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Fishery Bulletin 102(2) 



Discussion 



Age estimate determined from dorsal-fin spines 



Dorsal-fin spines appear to be useful for aging sailfish. 

 They are easily sampled without reducing the economic 

 value of the fish and can also be read easily (the growth 

 rings stand out clearly). In contrast, scales cannot be 

 used to age sailfish because scale deposition patterns 

 change as sailfish age (Nakumura, 1985), and otoliths are 

 extremely small and fragile and are often difficult to locate 

 (Radtke, 1983). Reading otoliths is more time consuming 

 and expensive than reading spines and spines can also 

 be easily stored for future re-examination (Compean- 

 Jimenez and Bard, 1983; Sun et al„ 2001, 2002). 



The problems associated with the fin-spine aging meth- 

 od used in this study were the possible existence of false 

 rings and the presence of the vascularized core which can 

 obscure early growth rings in larger fish. These problems 

 were also noted by Berkeley and Houde (1983), Hedge- 

 pet h and Jolley (1983), Tserpes and Tsimenides (1995), 

 Megalofonou (2000), and Sun et al. (2001, 2002). However, 

 Tserpes and Tsimenides (1995) and Megalofonou (2000) 

 noted that experienced readers can overcome the problem 

 of multiple rings by determining whether the rings are 

 continuous around the circumference of the entire spine 

 section and by judging their distance from the preceding 

 and following rings. We observed false rings in spines for 

 all age classes larger than age two, which we read with- 

 out problem by using these guidelines. The missing early 



growth rings in larger specimens were accounted for by 

 compiling ring radii statistics for younger specimens for 

 which at least the first or second ring was visible and by 

 comparing the radii of the first several visible rings of the 

 specimens that had missing early rings to the mean radii 

 and standard deviations of the compiled data. Similar ap- 

 proaches for solving the problem of missing rings have also 

 been used for Pacific blue marlin (Hill et al., 1989). 



Marginal increment ratio (MIR) analysis is the most 

 commonly applied method for age validation (Campana, 

 2001). The MIR analysis conducted for sailfish suggested 

 that one growth ring is formed each year from September 

 to November for males and from October to November for 

 females. Spawning for sailfish in the waters east of Taiwan 

 lasts from April through September (Chiang and Sun-). 

 This is exactly the period when growth is low, as indicated 

 by the narrow and translucent rings. Similar findings 

 have been reported for skipjack tunatAntoine et al., 1983), 

 swordfish (Ehrhardt, 1992; Tserpes and Tsimenides, 

 1995), and bigeye tuna (Sun et al., 2001). Although the 

 timing of annulus formation coincides with spawning sea- 

 son for sailfish in the eastern Taiwan, annulus deposition 



- Chiang, W. C, and C. L. Sun. 2000. Sexual maturity and sex 

 ratio of sailfish {Istiophorus platypterus) in the eastern Taiwan 

 waters. Abstracts of contributions presented at the 2000 

 annual meeting of the Fisheries Society of Taiwan, Keelung. 

 Taiwan, 16-17 December 2000, 15 p. The Fisheries Society of 

 Taiwan, 199 Hou-Ih Road, Keelung, 202 Taiwan. 



