300 
Fishery Bulletin 108(3) 
Table 6 
Posterior means and coefficients of variation (CV) in the natural logarithm for bulk catchability (q grosa ) under four different 
runs of the trawl survey catchability model with and without the Bayesian update and with and without the uncertainty factor 
applied. The values in the first column give the prior mean and CV for q gross that could be used in a stock assessment. WCVI =west 
coast of Vancouver Island; QCS = Queen Charlotte Sound; HS = Hecate Strait. WCHG=west coast of Haida Gwaii. 
Bayesian 
Bayesian 
update, 
update, 
uncertainty 
uncertainty 
factor, same 
factor, 
Bayesian 
Bayesian 
No Bayesian 
No Bayesian 
density in 
very high 
update, 
update, 
update, 
update, 
trawable and 
densities in 
uncertainty 
no uncertainty 
uncertainty 
no uncertainty 
untrawable 
untrawable 
Survey number 
factor 
factor 
factor 
factor 
areas 
areas 
Mean CV 
Mean CV 
Mean CV 
Mean CV 
Mean CV 
Mean CV 
and region 
WCVI 
groundfish 
0.070 
0.77 
0.067 
0.67 
0.068 
0.76 
0.065 
0.66 
0.145 
0.68 
0.046 
0.85 
QCS 
groundfish 
0.046 
0.80 
0.044 
0.71 
0.044 
0.79 
0.042 
0.70 
0.094 
0.71 
0.030 
0.89 
HS 
groundfish 
0.0067 
0.83 
0.0064 
0.74 
0.0064 
0.82 
0.0061 
0.73 
0.0137 
0.75 
0.0043 
0.91 
WCHG 
groundfish 
0.0021 
0.79 
0.0020 
0.69 
0.0021 
0.78 
0.0020 
0.68 
0.0044 
0.70 
0.0014 
0.87 
WCVI 
shrimp 
0.0030 
1.46 
0.0022 
0.81 
0.0072 
1.63 
0.0052 
0.94 
0.0062 
1.36 
0.0019 
1.51 
QCS 
shrimp 
0.00036 
2.74 
0.00026 
1.89 
0.00086 
2.92 
0.00063 
2.05 
0.00054 
2.52 
0.00029 
3.01 
U.S. triennial 
groundfish 
0.047 
1.73 
0.045 
1.62 
0.047 
1.70 
0.045 
1.61 
0.069 
1.50 
0.037 
1.87 
Table 7 
The posterior mean ratio and posterior coefficient of variation (CV) for the 
ratios for trawl-net catchability ( q net ) for the groundfish and shrimp trawl 
nets obtained from inputs provided by each of the captains. The posterior 
probability assigned to each captain’s inputs is given in the last column. 
Captain 
Mean ratio 
Mean CV 
Posterior probability assigned 
to each captain’s inputs 
1 
2.30 
0.52 
0.110 
2 
9.18 
0.45 
0.165 
3 
0.96 
0.15 
0.000 
4 
1.30 
0.10 
0.000 
5 
0.70 
0.38 
0.007 
6 
3.02 
0.19 
0.277 
7 
0.95 
0.10 
0.000 
8 
0.77 
0.25 
0.000 
9 
1.91 
0.25 
0.060 
10 
1.47 
0.15 
0.000 
11 
31.86 
0.11 
0.000 
12 
5.26 
0.26 
0.380 
counteract the problem of experts be- 
ing overly certain (Chrome et al., 1996; 
Martin et al., 2005; Uusitalo et al., 
2005). The use of highly precise prior 
distributions for q has at least two ad- 
verse consequences for fish stock as- 
sessment. First, when a highly precise 
prior for q is applied (e.g., CV<0.4), the 
estimates of quantities of interest (e.g., 
virgin biomass, B 0 ) can be highly pre- 
cise and exclude values consistent with 
stock assessment data (Boyer et al., 
2001). Second, simulation evaluations 
have shown that when highly precise 
priors for q (e.g., with prior CV<0.5) 
that are centered over values as little 
as 50% higher or lower than the ac- 
tual value, it takes many more years 
for stock assessment data to update 
precise priors than less precise priors 
centered over the same incorrect val- 
ues (McAllister and Kirkwood, 1998). 
Application of a multiplicative uncer- 
tainty factor with a median of 1 and a 
CV of no less than about 0.5 maintains 
