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Fishery Bulletin 108(3) 
lable areas in a surveyed area and the fraction of the 
surveyed area that is untrawlable. 
It can be shown that the expected value for the sur- 
vey biomass index when it is computed only from tows 
in trawlable habitat is 
E(I t ) 
dnet ^ ^ pop 
(fr + a (l~/r)) 
(3) 
where f T 
a 
S 
the fraction of the total survey area that is 
trawlable; 
the ratio of fish density in untrawlable to 
trawlable habitat; and 
the fraction of the total population biomass 
potentially susceptible to capture by survey 
gear ( B ) and that is on average present 
in the surveyed area. 
To briefly explore the implications of this equation let 
£ s = /r, s +a< 1 -/r, s )> < 4 > 
where f Ts = the fraction of sea bottom in a surveyed 
area that is trawlable in survey area s; 
and 
a = the ratio of target species density (t/km 2 ) in 
untrawlable to density in trawlable habitat. 
Although a may vary with survey area, we typically 
do not have data that would allow us to estimate these 
separate factors and a therefore was assumed not to 
depend on s. In contrast, it is common to have data on 
the fraction of trawlable area within each survey area 
and therefore one can thus compute factor g for each 
survey area. 
Should some experiment provide data on a, then the 
prior for a, P(a), could be statistically updated with the 
following equation: 
P(a\data)xP(a)P(data\a), (5) 
where P(a\data) = the posterior for a, given the data; 
and 
P(data\a) = the probability of the data, given 
a. 
Note that no such data were available for our case study 
application. f Ts can be treated as a beta random variable 
with binomial data on the number of trawlable sites for 
each area, or it can be fixed, if the number of trials in 
each area is very large. 
q availability-^* fraction of total exploitable 
species abundance in each surveyed region 
We developed a protocol to approximate the percentage of 
the coastwide target species exploitable biomass that is 
available to each of the surveys. This protocol computes 
and treats as a random variable the fraction of the total 
coastwide swept-area biomass in each survey area and 
accounts for potential differences between survey areas 
in the fraction of trawlable ground in each survey area. 
For this protocol, tow-by-tow data from the most exten- 
sive trawl survey (i.e., the “reference survey”) were used. 
The protocol is illustrated with bocaccio as an example 
and includes the following assumptions: 
1 The relative distribution of stock biomass among 
areas has been constant over a reference set of years 
and two or more groundfish trawl swept-area bio- 
mass estimates are available for all survey areas. 
For bocaccio this reference set covers the years from 
2003 to 2007. 
2 The proportion of untrawlable area to trawlable area 
within a surveyed region varies among regions and 
can be approximated from the observed frequencies 
of trawlable and untrawlable sites in the cumula- 
tive set of randomly allocated survey locations in 
each survey area. In fact, the locations found to be 
untrawlable are removed from the set of locations to 
be considered for research trawling in future years. 
However, in recent calculations we have found that 
the impact of this sequential removal of untrawlable 
locations on the estimated fraction of trawlable area 
is very small because the fraction of untrawlable 
areas is overall relatively small (less than about 
20 %). 
3 The ratio of target species density in trawlable and 
untrawlable areas (a) is the same across areas and 
time. 
4 The habitat for the target species is assumed to be 
the seabed area between a fixed depth range, e.g., 
100-300 m in bottom depth for bocaccio. 
Building on the above assumptions, we assume that 
the proportion of the coastwide target stock biomass 
available to each survey is the ratio of the swept-ar- 
ea biomass (adjusted for untrawlable area) estimated 
from trawl surveys during the reference-year period to 
coastwide stratified swept-area biomass of the target 
species (Table 2). For bocaccio, the coastwide swept- 
area biomass was the sum of the swept-area biomasses 
computed in each survey area from the DFO groundfish 
surveys plus a swept-area biomass estimate from the 
unsurveyed area not covered by the DFO groundfish 
surveys (Fig. IB). This coastwide swept-area biomass 
includes regions over 100-300 m in bottom depth on 
the outer coast or in Hecate Strait (see, for example, in 
Fig. IB the gap off the southwest coast of Haida Gwaii 
between the west coast of Haida Gwaii (WCHG) and 
QCS surveys). For the latter survey, a global estimate 
of species density was made across all trawl areas for 
the reference year period. 
For surveys other than the reference survey (e.g., for 
bocaccio in the West coast of Vancouver Island (WCVI) 
shrimp survey, Queen Charlotte Sound (QCS) shrimp 
survey, and U.S. triennial surveys), the biomass of the 
target species present in the areas covered by the sur- 
veys was computed from densities observed in the ref- 
