Chiang et al.: Analysis of sex-specific spawning biomass per recruit of Istiophorus platypterus in off eastern Taiwan 
269 
1400 
1200 - 
1000 - 
800 - 
600 - 
400 
200 - 
■ Female (n= 3532) 
Z Male (n=8791 ) 
80 
i—TTr-if—i » I F 1 ! 1 
95 110 125 
140 155 170 185 200 215 
260 
Lower iaw fork length 
Figure 2 
The length-frequency distributions (5-cm intervals) of female (black bars) 
and male (white bars) sailfish ( Istiophorus platypterus) in the waters off 
eastern Taiwan during the period from July 1998 to July 2005. Most fish 
caught were 160-190 cm in lower jaw fork length. 
0 
1 
8 
10 11 12 + 
Age in years 
Figure 3 
The age-frequency distributions for female (black bars) and male (white 
bars) sailfish ( Istiophorus platypterus) in the waters off eastern Taiwan 
during the period from July 1998 to July 2005. Most fish caught were 4-7 
years old. 
fish (446 females and 720 males). The 
range of lower jaw fork length was 
80-239 cm for females and 78-227 cm 
for males (Fig. 2). Age compositions of 
samples collected during the entire 
study period indicated that most sail- 
fish caught off eastern Taiwan are 
larger than the age at 50% matu- 
rity, and peak in length at 5 years 
(Fig. 3). Accordingly, the estimates 
of age at full recruitment (t c ) and the 
sample size during studying periods 
are shown in Table 3. The estimates of 
t varied from 5 to 7 years for different 
time periods. Few sampled fishes were 
older than 11 years for both sexes and 
hence age 12 and higher were com- 
bined into the 12+ group (Fig. 3). The 
empirical estimates of maximum lifes- 
pan (t max ) were 13 years for males and 
21 years for females. 
Mortality rates 
The estimates of F were 0.24/yr for 
females and 0.43/yr for males based 
on the samples collected during the 
entire study period. Based on boot- 
strap analysis, the standard deviation 
of F was 0.046/yr and 0.045/yr for 
females and males, respectively. In 
addition, 79% of bootstrap replicates 
of t c were 5 years old and few were 6 or 
7 years old. The values of M estimated 
from the Pauly’s empirical equation 
were 0.26/yr for females and 0.27/yr 
for males. In this study, therefore, F 
of 0.24/yr for females and 0.43/yr for 
males, t c of 5 years and M of 0.26/yr 
for females and 0.27/yr for males were 
set as the base case values for the sub- 
sequent analyses. Sensitivity analyses 
were also conducted for examining 
the results of Y/R and SPR analyses 
by assuming values of 0.20, 0.30, and 
0.35 for M. 
Y/R and SPR models and biological 
reference points 
In this study, selectivities for females and males were 
assumed to be dome-shaped, and the estimated selectiv- 
ity curves are shown in Figure 4. The estimates ofF CUR , 
F 0 j, and Y/R under various values of M are summarized 
in Table 4. For the base case, F CUR (0.24/yr for females 
and 0.43/yr for males) were substantially lower than the 
corresponding biological reference points F 0 1 (0.62 /yr for 
females and 0.79/yr for males). The estimates of Y/R cvn 
were 3.37 kg for females and 3.72 kg for males and the 
estimates of Y/R 0 x were 5.11 kg for females and 4.68 kg 
for males (Fig. 5). Even in the most conservative case, 
when M was assumed to be 0.2, F CUR was still lower 
than F 0 x for both females and males. 
The effect of varying t c on Y/R is shown in Figures 
6 and 7. At low levels of F, Y/R generally increased 
rapidly over the range of t c values tested. The values of 
t c that maximized the yield per recruit decreased with 
the magnitude of M and increased with the level of F 
but typically ranged between 2 and 5 years for females 
and males. 
