McAllister et al: Using experiments and expert judgment to model catchability of Pacific rockfishes 
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Implementation of the method for bocaccio 
In the q prior model for British Columbia bocaccio, 
we treated the factor a as a random variable, having 
a triangular prior distribution with a minimum of 1, 
maximum of 10, and mode at 3. f T is provided by survey 
area (Table 2) and treated as known because the number 
of sites sampled per survey area was high in all areas 
(300-1000 depending on the survey area). Shelf regions 
without trawl surveys (unshaded regions in Fig. IB) 
were excluded from the original set of groundfish sur- 
veys because the fraction of trawlable seabed was known 
to be very low. Thus, f T in areas where there are no 
surveys is presumed to be 0% 
The average and standard error (SE) in the average of 
the natural logarithm of the available estimates of an- 
nual swept-area biomass for each survey region (Table 
2) were computed and applied in the q prior model to 
generate from a lognormal density function samples of 
potential stock biomass in each region and the potential 
fraction of total stock biomass in each region. Some 
of these standard errors were very large and created 
large uncertainty in the fraction of stock biomass for 
each of the regions. Note that in instances in which the 
SE is less than 0.15, we recommend that this value be 
set to 0.15, because, in general, the minimum CV in a 
swept-area biomass, accounting for all sources of error 
variability (i.e., SE divided by the mean) for relative 
stock size, should be no less than 0.15 (the CV of a 
lognormal distribution is Vexp(a 2 )-1, where a is the SD 
in the natural logarithm of the random variable). The 
empirical values for SE may be low because of small 
sample sizes (e.g., n - 2 years) and chance. We believe 
that because of the highly clumped spatial distribution 
of bocaccio, longer time series would yield higher values 
for SE than were obtained when the empirical values 
happened to be less than 0.15. 
The U.S. triennial survey and the two shrimp surveys 
are contained within the WCVI and QCS groundfish 
surveys (Fig. IB). These larger surveys that contain the 
smaller, more localized ones are called here “containing 
surveys.” For the smaller or “contained” surveys, the 
random variable (RV) for I Ts was limited to the product 
of the fraction of area occupied by the contained sur- 
vey and the RV for I Ts for the containing survey. This 
computation presumes that bocaccio density in the con- 
tained survey is no larger than that in the containing 
survey and limits the biomass for the contained survey 
to no more than that expected if the density was the 
same between the contained and containing survey. 
In our application, we consulted with 12 commercial 
trawl captains — each with at least 10 years of experi- 
ence in trawling for rockfish. All captains had experi- 
ence (11-22 years) with types of trawls used in the DFO 
groundfish and U.S. triennial surveys, i.e., both ground- 
fish and shrimp trawl nets, and with total groundfish 
landings ranging from 6800 to 275,000 t. Captains 1-4 
were interviewed in groups of two and the remaining 
captains were interviewed separately. An attempt was 
made in each interview to provide the same explanation 
for the requested information, although the interview 
was conducted in an informal conversational manner. 
The format undoubtedly varied in subtle ways over 
the course of the 12 interviews. During our interviews 
with trawl captains, we characterized “typical survey 
fishing” as occurring on average at 150 m depth from 
June to July from 1 h after sunrise to 1 h before sun- 
set. This fixed interval of time was necessary because 
trawl captains preferred to answer the questions while 
considering specific fishing conditions (i.e., time, depth, 
season, etc.) 
For bocaccio, only one of the captains presumed 
that the a 6 proportion reflected the proportion of fish 
between the dead zone and the path of the net that 
are herded into the path of the net. The rest of the 
captains presumed that this proportion reflected the 
fraction of fish between the doors and the path of the 
net that are herded into the path of the net. The door- 
spread of the U.S. triennial survey Nor’ Eastern trawl 
net was not measured. We assumed it had the same 
ratio of wingtip to doorspread as that of the Atlantic 
western (WII) trawl net used in the DFO groundfish 
survey. 
For the q net interview questions, each captain was 
asked for catch estimates for each of the three nets. 
The nets are towed at different speeds, have different 
vertical openings and, perhaps most importantly, the 
mouth opening of the shrimp trawl is not configured, 
so that the headrope overhangs the footrope (known as 
a “cape”). The net parameters are as follows: 
1 DFO Atlantic Western trawl: towed at ~3 knots, and 
having a 3.7-m vertical opening; 
2 U.S. Nor’ Eastern trawl: towed at ~3 knots and 
having a 7.1-m vertical opening; and 
2 DFO shrimp trawl: towed at ~2 knots and having a 
2.7-m vertical opening. 
Most groundfish trawls have a shorter headrope 
than footrope so that the headrope precedes the foot- 
rope through the water providing a “cape” or “hood.” 
As a fish encounters the footrope, it cannot escape by 
swimming directly up. On the shrimp trawl, however, 
the headrope and footrope are virtually in line. Pre- 
sumably, when the bocaccio detect the proximity of the 
mouth opening of the shrimp trawl, the net front is 
already effectively a 2.7-m vertical “wall” of footrope, 
disturbed sediment, and headrope. It is reasonable to 
assume that some bocaccio would escape vertically. 
When a bocaccio encounters the groundfish footrope, 
however, it is surrounded on four sides (wings, cape, 
and the bottom). We assumed that the relatively large 
bocaccio did not escape through the net and that the 
probability of retention was 1. The value for this fac- 
tor, a 7n , depends on the net (a 71 for the AWII trawl, 
a 7 2 for the triennial Nor’Eastern, a 7 3 for the DFO 
shrimp trawl). Our approach derives catchability based 
on doorspread, and therefore the U.S. triennial and the 
WCVI shrimp trawl estimates first had to be altered 
by the ratio of wingspread to doorspread (a Sn ). 
