308 
Fishery Bulletin 113(3) 
NO 
ON 
CD 
O) 
c 
03 
-£= 
o 
ro 
a> 
GC 
20 40 60 80 100 200 400 600 800 1000 
Parameter magnifier (%) 
Figure 2 
Relative changes from the Monte Carlo simulation with data collected during 1998-2006 for female Japanese eel (Anguilla 
japojiica) in the Kao-Ping River in southern Taiwan. The relative changes are measured as percentages, for the means 
(shown in the left panels) and SDs (shown in the right panels) of fishery-mortality-based reference points(EBRPs) i n sce- 
narios 2-6 of the spawning-biomass-per-recruit model, where the mean and SD of the natural mortality (M and ejy), the 
von Bertalanffy growth coefficient ( K ), the asymptotic length ( L „,), and the multiplicative error in the growth curve ( qr) 
were under-specified from 5% to 95% and over-specified from 150% to 1000% (except forvalues of that went from 50% to 
200%). In each graph, the solid line indicates the fishing mortality rate at which the spawning biomass per recruit (SPR) is 
30% of the SPR when fishing mortality is zero, and the dashed line indicates the fishing mortality rate at which the SPR 
is 50% of the SPR when fishing mortality is zero. Because of the large variation in RC values for the different scenarios, 
the scales of the y-axes differ greatly to allow changes to be seen. Under=underspecified parameters; Over=overspecified 
parameters. 
Effects of multiplicative error in the length-weight 
relationship 
The changes in £pW resulted in nonsignificant changes 
in the mean and SD of the 4 Fbrp s . They also did not 
affect the composite risks of exceeding these Fbrp s . 
Discussion 
Effects of uncertainty in mean and SD of natural mortality 
Estimates of the mean of M are usually highly uncer- 
tain, possibly because of the lack of appropriate data 
for direct estimation, data such as those from tag-re- 
capture experiments (Vetter, 1988). As M increases, the 
YPR curve becomes flatter with a decreasing maximum 
value and decreasing slope at the origin (fig. 17.18 in 
Beverton and Holt, 1957). This change in the shape 
of YPR curve accounts for the nonlinear accelerating 
trend of F max and Fq.i found in our study. Uncertainty 
in the mean (bias) of M can also cause significant bi- 
ases in results of the model for SPR, particularly at 
a high level of fishing mortality (Goodyear, 1993). A 
higher mean of M led to a steeper SPR curve, but the 
general shape of the curve was not altered, explain- 
ing the lower sensitivity of ^ 30 % and F^,q% to M. On 
the other hand, the SD (imprecision) of M resulted 
