries) whenever land- 

 ings data allow. The sexual differences in body size and 

 growth that we observed reinforce the argument that 

 sex-specific, age-based stock assessments are needed 

 for swordfish. 



The greater probability of error in estimating the ag- 

 es of larger swordfish affects females disproportionately 

 as the larger-bodied fish. Sex-specific stock assessments 

 need to explicitly evaluate the greater uncertainty of 

 age estimates for the largest, mostly female, fish as well 

 as the more generally recognized influences of gear bias 

 on the capture efficiency (hence sex ratio) for male and 

 female swordfish of different sizes. 



Unresolved issues and future research needs 



In prior studies of swordfish age and growth there has 

 been the conspicuous need to validate the periodicity of 

 annulus formation for all age groups present in a fish- 

 ery, including the oldest age groups, which are typically 

 relegated to a catch-all plus group that complicates stock 

 assessment computations (Restrepo and Powers, 1991). 

 Validation of the age and growth of these largest, oldest 

 fish likewise has been a problem in our study, and our 

 estimates of growth-at-liberty for tag-recaptured fish 

 only provide insights into growth (not absolute age) of 

 juveniles and young adults. There is still great need 

 to verify the accuracy of age estimates for swordfish 

 in older age-groups. Radiometric aging (Watters et al., 



A 



Tagged fish # 1 

 Tagged fish #2 

 Tagged fish #3 

 VBGF-predicted curve 

 95% confidence bounds 



5 6 7 

 Age (yr) 



10 11 12 



Figure 5 



Fitted equation (based on standard von Bertalanffy growth formula, 

 VBGF) describing eye-to-fork length (in cm)-at-age (in yr) for swordfish 

 iXiphtas gladius) (both sexes pooled); calculated growth trajectories 

 for each of three tagged-recaptured swordfish (sexes unknown) are 

 indicated. 



2006), one alternative method for aging the hard parts 

 of large, old fish, is inappropriate for swordfish because 

 the oldest living tissue is reabsorbed within fin rays and 

 because their tiny otoliths provide insufficient mate- 

 rial for analysis of individual fish. Resolution of the 

 problem would almost certainly require a dedicated, 

 large-scale, and expensive conventional tagging effort 

 targeting large and old fish, preferably one in which a 

 rigorous protocol is implemented for accurately estimat- 

 ing size at initial capture and labeling body size at time 

 of release with a fluorescent biomarker. Such a study is 

 unlikely to be implemented unless greater reductions in 

 stock sizes, increases in value, or (more likely) both in 

 concert, justify the great cost of such an enterprise. Of 

 course, such a study could provide data on fish move- 

 ments whose importance might dwarf that of age and 

 growth validation. 



Geographic variation in growth rates is evident for 

 swordfish in regional Pacific fisheries. Size-at-age is 

 appreciably greater for swordfish caught in the region 

 of the Hawaii-based fishery (versus the Taiwanese fish- 

 ery) and the regional difference includes juvenile and 

 small adult, as well as larger adult fish. For example, 

 the mean length at age 1 of swordfish in the Taiwanese 

 fishery is about 96 cm lower-jaw-to-fork length, which 

 is equivalent to only 83 cm EFL (Sun et al., 2002), 

 whereas the mean length at age 1 in the Hawaii-based 

 fishery is 99 cm EFL. The results of our study indicate 

 that swordfish caught in the central North Pacific grow 

 in length at a rate faster than swordfish caught by 

 several other Pacific regional fisheries (Fig. 6), and this 



