272 
Fishery Bulletin 107(3) 
Fishing mortality (1/year) 
Figure 6 
Yield-per-recruit isopleths against fishing mortality per year and age at 
first capture ( t Q ) at different levels of natural mortality (M) for female 
sailfish ( Istiophorus platypterus) in the waters off eastern Taiwan. 
duced higher estimates ofF 0 x for sailfish in the water of 
eastern Taiwan. For scenarios E-J that incorporated the 
combinations of uncertainties in F, M, and t c , large varia- 
tions were observed for estimates of F 0 x when a higher 
level of uncertainty was assumed for M (scenarios F, H, 
and J). Even though combinations of parameters with 
higher uncertainties were considered, the upper bounds 
(the third quartile) of F CUR /F 0 x were lower than 1 for all 
scenarios. This result implied that it was highly unlike 
that F cur could exceed F 0 v In contrast, the effects of 
adding uncertainties in F and t c were relatively minor. 
Figure 11 shows the box plots of the estimates of 
-^SSB40> -^SSB25’ -^CUR^0.1SSB40, and F CUR /F 0 lggB25 ^ or SCe ' 
narios A-J (with uncertainty). Similarly, higher levels 
of uncertainties in M resulted in higher variations for 
the estimates of F gSB40 and F ggB25 (scenarios C, F, H, 
and J). Moreover, the estimates of F gSB40 and F ggB25 
were obviously higher than those of the base case when 
the uncertainty in t c was considered (scenarios D, G, H, 
I, and J), which might result from female fish younger 
than t c being less vulnerable to fishing gear. Therefore, 
higher values of SSB/R would be obtained when larger 
values of t c were selected and higher estimates of F gSB40 
and F SSB25 were revealed for these scenarios. For all 
scenarios, the medians of F CUR /F ggB40 and F CUR /F ggB25 
were 0.71 and 0.37, respectively, which were similar to 
those of the base case (Table 5). The upper boundaries 
of F CUR /F SSB40 and F CUR /F SSB25 were substantially lower 
than 1 for all the scenarios and no simulation runs 
were observed to have the values higher than 1. 
Discussion 
Mortality rates 
The Y/R curve is so flat-topped that maximum Y/R 
cannot be well defined and may not be attained under 
any practical fishing mortality rate (Fig. 5). Reference 
points such as F 0 1; F gSB25 , and F ggB40 have often been 
used to develop fishery management strategies. Previous 
studies have indicated that Y/R and SPR are sensitive 
to values of M (Griffiths, 1997; Kirchner, 2001; Sun 
et al., 2005). In this study, however, the estimates of 
F CU r were much lower than the target levels of F 0 x or 
F sgB40 except for the scenario when M was assumed at 
the lower bound of 0.2. Moreover, the estimates of F CUR 
were never larger than the threshold level of F ggB25 even 
