Chiang et al.: Analysis of sex-specific spawning biomass per recruit of Istiophorus platypterus in off eastern Taiwan 
275 
B 
ABODE FGH 
Scenario 
ABODE FGH 
Scenario 
ABODE FGH I J 
Scenario 
D 
0.7 
- 0.6 
o 
-L 
§ 0.5 
o 
0.4 
0.3 
ABODE FGH I 
Scenario 
FSgure 10 
Box plots of the estimates of F 0 1 and F CUR /F 0 i f° r scenarios incorporating the uncertainty in 
total mortality (Z), natural mortality (M) and age at first capture (t c ) for female (A, C) and 
male (B, B) sailfish (Istiophorus platypterus) in the waters off eastern Taiwan. The center 
line represents the median and the boxes represent the quartiles. The whiskers extend 1.5 
times the interquartile range. Open circles are outliers. Horizontal dash lines indicate the 
levels of reference points at base case. 
There is a possibility that the low F estimates may 
be an artifact of an influx of fish from outside the 
study area. Such a bias could occur if the probability 
that a fish will move from distant waters to within 
range of the eastern Taiwanese fleet is substantial and 
increases with the age of the fish (Sun et al., 2005). 
In principle, estimates for M could be obtained from 
research (such as tagging studies to determine M; 
Hampton, 2000). However, in the short to medium 
term, the values for M will have to be obtained from 
the results of studies for other stocks of sailfish. They 
have been similar to those obtained by Pauly’s (1980) 
method. This difference should be evaluated in relation 
to environmental factors. Although recent analyses 
of molecular markers do not support recognition of 
separate Atlantic and Indo-Pacific species of sailfish 
(Graves and McDowell, 2003), there is no evidence 
to indicate that sailfish become increasingly likely 
to migrate to Taiwan with increasing age. Although 
Prince et al. (2006) examined the tagging results in 
the eastern Pacific Ocean, they considered this spe- 
cies a single stock. It is unclear whether the sailfish 
population in the Pacific Ocean comprises a single or 
multiple stocks, and their regional or global abundance 
is unknown (Ehrhardt and Fitchett, 2006). A tagging 
program in which electronic and conventional tags are 
used to examine the spatial movement patterns and 
stock structure of sailfish in this geographical region 
would prove beneficial for the sustainable management 
of the species. 
There are no management measures for sailfish in 
the waters off eastern Taiwan at present. In the wa- 
ters off eastern Taiwan, sailfish are targeted by the 
gillnet fishery and caught incidentally in the longline, 
harpoon, and set net fisheries. This makes it difficult 
to effectively control the fishing effort of these fisheries 
for sailfish. In this case, the isopleths of SPR indicate 
that increasing t c to an age between six and seven 
years old would likely result in modest gains in terms 
of SPR (on the order of 40%) and also hedge against 
recruitment overfishing. For example, the target level 
of 40% SPR could be achieved at even double values of 
F 'cur if t e for female sailfish was larger than five years 
(the age at sexual maturity). Of course the efficacy of 
increasing t c would be mitigated by any substantial 
release mortality. At present little is known about the 
mortality rates of fish released from gillnet and long- 
line vessels operating off eastern Taiwan, and further 
