Chiang et al.: Age and growth of Istiophorus p/atypterus in waters off eastern Taiwan 



255 



100 -i 



80 



60 



40 



20 - 



□ Male (n=720) 

  Female (n=446) 



p p H , M ^ 



L*^ 



75 85 95 105 115 125 135 145 155 165 175 185 195 205 215 225 235 



Lower jaw fork length (cm) 



120 



90 



60 



30 



r? f P ^ 



2 16 20 24 28 32 36 40 44 48 52 56 60 



Round weight (kg) 



Figure 3 



The size-frequency distributions by 5-cm intervals (upper figure) and by 

 2-kg intervals (lower figure) for male and female sailfish (/. platypterus) 

 collected from the waters off eastern Taiwan. 



significantly higher than those from September through 

 November for males U-test, P<0.001) and from October 

 through November for females (f-test, P<0.001). Also, the 

 mean MIR in November was significantly lower than that 

 in December (f-tests, P CT <0.05, P 9 <0.05). The trends in the 

 monthly means of MIR when the data were split into ages 

 1-5 and 6+ were similar to those for all ages combined. 

 The results in Figures 5 and 6 indicate that one growth 

 ring is formed each year, most likely from September to 

 November for males and from October to November for 

 females. 



Figure 7 shows the sex-specific relationships between 

 LJFL and spine radius based on method 1 (linear regres- 

 sion) and method 2 (power function). The relationships for 

 males and females are significantly different (method 1: 



= 56.07, P<0.01; method 2: F, 



- = 59.93, P<0.01). 



According to AIC, the power function provides a better fit 

 to the data (4AIC = 38.57 and 30.96 for males and females, 



respectively). Therefore, the most parsimonious repre- 

 sentation of the data is the power function with separate 

 parameters for males and females. 



The mean back-calculated lengths-at-age obtained from 

 methods 1 and 2 are listed in Table 3. After the first year 

 of life, the growth rates of both sexes slow appreciably. 

 However, females still grow faster and consequently reach 

 larger sizes than males. The standard VB and the Rich- 

 ards function for males and females are shown in Figure 8 

 and the corresponding parameter estimates are listed in 

 Table 4. The growth curves for males differ significantly 

 from those for females (F=99.86 P<0.05 and P=107.38 

 P<0.05 for the standard VB curve [methods 1 and 2], and 

 P=144.01 P<0.05 and F=48.43 P<0.05 for the Richards 

 function [methods 1 and 2]). The Richards function pro- 

 vides a statistically superior fit to the data (log-likelihood 

 ratio test; P<0.001) when method 2 is used to back-calcu- 

 late length-at-age but not when method 1 is used. 



