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Table 3 
Relative distribution of fish in different sectors and parts of the kill zone of the gear as the gear approaches a stationary fish. 
The factors, proportion of the linear distance between the trawl doors that is within wingtips (a 3J ) and proportion in the dead 
zone (a 32 ) are used in the trawl survey catchability model. Distances are in meters. See Figure 3 for a schematic diagram of a 
trawl net. 
Net type 
Nominal 
door 
spread 
Nominal 
wing 
spread 
Nominal 
distance 
between 
doors, 
outside 
of wings 
Dead 
zone in 
herding 
area 
Effective 
herding 
zone 
Proportion 
remaining 
in herding 
zone 
(a 3 , 2 ) 
Proportion 
within 
wingtips 
(a 3,F 
Proportion 
removed by 
dead zone 
AWII trawl 
63.3 
14.4 
48.9 
6.0 
36.9 
0.583 
0.227 
0.190 
(groundfish) 
Nor’Eastern trawl 
58.9 
13.4 
45.5 
6.0 
33.5 
0.569 
0.228 
0.204 
(U.S. triennial) 
Shrimp trawl 
26.5 
10.6 
15.9 
6.0 
3.9 
0.147 
0.400 
0.453 
Table 4 
Relative proportions (P) of remaining fish in areas C and D1 (from columns 6 and 
7 in Table 3, Fig. 3). Both of these factors (a 4 and a 5 , respectively) are used in the 
trawl-survey catchability model. 
Net 
P between wingtips (a 4 ) 
P in herding zone (a 5 ) 
AWII trawl (groundfish) 
0.281 
0.719 
Nor’ Eastern trawl 
0.286 
0.714 
(U.S. triennial survey) 
Shrimp trawl 
0.731 
0.269 
(zone Dl, Fig. 3B, Tables 3, 4) 
(factor a 5n ) (see Eq. 10 for cap- 
tains who explicitly accounted for 
the dead zone and Eq. 11 for those 
who did not). 
Step 5 Determine the propor- 
tion of fish that will be herded 
from the path of the sweeps and 
bridles (zone Dl) into the path of 
net (inside the wingtips) (zone C). 
Question 3) What percentage of 
the fish in front of the bridles and 
sweeps would be herded into the 
path of the net ? 
The factor a 6 concerns the remaining fish in sweeps 
and bridles path and for this step the following assump- 
tions are made: 
1 fish initially in front of the net, stay in front of the 
net; and 
2 factor a 6 is the same for all nets. 
Step 6 Determine the proportions of fish that are cap- 
tured of those that end up in front of the net (a 7nc ). 
Question 4) What percentage of the fish that make it to 
area E will be captured and retained by the net ? 
Finally, of the fish that have ended up in front of the 
net (zone E in front of footrope, Fig. 3), what percentage 
will be captured and retained in the net? 
Step 1 Draw a value for the ratio of fish density in 
untrawlable areas to fish density in trawlable areas, a, 
from the density function for it (see Eq. 3 and below for 
specifications). 
Step 2 To generate a value for the fraction of total 
population biomass vulnerable to survey gear in each 
area, S s , draw a value for swept-area biomass in each 
of the eight coastal areas, using the lognormal density 
function and the empirical swept-area value as the 
median and the variance in the natural logarithm of 
the estimate (Eqs. 6-8, Table 2). 
Step 3 For each captain, draw a value for the proportion 
of fish below the headrope (a 4 ) using the parameters of 
the triangular distribution provided by each captain. 
Steps in the algorithm to compute a prior probability 
density function for estimates of catchability 
WinBUGS 1.4 (Lunn et al., 2000) was applied to synthe- 
size the inputs from the trawl captains and other techni- 
cal settings and to produce output density functions for 
the q gross values for each of the surveys. The steps of the 
algorithm applied are provided below. 
Step 4 For each captain, draw a value for the propor- 
tion of fish above the headrope that stay above the head- 
rope as the net approaches (a 2 ), using the parameters of 
the triangular distribution provided for each captain. 
For each captain, compute the proportion of fish en- 
tering the path of the net and doors from those in the 
water column that are in the path of the net and the 
doors (a 1 2 ), such that 
