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Fishery Bulletin 108(3) 
Discussion and conclusions 
We provide an approach to formulating a Bayesian 
prior for trawl survey catchability for rockfish that can 
integrate subjective judgment from several experts on 
factors affecting net catchability with data obtained 
from field experiments and trawl research surveys. The 
approach is useful for situations where a stock assess- 
ment model is to be fitted to one or more survey indices 
and although directed at rockfish surveys, it could be 
extended to other groundfish species. For this approach, 
data are used from a series of related trawl surveys 
covering most of the British Columbia continental shelf, 
all based on the same trawl gear, to provide estimates of 
the fraction of the population in each survey area. Bias 
correction factors are formulated and can be updated 
with experimental data that allow us to evaluate hypoth- 
esized average differences in target fish species density 
between trawlable and untrawlable areas and the frac- 
tion of area in the survey that is treated as untrawlable. 
The current approach presumes that experts all have 
experience with all of the types of nets that are used in 
the surveys or with nets that are very 
similar. The approach updates each 
captain’s inputs on the basis of the con- 
sistency with observations from experi- 
ments where the ratios of catch rates 
between different types of survey nets 
were evaluated. This updating process 
reduced the degree of uncertainty in 
the prior by considerably modifying the 
posterior distributions, particularly for 
the most poorly understood gear — the 
shrimp trawl. 
A few different procedures were ap- 
plied to counteract the adverse effects 
on stock assessment results that may 
result from the tendency of individual 
experts to provide distributions that 
are too certain. One procedure was 
to apply an uncertainty factor (Boyer 
et al., 2001); this is discussed further 
below. The second was to apply a mix- 
ture distribution approach to incorpo- 
rate judgment from different experts. 
However, this procedure may cause the 
resulting posterior distributions to be 
multimodal — a feature that may arise 
when the narrow distributions offered 
by the various experts fall into differ- 
ent modes. Such a result is less likely 
when there is a large number of con- 
tributing experts. 
Attempts to directly estimate catch- 
ability with trawl nets ( q net ) have met 
with limited success, particularly for 
rockfish. The principal difficulty lies 
in measuring the abundance of fish 
that is positioned in front of the net. 
Krieger and Sigler (1996) attempted to 
estimate catchability of Pacific ocean 
perch (S. alutu s) for a bottom trawl 
on the basis of observations from a 
submersible vessel. They reported es- 
timates for q net of 0.97-1.27 for trawl 
catchability based on wingtip spread. 
Using the AWII to calculate an ap- 
proximate ratio of doorspread to wing- 
spread of about 4.4 (AWII, Table 3), 
they determined a doorspread catch- 
ability of 0.22-0.29 for Pacific ocean 
