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Fishery Bulletin 99(2) 
involved in defining overfishing and rebuilding overfished 
stocks as required under U.S. law (Restrepo et al., 1998). 
Estimating selectivity patterns for commercial fishing is 
a central issue in use of most stock assessment models 
based on forward simulation calculations (e.g. Deriso et ah, 
1985; Methot, 1990; Fournier and Archibald, 1982; Jacobson 
et ah, 1994). Changes in fishery selectivity patterns over 
time may be difficult to measure if length or age composi- 
tion data are not available for some years. When fishery 
length or age composition data are available, they can often 
be explained equally well by many different assumptions 
about fishery selectivity and population length or age com- 
position. To understand this, consider the catch in number 
(C L ) of a single size group (length L) from a population. If 
the fishing mortality rate (F) is low and the selectivity for 
the size group is s L , then C L ~ N r F v s L . Even if F v is known, 
the resulting catch C L could be from a high N L and low s L , 
low N l and high s L , or an infinite number of intermediate 
combinations. Problems are compounded if the operation 
of the commercial fishery and selectivity parameters have 
changed over time (Sampson, 1993; Brodziak et al., 1997; 
Rogers et al., 1997) or if natural mortality is also a function 
of size or age. For example, Tagart et al. ( 1997) found that 
scarcity of large female fish in fishery length-composition 
data was explained equally well by two models. One model 
had constant natural mortality and fishery selectivity de- 
creased with size. The other model had constant fishery se- 
lectivity and natural mortality increased with size. 
In our study, we estimated fishery selectivities for the 
commercial bottom trawl fishery using a new approach that 
complements estimates from stock assessment models. Our 
approach is based on information available in many fish- 
eries, including data from bottom trawl surveys, informa- 
tion about bathymetry of fishing grounds, fishing effort 
data from logbooks, and length- or age-specific vulnerabil- 
ities to commercial fishing gear from field experiments. 
First, we used Jacobson and Hunter’s (1993) method with 
our bottom trawl survey and bathymetric data to estimate 
depth distributions for fish of different lengths. Next, we 
used a new method based on commercial fishing logbook 
data, bathymetric information, length-specific vulnerabili- 
ties (from field experiments with commercial fishing gear) 
and depth distributions to estimate fishery selectivities in 
the commercial bottom trawl fishery. Our approach may 
be useful whenever the geographic distribution of fish de- 
pends on size or age, when fishing effort is not randomly 
distributed geographically, and when both survey densities 
and commercial fishing effort data are available. 
Our results show clear differences in commercial fish- 
ery selectivities among species, areas, and over time. In 
addition, our analysis provides new information on depth 
distributions of sablefish and more precise understanding 
about depth distributions of Dover sole and shortspine 
and longspine thornyheads. 
MateriaSs and methods 
All depths in this study are measured in fathoms (fm). Our 
study area was the continental shelf and upper continen- 
tal slope at depths of 100-700 fm (equivalent to 183-1280 
m) along the west coast of the U.S. between 36°00' and 
48°30'N (Fig. 1). We divided the study area near the Ore- 
gon-California border into southern (36°00'N to 43°00'N) 
and northern (43°00'N to 48°30'N) subareas to account 
for geographic differences in groundfish habitat, bottom 
trawl fishery and logbook data, and to accommodate areas 
defined for management of the groundfish fishery. The 
boundary 43°00'N separates the Eureka and Columbia 
INPFC (International North Pacific Fisheries Commis- 
sion) management areas. 
Areas (km 2 ) of each 100-fm stratum (estimated from 
spherical projections at sea level) were the same as those 
used by the National Marine Fisheries Service (NMFS) to 
estimate fish density and swept area abundance (Lauth 1 ). 
The shallowest depth stratum in our study (100-199 fm) 
was relatively larger in the northern subarea (24%) than 
in the southern subarea (16%, Fig. 1; Tables 1 and 2). 
Fishing effort shifted into deep water earlier in the south 
(Tables 1 and 2 ). Fishing effort data from the southern subar- 
ea were collected mostly from California logbooks, whereas 
fishing effort data from the northern subarea were mostly 
collected from Oregon and Washington logbooks. 
Survey data 
Data from eight NMFS bottom trawl surveys on the 
upper continental slope in our study area were used to 
estimate depth distributions (Table 3). Each survey was 
conducted during October-December from the National 
Oceanic and Atmospheric Administration (NOAA) ship 
Miller Freemcm (e.g. Lauth, 1997a, 1997b; Lauth, 1999). As 
a group, the surveys covered the entire study area (Fig. 1). 
A NMFS standard Nor’eastern otter trawl net with a 
27.2-m headrope, 37.4-m groundgear, 89-mm codend mesh 
and a 32-mm mesh liner was used in each bottom trawl 
survey. In each survey, bottom trawl stations were allo- 
cated roughly in proportion to the area of 100-m depth 
strata (100-199, 200-299, 300-399, 400-499, 500-599, 
and 600-699 fm). Tows with poor gear performance, out- 
side the study area, and at depths greater than 699 fm or 
less than 100 fm were excluded from our study. Lengths 
of Dover sole captured in surveys were recorded as total 
length (TL) in mm. Lengths of other species were recorded 
as fork length (FL) in mm. 
Fishing effoit data 
Bottom trawl fishing effort data (hours towed) for the 
northern (Table 1) and southern area (Table 2) were 
obtained from logbooks submitted by commercial vessels 
operating out of ports in Washington (1985-97), Oregon 
( 1978-97), and California ( 1978-96). The fishing effort data 
in our study were nominal (as reported) hours towed for 
bottom trawl tows in which the catch of Dover sole, thorny- 
heads, or sablefish was greater than zero. 
1 Lauth, R. 1998. Personal commun. Alaska Fisheries Sci- 
ence Center, National Marine Fisheries Service, 7600 Sand 
Point Way, BIN C15700, Seattle, WA 98115-0070. 
