Chiang et al.: Analysis of sex-specific spawning biomass per recruit of Istiophorus platypterus in off eastern Taiwan 
273 
Figure 7 
Yield-per-recruit isopleths against fishing mortality per year and age at 
first capture ( t c ) at different levels of natural mortality (M) for male sailfish 
( Istiophorus platypterus) in the waters off eastern Taiwan. 
when M was assumed to be 0.2. This result 
would indicate that the stock of sailfish in the 
waters off eastern Taiwan appears to be mod- 
erately exploited and has relative low risk of 
being overfishing. 
Sex-specific per-recruit analyses 
and sensitivity analyses 
Most assessment methods require historic infor- 
mation on catch, effort, and catch-at-age (or 
catch-at-length). The lack of long-term fishery 
statistics usually makes it difficult to evaluate 
the status of populations exploited by small- 
scale fisheries or taken incidentally (e.g., Gov- 
ender, 1995; Barbieri et al., 1997; Griffiths, 
1997; Jones and Wells, 2001; Sun et al., 2002, 
2005). Per-recruit analyses, which require only 
parameters related to life span and mortality, 
combined with an analysis of catch curves, can 
become an alternative method for evaluating the 
status of a fishery. Because per-recruit analyses 
can provide the estimates of biological reference 
points and the catch curve analysis can yield 
10 ■ 
0 1 ■ — ■ — * 
0 0.2 0.4 0.6 0.8 1 
Fishing mortality (1/year) 
Figure 8 
Spawning potential ratio (expressed as a percentage) against 
fishing mortality for female sailfish (Istiophorus platypterus) in 
the waters off eastern Taiwan where age at first capture (t c ) = 
5 years (solid dots are SPR cvr levels; horizontal dashed lines 
are levels for the biological reference points of PgsB4o ar| d Fssb25’ 
and • = F cur ). 
