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Table 8 
The correlation between measures of expertise and the posterior placed on each trawl captain’s input. These correlations are 
presented to evaluate the presumption that the reliability of a trawl captain’s judgment on trawl-net catchability increases with 
experience (i.e., the posterior probability on the captain should be positively correlated with years of experience). See Results 
section for further details. 
Years of Total Total Posterior probability 
experience landings (t) bocaccio catch (t) obtained on a captain 
Years of experience 1 
Total landings (t) 0.66 1 
Total bocaccio catch (t) 0.70 0.85 1 
Posterior probability obtained on a captain -0.09 —0.29 -0.24 1 
the central tendency of the experts’ dis- 
tributions and gives a prior CV in q net 
for each expert that is no less than 0.5. 
Simulation evaluation has shown that 
prior CVs for q>0.5 enable stock assess- 
ment data to override a biased prior for 
q within relatively few (e.g., 5-10) years 
(McAllister and Kirkwood, 1998). The 
choice of a CV of 0.5 is somewhat arbi- 
trary but in our view necessary. 
A hierarchical meta-analysis of stock 
assessment data from different popu- 
lations of the same species group was 
applied to quantify the cross-stock cen- 
tral tendency and variability in q„ ross 
for rockfish in the U.S. triennial sur- 
vey (Millar and Methot, 2002). This 
approach has the advantage of avoid- 
ing expert judgment altogether and is 
tractable providing there is uniformity 
in the survey gear used in the differ- 
ent surveys. However, differences in be- 
havioral responses to trawl gear among 
species could limit the validity of the 
assumption of exchangeability, which 
must be made in hierarchical model- 
ing, and thus limit the applicability 
of the results as a prior distribution 
to an unsampled population. The log- 
transformed mode in the posterior pdf 
of “bulk” catchability equated to about 
1.27 between the wingtips. The ratio of 
doorspread to wingspread ratio for this 
survey is not available but is probably 
similar to the approximately 4.4:1 ratio 
of the AWII configuration used in the 
DFO groundfish surveys and translates 
to a doorspread catchability estimate of 
about 0.29. 
The priors provided in this study have 
higher CVs (0.8— 2.7) than previous priors on survey q difference is partly due to the high uncertainty in the 
obtained from different experts (ranging from about fraction of the population falling within each survey 
0. 4-0.7, e.g., Punt et al. 1993; Boyer et al., 2001). This area which did not apply to the other studies because 
A 
C 
Figure 10 
Probability density functions for trawl survey net catchability (q net ) for 
each captain for the (A) Atlantic western (AWII), (B) shrimp, and (C) 
Nor’Eastern trawl nets, based on inputs provided for each captain with 
Bayesian updating and with the uncertainty factor applied. 
