McAllister et al.: Using experiments and expert judgment to model catchability of Pacific rockfishes 
283 
analyses have improved the conceptual understand- 
ing of the processes contributing to trawl catchability 
during surveys and have shown how expert judgment 
and other information can be used to help form priors, 
no unifying framework exists that integrates the judg- 
ment from multiple experts with other available data 
on survey catchability. 
In this article, we present a synthetic approach for 
integrating inputs on technical parameters elicited from 
experts and survey data not used in a stock assessment 
to form a prior for q gross that can be used to improve the 
value of biomass estimates from trawl-swept areas for 
use in stock assessments. This approach can be applied 
to swept-area abundance estimates across a number of 
areas and over multiple types of trawl gear (e.g., shrimp 
and groundfish trawl nets). Each expert is assigned 
equal prior weight which is then updated from ratios of 
relative catch rates between different survey gear types. 
We use estimates of the fraction of the total population 
biomass that lies within the boundaries of each sur- 
vey area and that accounts for the fraction of the area 
within the boundary of each survey that is trawlable. A 
factor is applied to prevent the expert inputs from being 
overly certain. Finally, because this method is applied 
to different surveys of the same stock, these parameters 
are not independent (in some surveys the same gears 
were used) and there is spatial covariance in estimates 
of the fraction of biomass in the different survey areas 
(the prior pdf formed accounts for the correlations in 
the q gross parameters between surveys). We illustrate 
the method with an application to bocaccio ( Sebastes 
paucispinis) off British Columbia (B.C.) that relied upon 
technical information obtained from interviews with a 
dozen trawl captains. We show the sensitivity of the 
results to assumptions about potential differences in 
rockfish density between trawlable and untrawlable sub- 
strates, the amount of uncertainty in expert inputs, and 
how results from different experts should be integrated. 
The impact on the overall stock assessment results are 
illustrated by comparing results obtained with and 
without informative priors for q aross - 
Bocaccio in British Columbia were chosen to illus- 
trate the new method to formulate a prior for q gross 
because this species presents an instance in which the 
time series of abundance indexes available for stock 
assessment are mostly too short or imprecise to en- 
able estimation of parameters of population dynamics 
and abundance trends. An informative prior for survey 
q is essential to achieve these ends. Bocaccio range 
from the Alaska Peninsula to Baja California (Love et 
al., 2002). In British Columbia, adult bocaccio exhibit 
a widespread distribution mainly on the outer coast 
(Fig. 1). Most catches are taken close to the bottom 
over depths of 60-200 m near the break-in-slope of the 
continental shelf, as well as at the edges of troughs in 
Queen Charlotte Sound (QCS) and Hecate Strait (HS). 
Adult bocaccio can be semipelagic and are found over 
a variety of bottom types, although harvesters suggest 
they favour proximity to high relief and rocky bottom. 
In British Columbia, bocaccio are caught by trawl and 
hook-and-line gear along with many other groundfish 
species, including Pacific ocean perch (S. cilutus), yel- 
lowtail rockfish (S. flavidus), canary rockfish (S. pin- 
niger ), and lingcod ( Ophiodon elongatus). 
Indices from seven trawl surveys (Fig. 1) were used 
in our study. Four of the surveys, 1) the west coast of 
Vancouver Island groundfish (WCVI Gfish), 2) Queen 
Charlotte Sound groundfish (QCS Gfish), 3) Hecate 
Strait groundfish (HS Gfish), and 4) west coast of 
Haida Gwaii groundfish (WCHG Gfish) represent a 
set of nonoverlapping bottom trawl surveys that were 
started between 2003 and 2006 to collectively survey 
most of the B.C. coastal shelf between 50 and 500 m 
of bottom depth. The focus of these surveys was to 
provide relative indices of all groundfish species af- 
fected by the groundfish bottom trawl fishery in B.C. 
waters. For all four surveys, the Atlantic Western II 
groundfish bottom trawl was used and the surveys 
were conducted by the Canada Department of Fisher- 
ies and Oceans (DFO) staff on either the government 
research trawler (surveys 1 and 2) or chartered trawler 
(surveys 3 and 4). 
Two of the surveys, the WCVI shrimp (survey 5) and 
QCS shrimp (survey 6) are conducted by DFO staff on 
board the same DFO research trawler (Boutillier et al., 
1998). These surveys use a shrimp trawl and were de- 
signed to provide relative indices of shrimp abundance 
on two specific shrimp fishing grounds. For the seventh 
survey, the U.S. triennial survey a Nor’Eastern ground- 
fish bottom trawl was used. This survey was designed 
to monitor groundfish abundance in U.S. waters, but 
in some years covered a small portion of southern B.C. 
waters. This survey stopped covering Canadian waters 
after 2001. 
Methods 
General model structure for trawl survey 
catchability ( q gr0S5 ) 
See Table 1 for descriptions of all symbols used in this 
paper and Figure 2 for a schematic outline of the inputs, 
sub-models and outputs of the q prior model. We define 
catchability ( q gross ) as the ratio of biomass of rockfish in 
a particular survey area to the population biomass of a 
given rockfish population that is on average vulnerable 
to trawl survey gear on account of gear selectivity (i.e., 
the fully vulnerable population biomass). q gross is typi- 
cally considered to be the long-term average value and 
is applied as a scalar to the fully vulnerable popula- 
tion biomass (B ) modeled in a stock assessment model 
to predict the index of biomass obtained from a given 
trawl survey. The predicted swept-area biomass (/) is 
obtained from the product of B y and q aross - 
iy= Q gross X B y. 
For rockfish, it has been generally acknowledged that 
there are three main factors that may cause the value 
