294 
Fishery Bulletin 108(3) 
Table 5 
Biomass estimates of bocaccio ( Sebastes paucispinis) based on survey tows using the shrimp trawl survey net and the Atlantic 
western (WII) groundfish trawl survey net, Estimates were based on survey positions shown in Figures 4 and 5. Results are 
shown for Queen Charlotte Sound (2003-07 for shrimp trawl and 2004-05 for AWII groundfish) and west coast of Vancouver 
Island (2004, 2006). The ratio of catch rates between these two survey gears was used to screen the plausibility of values for 
trawl-net catchability for these two nets that was determined from the interviews of the trawl captains. 
Shrimp trawl 
AWII groundfish trawl 
Region 
Number of tows 
Biomass (kg) 
Number of tows 
Biomass (kg) 
Queen Charlotte Sound 
212 
4993 
52 
39,746 
West coast of Vancouver Island 
141 
5258 
66 
20,787 
The shrimp and groundfish survey gears were ap- 
plied in the same years in the survey area of the WC- 
VI shrimp survey and the QCS shrimp survey. The 
observed mean ratio of bocaccio density between the 
trawl and shrimp survey nets for QCS for the years 
2003, 2004, 2005, and 2007 was 8.76, with a SE in the 
natural logarithms of the estimates of 0.59 (Table 5, 
Fig. 4). The observed mean ratio for density estimates 
for the WCVI shrimp survey region between the ground- 
fish and shrimp nets for the years 2004 and 2006 was 
3.95, with a SE of 0.116 (Fig. 5). This latter SE was 
increased to 0.3 for the statistical estimation, because 
it was judged unlikely that the precision could be so 
high and there were only two years of survey data to 
provide this estimate. In each Monte Carlo iteration, 
the natural logarithms of the computed q net values cho- 
sen for the shrimp and groundfish surveys were taken 
and the logarithm of the q net for the shrimp survey was 
subtracted from the logarithm of the 
q net for the groundfish survey. This 
difference was used as the expected 
log ratio for these survey catch rates 
for these two types of trawl nets. A 
lognormal density function was then 
applied to compute the probability of 
the observed ratio, given the model 
predicted ratio of q net for these two 
nets: 
Figure 4 
Mapped zones and trawl tow positions (symbols) of the Queen Charlotte Sound 
groundfish and shrimp trawl surveys where the two surveys overlapped. These 
two surveys provided an unintended reference source of the catch-rate ratio 
data for these two survey gears that were then used to update the groundfish- 
to-shrimp trawl-net catchability ratios for each of the experts. Polygons sur- 
rounding the overlapped survey areas were connected by hand to delimit the 
outer boundaries. Open and closed circles indicate the absence and presence, 
respectively, of bocaccio ( Sebastes paucispinis) in the groundfish surveys. Open 
and closed triangles indicate the absence and presence, respectively, of bocaccio 
in the shrimp surveys. 
Ki-j = lo g <9 n =;, s )- 
log< s ) and i*j, 
r -°Ki-j 
log normal(lr s t a si _ 2 ). 
(15) 
where the subscripts i and j denote 
the DFO groundfish survey and 
shrimp survey nets; respectively, 
and 
r_ob si _j = the observed ratio of 
density values from 
groundfish and shrimp 
nets for survey area s. 
As indicated above, there are two 
observed ratios for bocaccio; one 
for the WCVI and one for QCS. For 
WCVI ,o si _j was 0.3, and for QCS, 
o s ■ • was 0.59. 
The WinBUGS results for bocac- 
cio were numerically stable after 
